Why do we sleep and dream?

Studies are shedding light on the mystery of sleep

ANYONE who has ever owned a hamster will know that the animal spends an inordinate amount of time sleeping. Depending on your point of view, that might be a good or bad thing. Hamsters are small and, broadly speaking, the smaller the animal, the more sleep it needs. While elephants need as little as three or four hours of sleep per day, tiny opossums will snooze for 18 to 20 hours. Flipper, however, doesn't seem to sleep at all--at least not in any way that humans know it. In the bottlenose dolphin, each side of the brain appears to take it in turns to do something that is equivalent to sleep but with only one eye closed.

Such animal curiosities give scientists clues to the function of sleep and thus what it is, and what it is for, in humans. It is clear that sleep is more than the absence of wakefulness--it is the reorganisation of the brain's neuronal activity. During sleep, the brain is very active and two different states, non-rapid eye movement (NREM) and rapid eye movement (REM) sleep occur. These are as different from each other as they are from wakefulness, which means humans have three very different states of being, rather than two. In each, most parts of the brain are active but in different ways.

When humans are deprived of sleep--something they spend about a third of their lives doing--they accumulate a sleep "debt". In other words, they must ultimately make up for some of the missed sleep if they are to continue to function. Clearly, something important is happening. But when only REM sleep is suppressed, either through brain lesions or some anti-depressant drugs, there are no obvious cognitive or physiological symptoms. So these two sleep states are likely to have different purposes.

There are many theories about the function of NREM sleep, with its reduction in brain metabolic activity. One is that it may conserve energy at some times of day in the same way hibernation does during particular seasons. Another role for NREM sleep is also suggested by recent work showing that sleep may have a role in allowing, or facilitating, the growth of new neurons in adults.

Jerome Siegel, of the University of California in Los Angeles, along with colleagues elsewhere, has suggested a third possibility. Writing in a special review of sleep in the current issue of Nature, Dr Siegel says that it may be related to defence against oxidative stress. Smaller animals have higher metabolic rates, and this results in the generation of higher levels of small, highly reactive molecules such as free radicals. Studies on rats show that when they are deprived of sleep, their oxidative stress increases. Dr Siegel proposes that animals with higher metabolic rates need more sleep to interrupt and protect against this kind of oxidative damage, as well as repairing any damage done.

Sleeping on it

REM sleep, which accounts for 20% of human sleep, is different from NREM sleep because the brain's metabolic activity remains high, as does its neuronal activity. Eyes and extremities twitch, and this is the point where humans are most likely to have illogical or bizarre thoughts, and vivid dreams--such as being eaten by an enormous snake.

One popular theory is that it is required for the consolidation of memories. The obvious flaw in this suggestion is that one does not have to have been eaten by an Anaconda or sat a maths exam naked in order to have this kind of dream. Indeed, recent studies suggest that only 1.4% of dream reports portray complete experienced memories. Typically fragments of memories are most common, although complete memories are more likely to be dreamed about after a traumatic experience. In one study, 65% of dream elements were linked to features of waking events. Such research, though, is difficult to do because it is based on subjective reports.

It seems likely that sleep is an important component of how memories are formed and shaped. Dreaming about newly learned material enhances its subsequent recall. However, the exact relationship between dreams and memory remains enigmatic. There is clearly more at work than the replaying of memories during sleep. Robert Stickgold of the Harvard Medical School thinks that memories are not being reactivated at all because brain-imaging studies show an area of the brain involved in memory recall is deactivated in sleep, especially REM sleep.

Allan Hobson, also of the Harvard Medical School, says that research suggests REM sleep is a physiological brain state that produces a distinctive and psychosis-like mental content, in which the sleeping brain can either generate its own perceptions or think about them, but it cannot do both.

However, dreams do simulate reality in several intriguing ways. One could be eaten by a snake in an exact replica of one's bathroom--that is, in a real, spatially coherent environment. And it also might be possible to have a discussion with the snake's owner, or experience the sensation of being swallowed, while maintaining a steady stream of perception throughout the whole dream. Such matters science is still struggling to understand. What exactly is the hamster dreaming during all those hours it spends asleep? That is one question that even science may never be able to answer.