All in a night's sleep | 快猫短视频

All in a night’s sleep

7 July 1990

For an activity that is so essential to our wellbeing, and that takes up so much of our daily lives, sleep remains something of an enigma

M0ST PEOPLE spend a third of their lives asleep. But why? Is it so that our body can repair itself? Is it part of a process in which our brain assimilates the information it has gathered during the day? Or is sleep a mechanism that has evolved to keep us out of harm鈥檚 way during the hours when we do not need to be out hunting for food, or reproducing? No one knows for sure, though each hypothesis has its supporters.

快猫短视频s are not even certain about the amount of sleep that humans need. Left undisturbed, most people spend around eight hours asleep each night. Some individuals, however, manage with much less 鈥� Margaret Thatcher, for one, seems to need little more than four hours sleep a night.

It is easier to say what sleep is than why we sleep. During sleep, our blood pressure falls, the pulse rate drops, respiration slows, body temperature falls, blood vessels in the skin become wider, our gastrointestinal tract sometimes becomes more active, most of our muscles relax, and in general our metabolic rate falls by about 20 per cent.

The organ that shows the clearest distinction between the states of sleeping and waking, however, is the brain. As the individual falls asleep, the brain becomes progressively passive, responding less and less to the outside world. 快猫短视频s who study sleep, therefore, tend to focus their researches on the brain.

Even in the 19th century, people appreciated the importance of the brain in sleep. Then, rival theories held that tiredness was caused either by a build up of blood 鈥渃ongealing鈥� in the brain during the day, or by blood draining away from the brain and causing 鈥渃erebral anaemia鈥�, which only sleep could cure.

In the early 20th century, researchers began to suggest that sleep is caused by natural chemicals 鈥� sleep substances 鈥� that accumulate in the brain. And by the 1930s, scientists were measuring changes in the electrical output of the brain. Their records showed that while the activity of the sleeping brain differs from that of the waking brain, it does not 鈥渟witch off鈥� during sleep.

Physiologists first measured the brain鈥檚 electrical activity in 1928. The brain contains billions of individual cells, called neurons (see Inside Science No 19). Neurons communicate with each other using both chemical and electrical signals or 鈥渕essages鈥�. An individual neuron can be excited by chemical messages from its neighbours to produce charged particles (ions) which travel down a nerve fibre, called an axon, to another cell. The pulse of electricity in turn can cause chemicals to be released which trigger a further electrical pulse, and so on.

By attaching electrodes to the scalp of volunteers, physiologists were able to record brain waves, as the electrical activity is known. Over a period of 30 years, the changing pattern of electrical activity during sleep gradually became clear. But scientists have taken much longer to begin to understand the chemical changes involved 鈥� their knowledge of these is still incomplete.

Brain waves

Measuring sleep

RESEARCHERS find that sleep involves five distinct patterns 鈥� four stages of successively deeper sleep known as non-rapid eye movement sleep (or NREM sleep), and a fifth stage known as rapid eye movement, or REM, sleep.

During NREM sleep, the brain waves tend to become slow and more regular, and the sleeper lies fairly still and breathes slowly and regularly. Any snoring that takes place will be during NREM sleep. The four levels of NREM sleep are known as stages 1, 2, 3 and 4. It is in stage 4, the deepest level of sleep, that the electrical activity is slowest.

REM sleep is a much lighter sleep. Its distinguishing feature is that the eyes intermittently dart about under closed eyelids; hence the term 鈥渞apid eye movement鈥�. At the onset of REM sleep, any snoring ceases, breathing becomes irregular, and both the flow of cerebral blood and brain temperature increase. There are also more body movements. The electrical activity of the brain is similar to that in stage 1 sleep.

The sleeper will progress through these five stages in cycles, each cycle lasting about 90 minutes. After falling asleep, a healthy adult will slip into stage 1 sleep, which gives way to 2, to 3 to 4; then the sequence goes into reverse. After the second period of stage 1 sleep, the cycle is completed by between 5 and 15 minutes of REM sleep. The sequence is repeated four or five times, with the portion of REM sleep increasing in each cycle while the NREM portion (especially stages 3 and 4) decreases. A person sleeping for around 8 hours will spend about two hours in REM sleep, and the remainder in NREM sleep.

This pattern of NREM and REM sleep, especially the 90 minute cycle, is remarkably similar from one human to another. Researchers can sometimes gain new insights into sleep problems, and also into the importance of sleep in the development of the brain, from deviations in this pattern.

For example, people with a condition called narcolepsy suffer an uncontrollable urge to fall asleep. No matter where they are or what they are doing, sleepiness will suddenly overtake them. Researchers have recorded the brain activity of sleeping narcoleptics. They found that narcoleptics do not start their night with NREM sleep, but instead go straight into REM sleep.

Young mammals experience a much greater proportion of REM sleep than normal adults. A newborn baby who sleeps around 16 hours a day will spend at least half of that in REM sleep. With premature babies, the percentage of REM sleep is even higher 鈥� around 75 per cent. A newborn kitten, puppy, rat or hamster experiences only REM sleep. A newborn guinea pig, however, has very little REM sleep.

Some researchers believe that REM sleep is essential for the human brain to mature before and after birth, and that this explains why babies need so much sleep. The small amount of time that the newborn guinea pig spends in REM sleep supports this hypothesis: compared with a helpless kitten or puppy, a guinea pig is 鈥渕ature鈥� at birth.

How much sleep?

Cradle to grave

A TEAM of American physiologists studying sleep patterns found that newborn babies sleep on average 16 hours in every 24. The sleep requirement, however, can be much less. The researchers carried out a detailed study of intellectual development in many babies, including those that stay awake for considerably more than the average 8 hours. They concluded that, provided the babies could sleep as and when they wanted, and for as long as they wanted, the amount of sleep taken during every 24 hour period has no affect on their subsequent development.

The same team found that 16-year-old students liked to sleep between 10 and 11 hours a night; older college students slept on average 8 hours a night. Some of the older students were able to adapt their sleep patterns to the demands of what they had to do the next day.

The need for sleep continues to decline as people age, to about seven hours a night on average for 45-60 year olds, and even less in old age. In particular, stage 4 sleep almost disappears after the age of about 50.

快猫短视频s have never come across a person who can go without sleep. A few individuals, usually in the interests of raising money for charity or seeing their name in The Guinness Book of Records, have attempted to stay awake for as long as possible. The current record holder is Robert McDonald, of California. In 1988 he held out for 18 days 21 hours 40 minutes during a rocking chair marathon.

Apart from the sheer difficulty of staying awake, such record-breakers report side effects such as bizarre hallucinations, paranoia, irritability, blurred vision, slurring of speech, and lapses in concentration and memory. The symptoms disappear after a few nights of normal sleep.

Without close medical supervision, however, it is impossible to tell if those taking part in 鈥渨akeathons鈥� are really awake all the time. Not many people realise that humans can not only take cat naps when their eyes are open, but may not even realise that they are doing so.

In the laboratory, researchers have regularly deprived volunteers of sleep for periods of about three or four days. Apart from sleepiness, the main problem is loss of concentration. The volunteers pass any test with flying colours, provided the tasks are very short and interesting. When asked to do long, repetitive tasks 鈥� equivalent to monitoring a production line in a factory or driving on a motorway 鈥� the sleepy volunteers make lots of mistakes.

People suffering from insomnia have trouble falling asleep and staying asleep throughout the night. The problem is real enough, but it may seem worse; time appears to pass very slowly when you lie awake at night, and insomniacs often think that they have been awake far longer than they really have. 快猫短视频s at Stanford University monitored the sleep patterns of some people who complained of very bad insomnia. Only about half of the insomniacs remained awake for more than 30 minutes in the course of the night.

Body chemistry

Sleep substances

IT MAY BE that some sleep problems are related to the production 鈥� or nonproduction 鈥� of sleep substances by the body. Recent research suggests that such chemicals modify and regulate sleep, rather than being its prime cause.

快猫短视频s at Harvard Medical School isolated one substance from human urine in the early 1980s; they called it Factor S. From 3000 litres of urine they extracted just 7 millionths of a gram of Factor S, but this was enough to make 500 doses. When the Harvard scientists gave a dose of Factor S to a rabbit it spent up to six hours in the deeper, NREM stages of sleep 鈥� much longer than it normally would.

Factor S is now known to be a muramyl peptide, the remains of bacteria after they have been digested and destroyed by the body鈥檚 immune system. The brain seems to incorporate this substance into its own biochemistry. 快猫短视频s have synthesised a similar chemical, muramyl dipeptide (MDP) and found that this also increases NREM sleep in rats, cats and monkeys, as well as rabbits. No human has yet volunteered to try it.

Another substance discovered within a mammalian brain is 鈥渄elta sleep- inducing peptide鈥�, better known as DSIP. It is found in the blood of sleeping rabbits, and given to other animals it rapidly induces NREM sleep, as well as increasing the amount of REM sleep. 快猫短视频s have tested a synthetic form of DSIP on human volunteers, but the effect on them was much less noticeable than in animals. This is probably because the synthetic version is not exactly equivalent to the natural product.

Our bodies produce other chemicals during sleep. In both children and adults, the output of human growth hormone surges during the first 3 hours of sleep. Nobody knows why this occurs, and it does not mean that people grow while they are asleep. Human growth hormone has several roles in the body, including speeding the rate of protein synthesis. However, stimulating cells to grow, which involves protein synthesis, requires a good supply of insulin to be present; and there is no surge of insulin in the early hours of sleep.

Mind games

Why do we dream?

WHEN people are woken during REM sleep they often say that they were dreaming, and have a clear memory of that dream. Sleep research has shown that everyone dreams, even though some people do not remember doing so. But what are dreams?

Clearly, dreaming involves electrical activity in the brain, and so does conscious thought. But is this just the untended machinery of thought ticking over while the owner is effectively not in control? Or is it an automatic process that occurs while nerve connections in the brain are being maintained biochemically, or while the flood of information from the previous day鈥檚 experiences is being collated, sorted and filed away for use? Nobody knows. But we do know that dreams are not just a filing process, nor are they simply random messages firing off among the brain鈥檚 neurons.

Dreams can be influenced by things that happen to the sleeper. For example, researchers sprayed a little cold water on to the back of one volunteer while he was in REM sleep and then woke him shortly after. The subject described a dream in which he was acting in a play. He reported that in his dream:

鈥淚 was walking behind the leading lady when she suddenly collapsed and water was dripping on her. I ran over to her and water was dripping on my back and head. The roof was leaking.鈥�

Problems and ideas as well as physical stimuli can become incorporated in dreams. In one classic experiment a researcher set 500 students a problem: 鈥淭he letters O,T,T,F,F form the beginning of an infinite sequence. Find a simple rule for determining any or all successive letters. According to your rule what would be the next two letters of the sequence?鈥�

Next day, only nine students had worked out the correct solution. Two had found the answer before they went to sleep; the remaining seven solved the problem in a dream. (The solution, by the way, is that O,T,T,F,F are the first letters of one, two, three, four, five. The next two letters in the sequence would be S,S.)

Nature鈥檚 cinema

A need for dreams

AMID all the uncertainties of research on sleep 鈥� and they are many 鈥� one possibility is that REM sleep is what we 鈥渘eed鈥� sleep for, and that NREM sleep is far less important for our wellbeing. Experiments suggest that our brains require a minimum 鈥渞ation鈥� of fantasy dreams each day. For example, in one study researchers deprived volunteers of REM sleep in the first half of the night. This affected people in very different ways.

Some subjects experienced an increase in REM sleep during the second half of the night. These were people who did not normally fantasise, or daydream, during their waking hours. After several nights deprived of half their REM sleep, they found themselves fantasising, seemingly to make up for the lost REM sleep. Subjects who ordinarily experienced a lot of waking fantasy continued to have their normal quota of REM sleep in the second half of the night, and said they did not notice any change in their daytime fantasising.

It seems that REM sleep, at least, serves a function in keeping us sane, although it is still not clear exactly how dreams serve that purpose. But REM sleep is only a quarter of the total, in adults. Why do we have so much NREM sleep? This is even less clear, and some researchers argue that such sleep may have evolved simply as a handy way to keep our bodies out of mischief in between meals.

This argument gains weight from the observation that prey animals, such as zebra, sleep much less than predators, such as lions. After eating its fill of meat, a lion may sleep for two or three days, more or less continuously. After all, why should the lion waste energy wandering about, or killing things, when its body has absorbed enough food to last for several days?

And maybe that is the answer to the ability some people have to sleep much less than others without suffering harmful effects. As long as they are getting their quota of REM sleep, and a reasonable amount of physical rest during the day, there really may be no reason why they should spend up to a third of their lives in bed.

A measure of sleep

RESEARCHERS studying sleeping and dreaming monitor and record the electrical activity of a person鈥檚 brain. They tape or glue one electrode to the scalp and another to the ear. Wires take the signals to a machine called an electroencephalograph, or EEG, which contains an amplifier to boost the signals (only a few millionths of a volt). The EEG also includes pens that move automatically across chart paper in response to the changes in the potential differences created by the electrical activity of the brain. The trace of the brain activity is called an electroencephalogram (also shortened to EEG).

The EEG of REM sleep, though, may look like that of stage 1 sleep. To distinguish between the two, researchers attach electrodes to the face that will pick up the rapid eye movements that are characteristic of this phase of sleep. Because the eye movements are intermittent 鈥� researchers also record impulses from the chin muscles. Muscles are extremely relaxed during REM sleep. Physiologists call the trace of eye movements an electrooculogram, or EOG; and that of muscle activity an electromyogram, or EMG.

The traces below show the changes in EEG through the stages of sleep into waking. Stage 2 sleep is deeper than stage 1, and so on. As the sleep deepens, the brain waves become slower and of greater amplitude. The REM stage of sleep is the lightest, but REM and dreaming do not occur at the beginning of the night鈥檚 sleep. The first period of REM normally begins about 60-70 minutes after the onset of sleep. Overall, REM sleep makes up about 25 per cent of the total.

Stage 1 accounts for about 5 per cent of sleep, and is really a transition phase from wakefulness to true sleep (stage 2 or deeper). The bulk of sleep, almost 50 per cent, is made up of stage 2 sleep. Stage 3 takes up about 7 per cent of the night, and mainly acts as a transition between stages 2 and 4 sleep.

The first period of stage 4 sleep is the deepest, and it is very difficult to rouse people from this phase of sleep. If they are wakened, they are likely to feel groggy and be incapable of coherent thought for 10 or 15 minutes. Stage 4 accounts for up to 13 per cent of sleep in young adults, but is almost totally absent in that of the elderly. As the night progresses, the depth of the periods of NREM sleep becomes less pronounced, and in the last couple of cycles there may be no stage 3 or stage 4 sleep.

Down to rhythms

OUR BODIES are paced by a circadian rhythm that seems to be kept in time by the light and dark cycle. The natural circadian rhythm, however, is longer than 24 hours. When volunteers agreed to live underground for a month with only artificial light available and no external cues, such as clocks, radios or television, they developed a circadian rhythm of about 25 hours. The experiment implies that it is external things such as light and the pressures of society that push people into a 24-hour rhythm.

This discovery has helped one group of patients suffering from sleeping difficulties. In 鈥渄elayed sleep phase syndrome鈥�, people have trouble going to sleep in the early part of the night. Only shortly before it is time to wake up do they fall into a sound sleep. Doctors reasoned that, because the natural body clock is set at around 25, not 24 hours, it is easier to go to sleep later than earlier. They therefore advised the patients to go to bed three hours later each day, starting with a bed time of 5 am on the first night. After six days, the patients were going to sleep at 11 pm, and their body clock was reset to the right time. Many were cured permanently of the syndrome.

When people move across time zones, during jet travel, they disrupt their normal circadian rhythms and develop jet lag. The body clock cannot reset enough in one jump to match the time zone it is now in, and needs about five days to get back to normal.

Jet lag is worst when travelling east because you experience sunset much earlier than your body expects, and are trying to persuade your body clock that it is time to go to sleep earlier than usual. Flying west involves trying to sleep later than normal, which is easier. If you have to take a long flight eastbound the best way to minimise jet lag is to get up earlier and earlier on the few days before you fly.

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