Sleep may have begun hundreds of millions of years ago as brain housekeeping
Sleep is found so widely across animals that it probably dates back hundreds of millions of years, and one leading idea is that it began as a way for the brain to flush out waste, including amyloid. Every night we cycle through distinct states roughly every 90 minutes, waking briefly more often than we realise.
Sleep is a state of dampened mental and physical activity in which consciousness shifts, muscles relax and response to the outside world drops, yet the brain stays active enough to react to loud or important sounds, unlike in a coma. Most body systems switch to repair mode, supporting immunity, nerves, bones and muscles, and slow-wave sleep brings bursts of growth hormone. The brain, which uses about 20% of the body's energy during quiet waking, burns much less, particularly in non-REM sleep, and replenishes its store of ATP.
Two sharply different states alternate. Non-REM comes first and deepens into slow-wave sleep, when temperature and heart rate fall. REM, sometimes called paradoxical sleep, is shorter and brings most dreams, fast desynchronised brain waves, darting eyes and near-paralysis of the body. A full cycle averages 90 minutes and repeats four to six times a night, running through stages N1, N2 and N3, back to N2 and then REM. Deep sleep dominates early on, while REM grows in the last two cycles before natural waking. Researchers track all this with polysomnography, which records brain waves, eye movements and muscle activity together.
Brief wakings are normal and cluster just after REM. EEG studies put nightly awake time at 0 to 1% for women and 0 to 2% for men, and one study found wakefulness rising from 3% in the first cycle to 13 or 14% in the fifth. Waking itself starts with a surge of activity in the thalamus that spreads across the cortex, and the stage one wakes from affects how groggy one feels.
Timing is steered by the circadian clock, called Process C and centred in the suprachiasmatic nucleus just above the optic chiasm, and by the homeostatic build-up of sleep pressure, Process S. Artificial light has reshaped human sleep: screens emit plenty of blue light associated with daytime, which disrupts release of melatonin. Many people sleep differently on workdays and days off, a habit linked to chronic desynchronisation of the body clock.
Source: Sleep
