To sleep, perchance to dream: Why do we and other animals sleep?

Professor Keith Kendrick

Sleep seems to be a universal imperative for all animal species, a fact that has been accepted by mankind for two and a half thousand years!

“Almost all other animals are observed to partake of sleep, aquatic, winged, and terrestrial creatures alike. For every kind of fish and the soft-shelled species have been seen sleeping, as has every other creature that has eyes.”

Aristotle (384-322 BC) On Sleep and Waking

We may all consider the fantastic tales of incredible periods of sleeping by “Rip van Winkle” and “Sleeping Beauty” as beyond the realms of reality but in fact we will all have spent 25 years of our lives asleep given a life-expectancy of 75. You might think that science would have paid considerable attention to understanding the mechanisms and purpose of an activity that consumes one-third of our lives. Nothing could be further from the truth however. While progress is now being made, science is still unsure about what the key functions of sleep and dreaming are. We prefer, not surprisingly, to concentrate on the waking two-thirds of our lives where we are conscious and can remember our experiences.

Our current culture denigrates the value of sleep. It is considered to be a useless vestige of past human societies that were unable to function during the hours of darkness. Our control of artificial illumination and constant enthusiasm for work and/or entertainment has led to the creation of a 24 hour day culture. The claim of being able to do without sleep is considered to be a highly positive characteristic much vaunted amongst politicians, business moguls, doctors, academics and even full-time party animals!

“Marxists get up early in the morning to further their cause. We must get up even earlier.” Margaret Thatcher

“If you can’t sleep, then get up and do something instead of lying there and worrying. It’s the worry that gets you, not the loss of sleep.” Dale Carnegie

“I never use an alarm clock. I can hardly wait until five a.m. In the army I always woke before reveille. I hate sleeping. It wastes time” Isaac Asimov

The inventor of the light bulb was also fast to engender negative attitudes towards sleep (a good marketing ploy one might argue!):

“Sleep is an acquired habit. Cells don’t sleep. Fish swim in the water all night. Even a horse doesn’t sleep. A man doesn’t need any sleep” Thomas Edison

Elsewhere, positive statements about the importance of sleep:

“Early to bed, and early to rise, makes a man healthy, wealthy and wise” Benjamin Franklin 1758

have been replaced by negative ones:

“Early to bed, and early to rise, makes a man healthy, wealthy and socially dead” Animaniacs

So irrelevant and forgetful has sleep become that it scarcely rates a mention or serious consideration in modern novels or films whereas in Shakespeare’s works it received frequent attention.

Is sleep and dreaming a useless vestige of our past when we could not control our external environment and, if so, is there selective evolutionary pressure towards reducing its longevity? Alternatively, if it does serve an important function, what is it and what potential impact is our persistent effort to do without it having on the quality of our lives?

What exactly is sleep and when do we dream?

Most of what we know about the biology of sleep has been discovered only in the last 50 years. In adult humans there are 5 defined phases of sleep which normally occur in 90 minute cycles together with transition phases prior to the initiation (falling off to sleep) and termination (waking) of sleep.

These different phases of sleep are defined primarily by changes in the general electrical activity of our brains (measured by an electroencephalograph and known as EEG) as well as by other factors such as heart rate, respiration, muscle activity, metabolic rate and body temperature. There are also, of course, associated biochemical changes occurring that mediate these physiological changes.

When we are awake and alert the global electrical activity of the brain shows a pattern of low voltage high frequency waves indicative of large amounts of relatively desynchronised activity (i.e. nerve cells in the cortex are mainly firing away independently of each other). The frequency of this activity is around 13-30Hz and is called a beta rhythm. Our heart, breathing and metabolic rates and muscle activity are also high during this time.

When we relax, close our eyes and lie down to sleep the frequency of our brain waves slows slightly to 8-13Hz and these are called alpha waves. At the same time our heart rate and breathing slows down and our muscles relax. We then fall asleep 10-20 minutes or so after lying down and enter Stage 1 of the sleep cycle where brain waves continue to decline in frequency and also increase in amplitude (the latter indicating increasing levels of synchronisation in brain activity – i.e. more nerve cells are firing together at the same time). At this stage the waves are primarily 3-7Hz and are known as theta waves. Your heart rate and breathing become slow and regular. This phase can only last between 10s and 10 minutes before you move into a slightly deeper Stage 2 sleep. If woken from Stage 1 sleep you might still consider that you were actually awake.

Stage 2 sleep is also characterised by theta waves in the brain but these are interspersed with spiked brain waves called K-complexes and sporadic bursts of wave activity called spindles. It is during this stage that you are really considered to be fully asleep and is marked by loss of movement and relative detachment from your sensory environment. This is also a relatively quick phase lasting 10-20 minutes.

Stage 3 sleep is marked by a mixture of theta waves in the brain with periods of very slow high amplitude waves called delta waves which are around 3Hz. The theta waves then progressively disappear to leave only delta ones and when this happens you are at the deepest level of sleep which is Stage 4. Here your muscles are completely relaxed, your heart, metabolic and breathing rates are slowed further and your blood pressure and core body temperature drop. If you are woken up at this stage you will feel rather disorientated for a few minutes.

After 30-40 minutes in Stage 4 sleep you come back up rapidly through Stages 3 and 2. Then, rather than returning to Stage 1, you enter another phase altogether, Stage 5, which is called either “paradoxical” sleep or rapid eye movement (REM) sleep. This stage of sleep was only discovered in 1953 and was called “paradoxical” because the brain EEG activity (bursts of alpha and beta waves) and heart rate and respiration is very similar to that seen when we are awake and yet “paradoxically” we are still asleep. It is also marked by the occurrence of the eyes flicking rapidly from side to side as if scanning an imaginary scene and this has led to the other, more widely used, name for this stage of sleep being “Rapid eye movement sleep” or REM for short. Another curious feature of this stage of sleep is that it causes widespread muscle paralysis with only the respiratory, heart and eye musculature being spared.

Two other features of REM sleep are that if we are woken from it we almost invariably (at least 80% of the time) will report vivid dreams and males will experience spontaneous and penile erections and females will also experience stiffening of their nipples, clitoral engorgement and vaginal lubrication.

For the first sleep cycle of the night we may only spend 10 minutes or so in REM sleep but with the subsequent three or so 90 minute cycles progressively longer periods are spent in stage 2 and REM sleep and by the end little or no time is spent in the deeper stages of sleep (i.e. stages 3 and 4). So in the final two sleep cycles before you wake up you will probably spend half or more of the time in REM sleep. When you wake up it is also likely that you will have just been in the REM stage of the cycle.

So if you want to make people feel a touch uncomfortable just inform them that for two hours every day they are virtually completely paralysed and show all the outward physical signs of strong sexual arousal!

Do we only dream during REM sleep?

For a considerable period it was thought that we only experienced dreams during REM sleep and this led to REM sleep also becoming known as “dream” sleep. However, it is now clear that we dream during all stages of sleep, although the types of dreams we experience vary. While people awoken from stages 3 and 4 of sleep may only report having been in the middle of a dream on 20% of so of occasions, this probably has more to do with the fact that being woken from these stages of deep sleep is very disorientating and they simply can’t remember what they were dreaming about. This might seem strange because such transient disorientation would not normally be expected to significantly disrupt memories of waking experiences. However, one important feature about our dreams is that they are usually instantly forgettable. It is almost as if we have evolved a system for making sure that we do not carry memories of our dreams into our waking lives!

So what do we dream about?

The dreams we experience during REM sleep tend to be vivid, emotional and may often appear rather disjointed and fantastical. However, usually aspects of the events that are dreamed about can be linked to actual happenings in the recent past and involve individuals we know, although things may appear to be mixed up slightly. While we often get the impression that events unfolding during dreams occur at high speed, it would appear that they are actually experienced in real time.

Dreams during the deeper stages of sleep are, perhaps surprisingly, much more mundane and realistic representations of routine aspects of life – which may also contribute to why people fail to report dream experiences when they are woken up during them. In these cases their urbanity may make them even more forgettable, or alternatively people may simply be too embarrassed to report having had them – after all we all like to boast to others of having complex and seemingly surreal dream experiences rather than that we had a meaningful dream about washing our teeth or doing the ironing!

Having said this, the contents of our dreams are variable and unpredictable although they almost all occur in the present and involve the dreamer. Dreams are much more highly egocentric than our waking experiences and thoughts.

While there is some evidence that the typical content of dreams can be influenced by the type of person you are (what kind of job you do, what you read etc) there are, perhaps surprisingly few reported sex differences. The only robust one across human cultures is that males dream of males more often than females do. So perhaps there is something to the male bonding thing after all and females don’t consider males important enough to dream about!

While we may have fond expectations that dreams are somehow portents of future events, new discoveries or the means whereby we can be communicated with by others, or even by alien or celestial beings, reality does not so far provide significant support for such possibilities. Unfortunately dreams may really be boring, all be it jazzed up, action replays of waking events that deserve to be instantly forgettable even if they may seem high pleasurable or extremely frightening at times.

One thing for sure though is that as dreamers we are confirmed pessimists and worriers. Approximately two-thirds of our dreams could be classified as nightmares emotionally (typically they involve falling or being attacked and evoke anxiety, fear, anger and frustration) and this does not necessarily reflect the nature of our characters when we are awake. This, of course, is another good reason why dreams should be instantly forgettable since we surely do not want to remember all these nightmare experiences that occur during sleep. However, the extent to which dreams are forgettable does depend on whether you are in a stable relationship with another individual or not. Single people seem to be more likely to remember their dreams than those with partners!

Are our dreams filled with sex?

For Freud, dreams reveal the contents of our unconscious which, at least in his view, is stuffed full of repressed sexual cravings. What is truly amazing is that Freud appeared totally ignorant of the fact that all humans regularly show physical signs of full sexual arousal when they dream – at least during REM sleep. If he had known this he would surely have trumpeted it as clear evidence of our sexual repressions being released in the context of our dreams.

If we believe the claims that all men think of sex at least once every six seconds when they are awake, then it would not perhaps be that surprising if I were to conclude that our REM-inspired dreams must be suffused with erotic and sexual content. However, this is actually not the case. Our night time episodes of sexual arousal are not generally filled with erotic dreams – a fact that would hopefully have been easy to derive from my previous statement that the majority of our dreams are in fact nightmares! These physical signs of sexual arousal during REM sleep even occur in young babies and in brain damaged individuals in a vegetative state. They can occur in men who are sexually impotent when they are awake and whereas alcohol and old age can have negative effects on the size of waking erections they have little effect on those occurring during sleep!

Once we rule out impure thoughts from the agenda, the question of what possible cause and function these nocturnal episodes of physical sexual arousal might serve becomes more difficult to answer.

One can I suppose consider that since you are most likely to wake up during REM sleep this might optimise the speed with which you can engage in sex with an appropriate partner when you wake up. However sex does not tend to be at the top of most people’s agenda when they first wake up.

A sleep scientist has once reportedly said that he had no idea where dreams came from but that the erect penis might serve as the antenna for receiving them! On a more serious note however it seems possible that it is the combination of vascular and neuromuscular changes that occur in our bodies during REM sleep that may inadvertently cause physical changes in the genitals and breasts. The only function proposed for them to date is that they represent a kind of automatic genital workout through helping with the maintenance of smooth muscle cells. Even this seems rather far fetched.

Lucid dreams

So far I have given the impression that our dreams are influenced by events that do actually occur to us during our waking lives, that they tend to be unpleasant and that we are not in control of them. This is not actually the whole story. There are what are termed “lucid” dreams which can occur during the REM stage but where we are aware of what is going on and can to some extent (but not completely) have control over dream content. Such dreams are, not surprisingly generally very pleasant and often have a strong sexual component.

The down side is that lucid dream experiences are rare and for most people it is necessary to develop some skills to experience them. It is also possible to over egg the emotional content of the dream and wake yourself up or slip back into normal REM dreaming. The best ways to practice are to rehearse what you want to dream about while you are awake and to routinely try to remember all your dreams when you wake up.

If you can develop lucid dreaming skills then you will be in good company. For example Samuel Pepys was clearly an aficionado and reports the following entry in his diary on 15th August 1665 about the King’s mistress Lady Castlemayne:

“…something put my last nights dream into my head, which I think was the best that ever was dreamed – which was, that I had my Lady Castlemayne in my arms and was admitted to use all the dalliance I desired with her, and then dreamed that this could not be awake but was only a dream.”

Are we viewing our dreams when we make rapid eye movements during REM sleep?

It has been proposed that eye movements during REM sleep might simply represent us attempting to view the scenes experienced during our dreams – but hopefully we don’t all spend our time during REM sleep picturing long baseline tennis rallies! Indeed, the eyes can sometimes be moving in different directions which would make for an interesting view of a tennis match! A more radical proposal is that such eye movements stir up the aqueous humour of the eye to help it transport oxygen from the blood to the iris and cornea (which are poorly vascularised). This is needed because in other stages of sleep the aqueous humour is static.

So how much do other animals sleep?

We have already established that adult humans in current societies that have control over environmental lighting conditions are expected to sleep on average for 8 hours, and usually during a single period at night. The other animal species nearest to this figure is the pig (7.8h), which provides an interesting juxtaposition! The animal species that sleep the least, and usually in a number of short episodes, are grazing ones that need to spend considerable periods of time eating and digesting, have a high risk of predation and live out in the open (giraffe 1.9h, horse 2.9h, donkey 3.1h, sheep 3.8h, cow 3.9h). Elephants also sleep very little for many of the same reasons (Asian Elephant 3.9h and African Elephant 3.3h).

Our primate relatives all sleep longer than us (Chimpanzee 9.7h, Squirrel monkey 9.9h, Baboon 10.3h, Rhesus monkey 11.8h) with one nocturnal New World species, the owl monkey, holding the primate record at 17.0h!

Hunter/predator animals at the top of the food chain also sleep for long periods (Tiger 15.8h, lion 13.5h, cat 12.1h, cheetah 12.1h, jaguar 10.8h, wolf/dog 10.6h).

Small rodents and other burrowing animals are also lengthy sleepers (star-nosed mole 10.3h, mouse 12.1h, rat 12.5h, gerbil 13.1h, hamster 14.3h, ferret 14.5h, squirrel 14.9h, opossum 16h – some door mice, such as the volcano mouse sleep for up to 17h a day).

The champion animal sleeper would appear to be the two-toed sloth at a staggering 20h a day - although closely followed by the brown bat at 19.9h. The three-toed sloth only sleeps a mere 14.4h a day although there is no serious suggestion the amount of sleep you need is determined by how many toes you have!

The total amount of REM stage sleep shown by each species is also rather variable and ranges from 2-60%. The platypus (8 out of 14h/57.1%) and the opossum (6.6 out of 16/41.3%) and ferret (6 out of 14.5h/41.4%) are some of the longest REM sleepers. Humans and most other primates spend around 25% of the time as adults in REM sleep (double this as babies). Dogs and cats spend 10-15% of the time and most of the short sleeping grazing species only 5-10%. The shortest REM sleepers are dolphins and whales at 0.2 out of 10h (only 2%). And just in case you were wondering, yes other animals do experience paralysis and get erections during REM sleep!

From the above, a rough conclusion is that how much each species needs to sleep is regulated primarily by its lifestyle, predation risk and size. From this one might conclude that if current trends continue in human cultures then given 1000 years of selection for genes promoting long periods of wakefulness we may ultimately aspire to be more like a sheep than a pig. Gene therapy might of course produce this outcome rather more quickly. This conclusion also assumes that there are no major benefits afforded to us by sleep. As we will see, such a conclusion may well be unwarranted.

What determines how long different species spend in REM sleep as adults is difficult to conclude, although there is a reasonable correlation between the amount of time you spend in REM sleep and how long you sleep overall. Also, species that give birth to immature young (humans, pigs, cats dogs for example), that often require lengthy periods to develop fully, tend to spend more time in REM sleep as adults than animals born fully developed (sheep, horses, goats, cows etc). The young from these late developer species usually spend up to twice the amount of time in REM sleep when they are first born than as adults. It would therefore appear that REM sleep may be important for stimulating brain development.

Sleeping with one-eye open

Since Aristotle’s observation that all animals sleep is correct, and during sleep there are periods of muscle relaxation and even paralysis during REM sleep, the more astute among you will have spotted that sleep could be life-threatening for some species. Birds often need to sleep on precarious perches, or even on the wing. Marine mammals such as dolphins and whales need to come up to the surface at regular intervals to breathe. How can they sleep without risk of death through falling to the ground or drowning?

The answer to this is effectively the old adage “sleeping with one eye open”; although in this case it would be more accurate to say “sleeping with one half of the brain active”. Birds and dolphins have the capacity to have one half of their brain sleeping while the other half is awake. This keeps the birds from falling off perches or out of the sky and dolphins and whales from drowning. It is also a great way to be constantly alert for detecting predators. Indeed, research has shown quite elegantly that where birds are perched in a row the individual on the end of the row (and therefore potentially most at risk from a predator) keeps the eye facing outwards open!

Animals capable of this “uni-hemispheric” sleep have visual systems that are totally crossed – that is all the optic nerve fibres projecting from each eye project to the opposite side of the brain. Thus, the right eye communicates exclusively with the left side of the brain and vice versa. If this were not the case it would be difficult to sleep with one eye open!

These animal species with uni-hemispheric sleep also show very little REM sleep and when they are in this stage it only lasts a few seconds – otherwise the paralysis associated with REM would cause them severe problems.

With humans and many other terrestrial mammals the optic nerve fibres from each eye communicate to varying degrees with both sides of the brain so we are not really designed appropriately for sleeping this way. Also, the different sides of our brains are specialised for performing certain functions (like language in the left brain hemisphere and recognising faces on the right). Thus any idea that we can perfect staying awake 24h a day by adopting uni-hemispheric sleep is somewhat of a non-starter. True we could wear special dark glasses or contact lenses obscuring the part of each eye connected with the same brain hemisphere (a different set of glasses for each half of the day). However, this would still mean being unable to talk with people and understand speech for one half of the day and then being unable to recognise who it is that is talking to you for the other half! A challenging and very frustrating kind of existence one feels!

Humans can also fall asleep on their feet or sitting down so what stops us falling over? The simple answer to this is that our bodies have a built in safety device that prevents us initiating the full sleep cycle in anything but a horizontal position. This may partly explain why astronauts have difficultly in sleeping in space, where lack of gravity may alter the body’s perception of the difference between vertical and horizontal. This also means that falling asleep in your chair in front of the television, or in an economy seat on a transatlantic flight, is not giving you the proper quality of sleep. Chair-bound naps will also act to reduce the amount and quality of sleep that you do get when you finally make it to the horizontal position. In the mean time all we can do is to turn the television off and go to bed, rather than sleep sitting down, and either hope that Richard Branson can find an economical way of giving all long-haul passengers access to beds, or find the necessary funding to travel 1st Class!

Do other animals dream?

All species whose sleep has been studied seem to have at least some REM sleep and many animal keepers will have seen their dogs and cats showing sporadic movements and vocalising as if chasing imaginary prey or perhaps even being chased themselves. Since REM causes paralysis these latter observations may often reflect dreaming in non-REM stages of sleep but it also seems that some small movements indicative of dreaming can sometimes be seen during REM.

However, since we can’t ask other animals to report their dreams to us, proof of their ability to experience them must come from other quarters. The most convincing evidence comes from animal experiments carried out by scientists trying to identify parts of the brain controlling body paralysis during REM sleep. Here they have found that this can be achieved by damaging a part of the brainstem called the pons. When this is done and animals such as cats go into REM sleep they get up and start hunting imaginary mice while they are still asleep. In this state they are totally unresponsive to actual external sensory stimuli like their favourite foods i.e. they are really completely asleep.

So here we see both convincing evidence for the conclusion that animals dream like us and also for why we need to be paralysed during REM sleep. If we were not paralysed at this time then we would all be getting up and acting out our dreams and the world would be a very strange and hazardous place! Sleep walking does occur of course in humans during non-REM stages of sleep but is quite rare. The probable reason why we don’t act out non-REM dreams is that, unlike in the REM stage, our bodies are far from being in an awake state.

What regulates sleep?

This is a very big topic and there are still many things that we don’t know about.

The main process whereby the brain controls the occurrence and stages of sleep through centres in the brainstem that regulate our state of arousal and also influence thalamic and cortical functions. Control over their cyclicity is exerted by the various clock centres within the brain such as the pineal gland and the suprachiasmatic nucleus which are regulated by light/dark cycles. I will be talking in more detail about these in my next lecture (Biological clocks: human and animal concepts of time – 26th February 2004). In particular these biological clocks regulate the release of the hormone melatonin which is high during the hours of darkness and low when we are awake. When its levels are high it promotes sleep.

However, most of the brain’s major neurotransmitter systems are involved in some aspect of sleep control and particularly those associated with brain arousal systems (such as serotonin, dopamine and noradrenaline) as well as its inhibitory systems (adenosine and gamma-aminobutyric acid). In short drugs which increase levels of the arousal transmitters, or decrease those of the inhibitory systems, keep you awake, and ones that boost the inhibitory systems (notably barbiturates) make you go to sleep.

There is a specific relationship between levels of another key neurotransmitter, acetylcholine, and REM sleep. High levels of acetylcholine are found in the brainstem and other brain areas during REM sleep suggesting that it may be important for regulating this stage of sleep. Paralysis during REM sleep is caused by stimulation of the brain inhibitory transmitter, GABA, acting to suppress the output of our brain motor control centres

What wakes us up?

You may feel like “death warmed up” when you first wake up but your body has been going through a complicated count-down programme to try to make your launch into a new waking day as effortless as possible. About an hour before you wake up the pituitary gland at the base of your brain is stimulated by the brain to release increasing amounts of adrenocortical hormone (ACTH). This influences a number of other hormones in the body such as cortisol release from the adrenal gland in a way similar to that seen if you have just encountered something that frightened you! The end result is that your body shifts into a state of higher arousal and sensitivity to external stimuli in the same way as the experience of fear would. No wonder we often wake up with a start!

Many of us are very adept at setting our own internal alarm clocks and can trigger these ACTH changes an hour before whatever time we want to wake up. For the rest of the population that can’t do this well and have to rely on external alarm clocks, these anticipatory ACTH changes may not have kicked in and its no wonder it takes so long to adjust to a waking state – the term “walking zombies” tends to spring to mind.

There has been recent great excitement at the discovery of a small peptide that is released within parts of the hypothalamus (which is important for regulating pituitary hormone release). This peptide orexin or hypocretin was originally thought to be important for appetite control but mice that lacked expression of the gene controlling its production showed narcolepsy (i.e. they tended to fall asleep spontaneously at all times of the day). It is now thought that it is necessary to have high levels of orexin in your brain to stay awake and that when these fall you fall asleep until they rise again and cause you to wake up. This discovery has given rise to intense speculation that it will soon be possible to produce a drug targeting orexin production that will allow anyone to stay awake 24h a day.

Orexin levels also tend to be high during REM sleep which again underlines how close this stage of sleep is to being awake.

Can we and other animals survive without sleep?

The simple answer to this is NO! What animal research has been done on this shows quite clearly that around 2-3 weeks of sleep deprivation will normally kill you. Early researchers investigating the effects of sleep deprivation in animals even concluded that sleep was more important for survival than food.

The progressive symptoms leading to death by sleep deprivation are consistent across species and very unpleasant. Prior to death they have a debilitated appearance, skin lesions, increased food intake, weight loss, increased metabolic rate, increased levels of the transmitter noradrenaline and reduced body temperature. One of the most significant effects of sleep deprivation is a progressive breakdown of your immune defences making you more susceptible to infection by bacteria, viruses and parasites. Indeed, one hypothesis is that the main cause of death following prolonged sleep deprivation is as a result of succumbing to such infections due to reduced immunocompetence.

The perils of sleep deprivation

The reported human record for staying awake is 18 days. The individual concerned turned into a psychological, emotional and physical wreck during the deprivation but appeared to have no permanent problems as a result of it. However, it seems more likely that longer term problems may result from constant minor sleep deprivation (only being able to sleep 4-5h a night for example) than as a result of the occasional experience of complete deprivation. What is really alarming is that this issue is not really given that much consideration.

Attention, motor co-ordination, learning and memory, emotional and psychological state, sexual drive and fertility, appetite, growth, metabolic rate, immunocompetence, life expectancy…. Actually it’s easier to say simply that pretty much all critical functional aspects of biological beings are influenced adversely by not getting enough sleep. Sleep deprivation can even shrink certain parts of your brain!

Given these dramatic effects of sleep deprivation it is no wonder that it is used as a popular form of torture and even as a weapon by opposing armies. On a more domestic front however one of the greatest contributors to sleep deprivation in parents are crying babies! As one lovely quote puts it:

“Sleep deprivation is a popular torture device used by the Indonesian secret police and small babies” John O’Farrell (2000) The Best a Man Can Get.

You may reasonably argue that sleep deprivation is an unavoidable consequence of most people’s lives at some stage. If this is the case then the easy remedies are to catch up with sleep at the weekends (or whenever you can) and to try taking short naps in the afternoon. Apparently a 15 minute nap after lunch can do wonders for your subsequent alertness – longer naps at this time are not as effective! Something for employers to consider perhaps!

Driving under the influence of sleep deprivation

While we are concerned about stopping people from driving while under the influence of alcohol, and our legal system is empowered to imprison or fine people who do, less attention is paid to the impact that sleep deprived individuals have on our roads. Even moderately sleep deprived individuals can show all the slowing of reaction times and lack of attention typically seen in people with just over the legal limit of alcohol in their blood! Current estimates in the USA are that at least 10% of all fatal crashes are caused by sleepiness (50% of crashes involving lorries) and a 1994 report by the US National Commission on Sleep Disorders concluded that driver fatigue contributed to 54% of all vehicle accidents in the USA (about a total of 100,000 accidents). Similar surveys in the UK have come up with lower figures of 16 and 20%. Other surveys have reported that 29% of all drivers had admitted to coming close to falling asleep at the wheel within a 12 month period. If sleep deprivation is combined with even low levels of alcohol consumption then the situation is compounded.

The long and short of it is that an individual driving a vehicle after a night without sleep, or even after a few days where they can only sleep for 2-3h, is probably pretty much equivalent in slowed reaction times and sluggish attention to someone who has drunk five pints of beer and is well over the legal limit for blood alcohol levels. However, unlike for alcohol consumption, there are no standard legal penalties for driving while under the influence of lack of sleep!

Growth, repair and immune function

I have already pointed out that death following prolonged sleep deprivation is probably caused by a breakdown in your immune system. However, it is likely that chronic sleep deprivation also depresses immune responses and contributes to increased likelihood of contracting all those coughs and colds and other things doing the rounds of your office, factory or school. Sleep is clearly a time when your immunological defences get a boost and when the substances promoting these beneficial changes are normally at their highest concentrations.

A somewhat alarming study conducted on rodents nearly 15 years ago first reported evidence that even a period of 7h sleep deprivation resulted in animals failing to make antibodies in response to a flu vaccine. So maybe the effectiveness of a routine vaccination could be affected if given after a night without sleep or following a period of chronic sleep deprivation! This study was finally repeated in humans and the results published briefly in the Journal of the American Medial Association (JAMA) in 2002. Two groups of young men(average age 23 years old) were used who either got a regulation 7.5-8.5h sleep per night or who were restricted to only 4h sleep a night for 6 nights. The sleep deprived group produced significantly reduced antibody titres to a flu vaccine (Spiegel et al 2002).

Many critical hormones are produced at their peak concentrations while we are asleep. For males this includes testosterone and this should perhaps serve as a warning to those men who think that sleep is a needless encumbrance to their social and sex lives. Of more general importance is growth hormone. Not only is this mainly released in the body during sleep it is also particularly restricted to the deepest levels of our sleep cycle (primarily stage 4). Growth hormone is not just important for growth but also contributes to repairing damage to the body. So restricted or poor quality sleep can have a negative impact on both body growth and repair. Growth factors also play a major role in promoting new synapse formation in the brain which is essential for learning and memory.

As you would expect the individuals who are most at risk from these hormonal insufficiencies through poor quality sleep and sleep deprivation are the young. However, old age also disrupts sleep cycles and reduces the amount of time spent in stage 4 sleep. This may well contribute to the body shrinking in size as we get older and to longer periods needed for tissue repair.

Education and adolescence

As we have just seen the right duration and quality of sleep is of particular importance for developing individuals. Babies start life sleeping up to 16h a day. While adolescents probably consider themselves in need of less sleep than the old-aged fogies who try to control their lives, research suggests that they still need nearer to 10h of sleep than the 8h recommended for adults. Although many parents may consider their teenage offspring’s weekend habits of lying in bed until 11 or 12 in the morning as laziness or rebellion against the establishment, it may also be their simple attempt at catching up on the effects of sleep deprivation during the week.

If one assumes that the average school child needs to be up by 7.30 to get to school by 9 (many after all have to travel some distance to get to their schools) then this would mean that they should be in bed and asleep by 9.30 at night. One wonders how many growing 13-18 years olds in the throws of their SATs and GCSEs do this?

So does sleep deprivation or poor quality sleep actually influence educational achievement? There is little doubt that it can and does. It may not be “cool” to go to bed early and get a regulation 10 hours of sleep, but you might end up spending the next 50 years in a dead-end job paying for skipping sleep to maintain your appropriate level of “street cred”.

If you go for a quick browse on the internet for advice on the best strategies for passing SATs and GCSE exams you will find that getting sufficient sleep in the build up to exams generally figures as the number one piece of advice. Studies in the USA in particular, where changes in school starting times resulted in teenage children losing 30-40 minutes of sleep a day, found that this produced significant effects on their learning abilities.

Caffeine and sugar junkies

The human race is awash with caffeine. This plant toxin (evolved by some plants to poison insect parasites!) is one of the favourite stimulant tipples of the human race and it is present in coffee, tea, soft drinks, chocolate, medicines and even over the counter in tablet form. We are the only species that consumes this drug and we excrete so much of it (average blood levels are around 10mg/Kg) that it can be used as a reliable way to detect the spread of human effluent out into the sea! Approximately half of the population of the USA consumes 300mg of caffeine a day (approximately 3 large cups of filter coffee). Coffee is the second largest consumer commodity in the World (the first is oil).

Caffeine is effectively an addictive drug like amphetamines, cocaine and heroin, although much less potent. The main reason why we consume so much of it is that, as a stimulant, it gives us a buzz. Soft-drink manufacturers argue that it is only the taste enhancing properties of caffeine that prompts them to use it although recent studies have not found strong evidence that consumers can tell the difference between normal and caffeine-free versions of their favourite beverages. Not surprisingly the drinks industry considers such studies flawed! For those individuals that chose to partake of high caffeine content Energy drinks such as Red Bull (which has about the same amount of caffeine as a cup of coffee but combined with about 15g of sugar to make it an even more potent arousing cocktail) it is even clearer that it is the stimulatory effects of the drinks rather than their enhanced taste that is the primary driver for the consumer. Taken in large amounts in combination with alcohol it can produce hyperactive drunks who can end up being extremely aggressive and dangerous to others.

Caffeine has a half-life (the time taken for the body to get rid of half of what has been taken in) of 6-7h and so even a coffee or tea taken in the middle of the afternoon will result in significant caffeine levels in your blood when you go to bed.

So why does caffeine make you more alert and disrupt your attempts to sleep? It seems to act by influencing the ability of adenosine to act on its receptors. I have already mentioned that increased levels of the transmitter adenosine are important for promoting sleep. For adenosine to act it needs to bind to receptors on nerve cells to change their activity and promote sleep. What caffeine does is to nip in and bind to these receptors but without changing the activity of the cells. The net result is that even if your body is producing adenosine like mad to try to get you into the land of nod, the caffeine occupancy of its receptors means that it cannot act.

Sleep disorders

Most of us have problems sleeping at some time in our lives. It is estimated that 35 million Americans complain of chronic insomnia, 65 million of them suffer from significant sleep disorders and 25% of their whole population take medication to help them sleep. In the majority of cases problems may simply be the result of stress, too much caffeine, alcohol or sugar or getting old. Any activity that prevents you from lowering your heart, respiration and metabolic rates and body temperature will also be potentially disruptive. One of the most obvious examples of such activities is eating a large hot meal before going to bed.


This is a frightening disorder where individuals suddenly fall asleep with no warning and in any situation. It is effectively very debilitating in humans and can also be seen in some other animal species such as dogs. Recent experiments with transgenic mice which have the orexin gene deleted (so the animals are unable to make this peptide) have shown that they are narcoleptic. It has since been confirmed that human narcoleptics have deficiencies in their orexin levels and this is why they are unable to stay awake all the time. It is thought that these deficiencies may be caused by an autoimmune problem.


This is another big and somewhat sensitive topic. We snore because relaxation of musculature in the back of the throat allows the soft tissue there (notably the uvula which is the tissue dangling at the back of your throat and which helps to keep food in your throat away from the cavity behind your nose) to vibrate as a result of associated narrowing of the airways increasing the speed that air has to travel through them. Vibrations in the soft palate and base of the tongue also contribute. Average snores are 70-90 decibels!

Snoring is a big disrupter of sleep both for the snorer themselves and for anyone sleeping in the near vicinity. A study of trainee doctors found that while only 13% of non-snorers failed their exams, 42% of the frequent snorers did!

Being male, overweight and drinking too much alcohol all contribute to how much you snore. The remedies to snoring are diverse and often not very effective (losing weight and not drinking too much alcohol can cause marked improvements). Allergies and colds will also exacerbate snoring since they also narrow the respiratory tract. Sleeping on your side rather than your back can help, or taking decongestants. Nasal-dilator strips (a springy, self-adhesive strip that sticks over the nose) can also be quite effective since they dilate the nostrils and reduced resistance to air flow.

A novel suggestion is to give the throat musculature practice workouts during the day to strengthen it. The two-week regime proposed includes pressing the tongue against the lower teeth for 2-3 minutes and holding a pencil firmly between the teeth for 10 minutes every day.

Sleep apnoea

This is pretty much an extreme version of the snoring problem. It occurs when your whole upper airway is temporarily obstructed during sleep and you cannot breathe at all. This can last up to 10 seconds or so and while physiological alarm bells eventually ring to make you gasp and re-open your airways. You may not wake up as a result of this but it is very disruptive to the quality of sleep and can happen as much as a hundred times a night. This condition is quite common and occurs in about 10% of middle-aged men. It is not easy to diagnose without being hooked up to proper physiological monitoring equipment while you sleep. However, if you are a heavy snorer, constantly wake up feeling shattered, depressed and unrefreshed by sleep and with a splitting headache it might be a good idea to consult your doctor (unless of course this is simply the result of heavy alcohol consumption!)

So why do we sleep and dream?

There are a bewildering number of theories about what basic functions sleep and dreams play. There is likely to be some element of truth in all of them because there are amazing numbers of changes going on in the brain and other parts of our bodies during sleep. It is also quite possible that sleep has slowly evolved to sub-serve all sorts of different purposes from those that originally spurred its development in biological species.

Saving energy?

This is the safest simple theory since there are varying amounts of metabolic savings to be had by sleeping rather than staying awake. However, one general problem with this is the question of why we need REM sleep where metabolic savings are minimal. If saving energy was the main purpose of sleep then we might expect to spend more time in the deepest levels of sleep and be closer to a state of hibernation (where there are definitely big energy savings to be had).

Recharging the batteries?

This is an attractive idea proposing that our bodily functions need a re-charge period to refresh them each night. Actually there is not much general support for this idea except for the fact that a number of key substances which boost growth, repair and the immune system are released optimally during sleep. Without this release, bodily functions can become impaired but it is not really sleep specifically recharging our batteries.

Re-setting the clocks?

This is another reasonable idea given that left to their own devices our biological clocks might run fast or slow or even stop. However, our clocks are more disrupted by changes in our external light/dark environment than by when we sleep. As with the re-charging the batteries concept many substances that influence sleep and waking, like melatonin, are also critical to the running of our biological clocks.

Putting out the trash?

This idea has been co-proposed by Francis Crick (of DNA fame)(Crick and Mitchison, 1983). In essence it proposes that dreaming activities during sleep are the brain selecting its garbage files for transfer to the waste bin. This is why, they suggest, dreams are so instantly forgettable (we don’t want to remember the garbage – except perhaps on garbage collection day!) and why sleep and dreaming makes us wake up refreshed and ready to take in further new information unhindered by a brain cluttered with useless past information. This is a nice idea but there is no real experimental support for it. Indeed it is difficult to come up with an easy way to test the hypothesis. Also, as we will see from the next idea about the functions of dreams, there is strong evidence that they may help improve our memories rather than help us to forget.

Consolidating memories and gaining insights?

The idea that thinking, learning and decision making processes in our brains really benefit, as the saying goes, from “sleeping on it” has been around for a long time. In recent years experimental support for the positive impact of sleep/dreaming on being able to remember things better has been impressive (see Macquet, 2001 and Stickgold et al 2001). There have been many experiments conducted on humans and other animals showing that sleep deprivation stops you from being able to consolidate memories (i.e. store them in such a way that they can be remembered for a long time). Work has also shown that REM and non-REM stages of sleep seem to benefit consolidation of different types of memories.

In REM sleep we seem to improve consolidation of procedural memories (memories of sequences of actions/skills etc – see my last lecture: “Memories are made of this: but what about intellect?” – December 2003). In non-REM sleep we improve consolidation of declarative memories (memories of events). Also some of the brain rhythms seen during different stages of sleep, such as the theta rhythm, have been shown to be important for learning in the waking brain.

The most startling evidence for this idea has been from animals where recordings from brain regions associated with memory, such as the hippocampus, have shown that virtually identical activity sequences can be observed being reproduced by nerve circuits in the sleeping brain as seen when then animal is learning a task while fully awake. This really cements the whole idea of dreaming as being a series of action replays of events during the day.

A study just published in Nature this month has provided evidence that sleep may even promote the process of gaining insight into discovering hidden rules. So perhaps the Newton sleeping under the apple tree idea is not too far-fetched after all! (Wagner et al, 2004).

However, we have to be careful about taking on theses ideas of sleep fuelling learning and insight too enthusiastically. As you will have seen from my data for how long other species sleep and spend in REM, humans are by no means exceptional in any respect. Surely if sleeping and dreaming were that important for memory and insight, humans would have to be quantitatively different from all other species. The only way out of this would be to postulate that the human brain does things in a qualitatively different way from other species, but again so far there is little evidence for this in relation to sleep. Indeed, our dreams tend to be more visual than auditory and so our greatest human cognitive asset, language, does not figure that much in our dreams.

Trying to find a unified explanation

My own personal view of this much-debated subject is that while sleep has come to make a contribution to all these areas of biological functioning its origins may be quite simple. When it comes down to it there is no sense in any species maintaining a waking state when there is nothing useful that can be done to boost its energy levels and reproduce itself.

While there are not substantial metabolic savings to be had even during the deepest levels of sleep compared with simple inactivity the savings are still worthwhile compared with a fully active state. Also, for any organism that is habitually on sensory alert and highly active for half the time in order to find resources to grow and reproduce, it would be difficult to suddenly switch to being inactive for long periods of time but still maintain the same level of awareness. Sleep solves this conundrum very nicely!

However, this argument would seem to lead to a conclusion that a simple deep hibernation state would be more useful than the complex sleep cycles that we have adopted. As already mentioned, there would certainly be greater metabolic savings to be had. While this is true, the problem is that although we need to shut down and make energy savings when there is nothing useful to be done to promote reproduction and survival, we are at our most vulnerable to other potential predators or competitors who are adapted to be active at the times of day that we are not. We are therefore constrained to adopt a sleeping with one eye open strategy. While some species have, for a variety of reasons, developed strategies of letting the two sides of their brain sleep independently this is not an entirely satisfactory way of going about things if one can do it all at one go. This is because functional competence with only half a brain working is clearly not that great (dolphins just end up swimming in slow circles in this state). The effective solution is REM sleep.

During REM we are ready and able to wake and go straight into action if changes in external circumstances demand. When we first find somewhere to go to sleep we have a high degree of confidence that we are safe to do so and can afford to go into a deep level of energy shut down. However, the longer the period of time we are divorced from detecting changes in our environment the greater is the risk that something might have changed that would represent a threat to our survival. A good way around this is therefore to systematically increase the length of time we spend in a state of readiness for action during sleep by staying in the REM stage more and more as the night progresses.

So why do we dream? A simple answer to this is because the brain is in a state of sensory deprivation. When we are asleep we are about half as sensitive to detecting external sensory stimuli as when we are awake. If any of us are placed in experimental conditions of sensory deprivation (basically suspended in a Michelin man suit in a swimming pool) our brains go hay wire and generate all kinds of uncontrolled sensory experiences (rather like dreams). In short, dreams may simply be the brains normal reaction to being cut off from the external world.

So why would dreams improve memory consolidation if they are simple hallucinations? The answer to this could simply be that cut off from the external world dream hallucinations are indeed primarily a re-run of recent events that have been experienced. There is also the potential advantage of reduced levels of interference from external events – experience of interference/noise during the consolidation phase of any memory system is generally bad news for being able to retain information.

Whatever the answer may be, you can be assured that more and more scientists are joining in the search for answers as to what sleep is about. The fact that it has now brought in the large number of scientists engaged in trying to understand brain mechanisms of learning and memory may hopefully accelerate this process.

So could we evolve to do without sleep?

In theory I think the answer to this is probably yes. However, it would not be a trivial process since we would have to extensively re-programme many central biological processes to occur when we are awake as opposed to when we are asleep. Whether, we could deal with the increased psychological and emotional strains of a 24h waking existence is also questionable. Finally, removing sleep from our repertoire would also be taking away one of our most basic pleasures – even if we don’t remember that much about what happens during it.

Some final overall conclusions:
Don’t miss out on your sleep
Get horizontal when you want to sleep
Try to remember and enjoy your sleep and dreams
Don’t read too much into them
If sleep deprived try a 15min afternoon nap
Regulate your caffeine, sugar and alcohol intake
Don’t miss out on sleep and drive
Don’t disturb sleeping teenagers at weekends!
Sleeping may help solve some of your problems
Selected references:

Crick F and Mitchison G. (1983) The function of dream sleep. Nature 304:111-113.

Macquet P (2001) The role of sleep in learning and memory. Science 294:1048-1051.

Martin, P (2003) Counting Sheep: the science and pleasures of sleep and dreams. Flamingo, London (available from Amazon).

Sleep and dreaming: Scientific advances and reconsiderations (2003) edited by Pace-Schott, EF, Solms, M, Blagrove M and Harnad S. Cambridge University Press, Cambridge

Spiegel KS, Sheridan JF and Van Cauter EV (2002) Effect of sleep deprivation on responses to immunization. JAMA 288:1471-1472.

Stickgold R et al (2001) Sleep, learning, and dreams: off-line memory reprocessing. Science 294:1052-1057.

Wagner U et al (2004) Sleep inspires insight. Nature 427:352-355.

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