Do you know why we need to sleep?

Prof. Know Why answers:

There are conflicting theories about the “purpose of sleep” – that is, why do we need to sleep? But the fact that we spend about eight hours asleep of every 24 hours, suggests that there are powerful evolutionary reasons for its continuing existence. Most theories center on the beneficial effects that sleep has on the brain. This is because; there is little evidence of any direct physiological effects on the body. Some scientists believe that sleep is a time for the immune system to regenerate, while one theory suggests that sleep evolved as a way of coping with limited supplies of food.

The dominant theory is that sleep is a time for the brain to store memory. The idea is that during sleep the brain, in effect, goes offline to file the events of the day. Another theory is that the brain is a complex organ that needs the downtime provided by sleep to recover from the stresses of waking hours. “While we are awake, the higher centers of the brain are working flat out,” says Professor Jim Horne, who runs the sleep laboratory at Loughborough University. “Even when you are lying down, the brain is in a state of quiet readiness, ready to respond. The only time it can rest is during sleep.”

Neuroscientists argue that at least one vital function of sleep is bound up with learning and memory. A cascade of new findings, in animals and humans, suggest that sleep plays a critical role in flagging and storing important memories, both intellectual and physical, and perhaps in seeing subtle connections that were invisible during waking.

In a study which was published last year, the participants were given an exercise to study pairs of Easter eggs on a computer screen and memorize a day later, how the computer has arranged them. The researchers at Harvard and McGill Universities, who conducted the study, reported that participants who slept after playing this game scored significantly higher on a retest than those who did not sleep. While asleep they apparently figured out what they didn’t while awake - the structure of the simple hierarchy that linked the pairs.

“We think what’s happening during sleep is that you open the aperture of memory and are able to see this bigger picture,” said the study’s senior author, Matthew Walker, a neuroscientist who is now at the University of California, Berkeley. He added that many such insights occur “only when you enter this wonder-world of sleep”.

Researchers, who champion these brain theories, point to the fact that behavior changes significantly with sleeplessness. A person who is lacking sleep takes more risks, has slower mental processes and reacts to events with more emotion and less logic. One theory is that brain circuitry needs to be repaired and maintained regularly and that when this maintenance work is not carried out during sleep, the workings go awry, with communications slowing down or being wrongly routed, if you don’t get enough sleep.

So, how much sleep do you need? Some researches suggest that while seven to eight hours a night is healthy, under five hours or more than eight is unhealthy, and linked to disorders such as heart disease, depression, diabetes and high blood pressure.

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