Scientists have long been puzzled by the purpose of sleep
WHAT makes you tired? Is it the build-up of something during wakefulness that sleep removes? Or is it the depletion of something during wakefulness that sleep restores? The purpose of sleep is a hot topic within the hot field of sleep research. ''It's the core question for our field,'' says Dr Gary Richardson of Boston's Brigham and Women's Hospital sleep laboratory. ''The burning question is, 'What part of the brain isbenefiting from sleep, and how?' '' Virtually all animals sleep, including birds and reptiles. But it was only in the last decade that a Chicago researcher, Alan Rechtschaffen, demonstrated that animals need sleep to survive. Sleep-deprived animals eventually die because the brain centresthat regulate body temperature break down.
Other clues about sleep's purpose: Prolonged wakefulness creates a ''pressure'' for slow-wave sleep, the dreamless state not associated with rapid-eye-movement, or REM, sleep. The longer you stay awake, the greater the duration and depth of slow-wave sleep.
Depriving people of REM sleep creates difficulty in memory and concentration, while those deprived of slow-wave sleep suffer aches, pains and fatigue. ''This suggests there are restorative functions of sleep and there are probably physical versus mentalcomponents,'' says Dr Sanford Auerbach, director of Boston University Medical Centre's sleep disorders centre.
The longer experimental subjects are kept awake, the more growth hormone they secrete when allowed to sleep. Growth hormone, secreted by the hypothalamus deep within the brain, controls fat metabolism, allowing the body to chug along without sugars during periods of fasting.
This last observation may be an important clue. Humans are unusual among animals in sleeping in long, often eight-hour segments. One hypothesis is that the growth hormone/sleep relationship evolved to permit humans longer periods of sleep - and thus longer, uninterrupted periods of wakefulness to complete complex tasks.
''Higher intelligence required a longer period of uninterrupted consciousness,'' Dr Richardson says. ''It's what allowed us to do things like invent the light bulb.'' Researchers hope to discover precisely what happens to brain cells during sleep. The answer has eluded them, but it could help them understand why some individuals need more sleep than others and how the brain functions during wakefulness.