But new research shows that this basic drive of nature is something we ignore at our peril, as Michael Church reports
Why do we sleep? The traditional assumption - that it's to rest the muscles and the mind - has not been confirmed by science. We simply don't know why the body must lie still, and the mind draw inwards, for eight hours a day.
Yet the brain is more active in sleep than when we're awake, and the body repairs and renews itself in ways which only sleep makes possible. Why do we feel lethargic when a fever's coming on? Because our body needs to focus its energy, and it fights infection better in that state. We may not know why we're designed to spend one-third of our lives asleep, but our bodies instinctively make brilliant use of it.
Mankind has always tried to understand sleep. Hypnos was the Ancient Greeks' god of sleep, and Thanatos their god of death, and they were believed to be twin offspring of Mother Night. But the Greeks were the first to offer a scientific explanation of sleep. In the 5th century bc, the philosopher Alcmaeon suggested that it was caused by blood filling the brain, and that we woke when the blood left it. Plato and Aristotle thought vapours from food decomposing in the stomach rose to the brain and caused sleep - an idea which wasn't displaced until 16th-century scientists showed it was anatomically impossible. Victorian theorists assumed sleep to be pure negativity, like a computer shutting down. Only in the 1950s did scientists realise that the brain actively puts itself to sleep and that it was worth studying while in that state.
Sleep is studied with the aid of a machine called an electroencephalograph (EEG). Using electrodes placed on the scalp, the EEG monitors the brain's varying patterns of electrical activity and shows them on paper as brain waves. Eye movements cause part of this activity and in 1952, quite by accident, American researchers discovered there were periods during sleep when these movements suddenly became rapid, and that these followed a regular 90-minute cycle. William Dement, one of the researchers, identified five stages in this cycle, which begins with quiet restfulness, with waves of lower frequency. After a few minutes, the muscles start to relax. If you're trying to sleep in a sitting position your head will slump forward, hence the saying "nodding off".
In stage two your eyes are still, your muscles are relaxed and it is harder to waken you. In stage three, which lasts only a few minutes, your sleep becomes deeper, after which stage four kicks in, where your sleep is deepest and the waves are at their lowest frequency. After half an hour of this, rapid eye movement (REM) sleep begins and the waves become so frenzied that the EEG trace alone would suggest you are awake, but your muscles are now paralysed, your body completely still. The brain is not reacting to external stimuli - it's generating its own stimuli. Your pulse rate and blood pressure have risen; if you are a male, your penis will be erect, and if female, your nipples erect. You will probably be dreaming, though, oddly, your dreams will probably not be about sex.
Clocking up sleep debt
William Dement saw immediately that the brain valued REM sleep as a thing in itself. He thought this discovery might confirm Sigmund Freud's ideas about the importance of dreams, but it propelled him instead towards other discoveries, of far greater significance.
He and his fellow researchers decided to look into the huge question of sleep deprivation. Why did the Exxon Valdez oil tanker run aground off the shores of Alaska, causing America's worst-ever spill? Why did the space shuttle Challenger explode shortly after its launch? Why did the Three Mile Island nuclear power station come close to meltdown, and the Chernobyl one blow up? Bad decisions, provoked by extreme fatigue, were the answer in every case. Daytime sleepiness, William Dement realised, was not regarded as part of sleep-scientists' domain, though he had long known that sleep deprivation was a favourite torture tool in the Korean War (and that in imperial China it had been a method of slow execution). He also knew that it caused at least as many road deaths as alcohol, and when combined with alcohol was an even greater killer.
William Dement was led to propound his theory of "sleep debt". "The feeling of being tired and needing sleep is a basic drive of nature," he says.
"Like bricks in a backpack, accumulated sleep drive is a burden that weighs down on you. Every hour that you are awake adds another brick to the backpack: the brain's sleep load increases until you go to sleep, when the load starts to lighten. All wakefulness is sleep deprivation. Generally, people need to sleep one hour for every two hours awake."
By measuring the length of time it took people to fall asleep, he found he could accurately measure the weight of a person's pack. And he coined the term "sleep debt", because lost sleep was like a monetary debt, which the brain demanded be paid back. As our debt grows, our energy, mood and judgment get undermined, and you can't cancel out a large debt with one long night's sleep. Most people collapse after four days without sleep - which would add 32 hours to whatever debt they already had - but they can temporarily mask this need with the aid of stimulants or activities which excite them.
However, William Dement also realised something else: with no sleep debt at all we could not sleep. And his next realisation - inspired by his observation of those who had jet-lag - was a counterweight to this fact.
Despite extreme tiredness after a long and sleepless flight, we may suddenly find ourselves wide awake in the middle of the night. This is because we are activated by our inbuilt biological clock. This is so accurate that we often wake up a few minutes before our alarm clock goes off. (All animals have this bio-clock. Laboratory rats, with no awareness of external time, climb on to their exercise wheel at the same minute every day.) Our bio-clock is set to galvanise us twice a day: early in the morning and mid-afternoon, when the accumulated sleep debt is threatening to overwhelm us. By keeping us awake through the day the clock ensures there will be enough accumulated debt to make us sleep through the night.
These two forces - one inducing sleep and the other inducing wakefulness - make a beautifully balanced mechanism.
Yet the bio-clock is not quite perfect for our needs. Remarkably, research has revealed that it follows a 25-hour cycle. Being "slow", it needs a corrective, which comes in the form of our spinning planet. The Earth itself sets the tempo of our days, and does so with the aid of light.
Sunrise is our cue to wake and sunset is our cue to sleep. Just how potent this cue is, has been shown by a surprising experiment: light shone on the skin on the back of the knees has been enough to reset the circadian or 24-hour bio-clock. This, believe it or not, has been proposed as a cure for jet-lag, but the finding has serious implications.
When our artificial light came from fire, it was not bright enough to reset our clocks. But with Thomas Edison's invention of the light bulb in 1879, everything changed. Where people had previously gone to bed when night fell - or, at most, sat up with a dim oil lamp - they could now turn night into day. (Thomas Edison, like Lady Thatcher, prided himself on only needing four hours' sleep a night and believed any more was bad for you. Yet he regularly caught up on the quiet. He discovered, as stressed executives are finding out today, that a short afternoon cat-nap can be wonderfully restorative.) In William Dement's words, we all now live in an "electric cave", which obliterates the ancient schedule, but our bodies haven't forgotten that schedule. Hence the problems so many of us have with sleeping, and hence our feeling of wellbeing when we go into the wilderness and sleep under the stars. As he observes: "In just a few decades of technological innovation we have managed totally to overthrow our magnificently evolved biological clocks, and the complex bio-rhythms they regulate."
Disorders and modern society
Though our muscles can still keep going, sleep loss impairs the way our brain functions and sleep disorders of one sort or another now affect almost everyone in the developed world. These disorders range from apnoea (a temporary inability to breathe) and narcolepsy (the sudden and uncontrollable onset of deep sleep), to that catch-all word covering a multitude of problems, "insomnia". And the biggest sufferers from this self-inflicted plague are children. As Paul Martin points out in his book Counting Sheep: "In many homes sleep is regarded as a maintenance activity to be squeezed into the busy schedule when everything else has been done."
In wealthy societies, children's bedrooms have become fun palaces, rather than places of rest, with the phone, television, computer games, blaring stereos and the internet providing round-the-clock stimulation.
Child psychologists are increasingly finding links between inadequate sleep, daytime drowsiness, and behavioural problems at school. "Tired children," says Martin, "try to resist their fatigue by becoming even more active. The tired child becomes irritable, fidgety, inattentive, disruptive, and generally bad company, but without necessarily looking obviously tired."
Lack of sleep is now thought to be a contributory factor in attention-deficithyperactivity disorder (ADHD). Ironically, the drug Ritalin, which is used to suppress the symptoms of ADHD, exacerbates the problem, since one of its side-effects is to disrupt sleep.
Teenagers, whose need for sleep is greater than that of younger children, are at risk from their peer-pressured lifestyle. Lying in at weekends and during holidays is, for them, an essential antidote. And if parents have a blind spot about this, so have doctors, whose training gives barely a mention to the importance of good-quality sleep for general health or to the consequences of sleep-deprivation. It is not conducive to patient welfare that the average hospital ward should be so inimical to sleep. And it is an indication of our national backwardness in this sphere that, while the USboasts 6,000 sleep-disorder units, we in Britain make do with a mere 150.
However, if deprivation is the down side, dreams - unless they are nightmares - are sleep's cheerful aspect, and we spend on average two hours dreaming each night (babies are thought to dream for eight hours). It is believed by many that the difference between our dreaming and awakened states is less great than is generally supposed. However, scientists are still seeking links between the two. One common piece of evidence of a link is the way we assimilate into our dreams things that are physically happening to us. If in reality we're being sprinkled with water, we may translate that into a dream about a rainstorm.
Many sleep researchers believe the purpose of dreams is to process and consolidate memory, but people have always tried to discover meaning in them. Some cultures regard them as the direct experiences of the wandering soul, which leaves the body during sleep and visits the places seen in the dream. Sir James Frazer, in his anthropological study The Golden Bough, noted that in some Brazilian villages it was considered dangerous to suddenly waken a sleeper, lest his soul should not have time to get back to his body. The ancient Babylonians, Assyrians, Egyptians and Greeks all used dreams, which were considered messages from the gods, to predict the future. For Sigmund Freud, in contrast, dreams were "letters to ourselves" - the expression of our unconscious sexual desires.
Artists have long mined dreams for subject-matter. One May morning in 1965, Sir Paul McCartney woke up from a dream with the melody of "Yesterday" running through his mind. Charlie Chaplin "composed" his film music in a similar way, while Mary Shelley claimed that the idea for Frankenstein came to her in a dream. William Blake's engraving technique was revealed to him in a dream by his dead brother, and Robert Louis Stevenson relied so heavily for inspiration on the inhabitants of his dreams that he gratefully invented a name for them. "God bless the Brownies," he wrote, "who do one-half of my work for me while I am fast asleep."
It makes sense that these wonderful dreams we have, which we often only remember during the first few seconds of waking, should take place during that thing which the body instinctively prizes so much, REM sleep. We may spend eight hours or so of every day asleep, but it is still virgin territory, awaiting discovery.
"Dreams are real while they last - can we say more of life?" - Psychologist Havelock Ellis
"If someone were to tell me I had 20 years left and ask me how I'd like to spend them, I'd reply: 'Give me two hours a day of activity, and I'll take the other 22 in dreams - provided I can remember them'." - Film-maker Luis Bunuel
"God bless the inventor of sleep, the cloak that covers all man's thoughts, the food that cures all hunger, the water that quenches all thirst, the fire that warms the cold, the cold that cools the heat; the common coin, in short, that can purchase all things, the balancing weight that levels the shepherd with the king, and the simple with the wise." - Sancho Panza in Don Quixote by Miguel de Cervantes Saavedra
"The bed is our whole life. It is there that we are born, there that we make love, and there that we die."- Guy de Maupassant
The Promise of Sleep: The Scientific Connection between Health, Happiness, and a Good Night's Sleep By William C Dement, Pan
Counting Sheep: The Science and Pleasures of Sleep and Dreams By Paul Martin, Flamingo
The Interpretation of Dreams By Sigmund Freud, Avon
The Good Sleep Guide By Dr Timothy J Sharp, Penguin
Doctor ... I Can't Sleep By Dr Adrian Williams, Amberwood
This is a short handbook by the director of Guy's and St Thomas' Hospital Trust sleep disorder centre.
Loughborough Sleep Research Centre www.lboro.ac.ukdepartmentshugroupssleep
British Sleep Society www.sleeping.org.uk
* Contact the London Sleep Centre for more information about sleep disorders
Average sleeping times a day for: * two-toed sloths - 20 hours;
* armadillos and bats - 19hours;
* lizards - 16 hours;
* cats, mice, and hamsters - 14 hours;
* gorillas - 12 hours;
* starlings - 9 hours;
* humans and moles - 8hours;
* horses, cows, elephants, sheep - 4 hours.
Dolphins, being mammals and needing 8 hours' sleep a day, avoid drowning by having only half the brain asleep at a time. Each half of the brain takes it in turns to sleep, while the other half directs them to the surface to breathe. The "sleep" of plants also follows a 24-hour cycle.
Time for bed
* Don't eat late at night or drink caffeine in the evening, as these things, like alcohol and nicotine, disrupt sleep.
* Get a comfortable mattress and good pillows.
* Make sure your bedroom is not too hot.
* Take a bath an hour before going to bed, not just before.
* Go to bed only when sleepy.
* Use your bed for getting into a relaxed state and for sleeping. Don't eat or watch television while lying in bed.
* Try to set up a regular sleep routine and, if necessary, pay off your accumulated sleep debt by going to bed half an hour earlier than usual for a few weeks.
* Don't feel guilty about napping during the day.