Why most of us spend one third of our lives asleep


THE average person aims to sleep eight out of 24 hours and therefore, in a typical lifespan of 75 years, he/she sleeps for 25 years. Despite the fact that we devote such a large fraction of our time to the activity, many mysteries still remain about the nature and function of sleep.

The average person sleeps for eight hours every night and, if he/she sleeps for substantially less time than this, feels poorly. Getting "a good night's sleep" has always been a cornerstone of preventive medicine and mothers' lore.

Some very high achievers e.g., Margaret Thatcher, resent the idea of spending one third of their lives asleep and train themselves to get by with significantly less sleep than the average person. Of course, a minority of "ordinary" people also get by perfectly well with a couple or several hours less sleep every night than the average eight hour figure.

The surrealist painter Salvador Dali claimed to sleep for only three or four hours every night and to compensate for this by taking short naps during the day. He liked to doze off holding a spoon poised over a tin plate. When the spoon slipped and hit the plate he would awaken a new man".

Conor Cruise O'Brien made short work of his recent large book on Burke by starting work on it every day at 4 a.m. He recounted wryly that a friend, on hearing of this work practice, remarked that it explained a certain inhuman quality in his writings. On the other hand, Einstein liked to sleep about 14 hours a day, a statistic that most of us would find more comforting.

When we go to bed at night what causes us to fall asleep? There were two theories in the 19th century, both based on blood circulation.

One theory said that, on lying down, the brain becomes engorged with blood and this causes sleep. The other theory held the opposite - that blood drained away from the brain and that this caused sleep.

Insomniacs could therefore be offered contradictory advice. One physician would advise sleeping without pillows in order to allow blood easier access to the brain. Another physician would advise using several pillows in order to help blood drain away from the brain.

Early this century two Frenchmen, Legendre and Pieron, did an interesting experiment with pairs of dogs One member of each pair was kept awake for as long as two weeks while the other dog was allowed sleep as normal. Then the researchers injected cerebrospinal fluid from the sleep deprived dogs into the well rested dogs, which quickly fell into a deep and unusually long slumber.

The researchers concluded that a sleep substance, which they termed hypnotoxin, had accumulated in the cerebrospinal fluid of sleep deprived dogs during their long period of wakefulness.

The work of the French men was succeeded by an alternative theory that sleep is induced by a changed pattern of electrical activity in the brain, the Neural Theory. It is now known that there are several areas deep within the brain geared specifically to regulate sleep.

Sleep is not a continuous homogenous phase in which the brain is inactive. A machine called an electroencephalograph, which records electrical activity in the brain, has shown that there are two major phases of sleep.

One phase is called slow wave: sleep, in which the sleeper appears to be in his most restful state. The next phase is marked by high frequency waves resembling those noted during wakefulness. This phase is also characterised by flurries of rapid eye movements (REM) and is called REM sleep.

Sleep begins as a brief light slumber lasting about 10 minutes. Then begins the first major sleep stage - deep slow wave sleep. After an hour and a half there is a transition back to light slumber followed by a phase off REM sleep. Over the course of eight hours this cycle recurs four or five times.

Mammals, other than humans, generally have REM sleep periods and probably dream at these times. They show pronounced activation of the visual cortex during REM sleep and, in humans, this corresponds to experiencing visual sensations (dreams).

The chemical nature of hypnotoxin, the sleep inducing substance from the cerebrospinal fluid, has now been worked out. Technically it is called a muramyl peptide, a class of substance contained in bacterial cell walls.

It has a very potent sleep inducing effect. As little as one billionth of a gramme induces deep sleep for several hours in rabbits. The muramyl peptides seem to exert their sleep inducing effects by initiating a complex sequence of biochemical reactions in the body, including activating the immune system.

The activation of the immune system includes the activation of macrophages, a freely mobile cell in the body that attacks and breaks down invading bacteria.

The fact that hypnotoxin is a muramyl peptide, a class of compound found in bacteria, is a fairly obvious clue that bacterial "play an important role in regulating our patterns of sleep.

Bacteria are far more than just carriers of disease; they have useful symbiotic relations with many mammalian species including man. Most of us carry about a kilogramme of bacteria - in our intestinal tracts where they help synthesise vitamin K and from where many of them pass into the body through the intestinal wall.

When bacteria enter the body they are broken down by the macrophages and muramyl peptides are released. This is a continuous process and probably acts to regulate everyday sleep. This bacterial theory would also explain the excess sleep that often overwhelms the body during its immune response to an infectious disease.

Sleep is essential to ensure not only our good health but even life itself. No mammal can survive without sleep.

Yet it is not entirely clear why sleep is so essential. It is known that our physical bodies do not need to spend one third of theirs time in a "switched off" mode in order to be optimally active for, the remainder of the time. The physical resting of our muscles during sleep is probably of some: benefit, but we do not need anything like eight hours rest perk day in order to achieve this physical benefit.

The main benefits of sleep, seem to be psychological. About one quarter of a night's sleep is spent in REM sleep and it is during this phase that we have our dreams. Dreaming is an essential process in which we clarify and make sense of the various experiences we have during our wakeful hours.

If we are prevented from dreaming, the programmes in our central computer, the brain, become mixed up and we suffer grave distress. Experimental animals systematically deprived of REM sleep quickly develop serious stress problems and, if the dream deprivation is continued, the animals become completely deranged.

So do not begrudge the fact that you spend so much time asleep. The bottom line seems to be the only way to ensure that we are reasonably sensible when awake is to spend about one third of our time asleep.