Under the Microscope: Most people spend about one third of their lives asleep, but science still has much to learn about why we need so much sleep, writes Prof William Reville.
There are many hypotheses as to why sleep is necessary, for example so the body can rest and repair itself; so the brain can categorise and assimilate the information it has gathered during the day; a mechanism that evolved to keep us safe during the dark hours when we were not hunting; and so on. No one knows for sure. A simple and clear treatment of the subject can be found in Inside Science No. 36 and in the New Scientist, July 7, 1990, written by Mary Gribbin.
There is no doubt that sleep is important. Try going without it. Laboratory studies have shown that if you deprive rats of sleep, they eat more but nevertheless they waste away. Their body temperature goes awry and they die within three weeks. Some people suffer from insomnia. About one person in four suffers from occasional bouts, while one in 10 has chronic insomnia. Chronic insomniacs are more anxious and depressed than average and are more prone to alcohol problems.
The amount of sleep we take depends on age. Newborn babies sleep about 16 hours a day. By adulthood, the average sleep taken is about eight hours a day and this declines to six hours in the elderly. However, there can be a difference between the amount of sleep we take and the amount we strictly need. Margaret Thatcher was reputed to get by well on four hours sleep per night. Also, extensive studies have shown no ill-effects in the development of babies that stay awake considerably more than eight hours per day, so long as they can sleep when they want and for as long as they want.
As we fall asleep, the brain becomes increasingly more passive and less responsive to the outside world. Blood pressure, pulse rate and body temperature fall, gastrointestinal tract activity increases, muscles relax and the rate of metabolism falls by about 20 per cent. The brain shows the most clear-cut changes during sleep. Electrical activity in the brain (brain waves), can be recorded by placing electrodes on the scalp. Wires carry the electrical signals to a machine called an electroencephalograph and the electrical activity of the brain is traced out on chart paper. The trace of activity is called an electroencephalogram (EEG).
Researchers have categorised five stages of sleep - a rapid eye movement (REM) phase and four non-rapid eye movement (NREM) phases. When you go to sleep, you first go through phases one to four NREM sleep, during which the brain waves slow down and you lie still and breathe regularly. REM sleep is much lighter and the eyes dart about under the closed eyelids. Breathing becomes irregular and there is more body movement than in NREM sleep. Electrical activity in REM sleep speeds up to stage one NREM activity, which is a transition phase between wakefulness and sleep.
The sleeper goes through several of these five-stage cycles, each lasting about 90 minutes. The proportion of REM sleep increases in each cycle and, overall, if you sleep for eight hours, you will spend about two hours in REM sleep. However, the proportion of sleep time spent in REM sleep varies with age. A newborn baby spends about half its sleep in REM sleep. Premature babies spend about 75 percent of their sleep time in REM sleep. Newborn kitten or puppy sleep is 100 per cent REM.
Everyone dreams and dreaming takes place during REM sleep. It is still not clear what function dreaming has, but many researchers believe that REM sleep is the most important phase of sleep. Some research suggests that we require a minimum amount of fantasy dreaming every day. Volunteers who had their REM sleep disrupted during the first half of their night's sleep reported an increased amount of fantasising (daydreaming) during waking hours, apparently to compensate for lost REM sleep.
Narcolepsy is a sleep disorder primarily characterised by intermittent uncontrollable episodes of falling asleep during the daytime. It affects about 25 people in every 100,000. Research has shown that narcoleptics don't start sleep with NREM sleep but go straight into REM sleep.
Dreams are much more than a simple filing process of our daily experiences. Ideas and problems can be worked on in our dreams. For example, the famous German chemist Friedrich Kekule (1829-1896), who worked out the molecular structure of benzene and some other molecules, reported that the solution to his conscious ponderings often appeared to him in dreams when he dozed off in weariness.
If REM sleep keeps us sane, what is NREM sleep for? This may have evolved just to keep us out of harm's way and to conserve energy during periods when our bellies are full. In nature we note that predator animals sleep more than prey animals.
All animals have periods when they do very little. Insects and fish rest, amphibians and reptiles sleep, and all mammals display REM sleep. Interestingly, in some sea-going mammals - dolphins, pilot whales, porpoises and the sea cow - only one hemisphere of the brain sleeps at a time. This allows the animal to keep swimming to the surface and breathing. The bottom line seems to be that complicated brains evolved REM sleep.
William Reville is Associate Professor of Biochemistry and Director of Microscopy at University College Cork.