Why We Sleep

Memory consolidation, synaptic rebalancing, waste clearance, energy and repair: the leading theories of why every animal studied sleeps — and what happens when we don't get enough.

Sleep is risky — a sleeping animal can't watch for predators — yet it is found in mammals, birds, reptiles, fish and even fruit flies, worms and jellyfish (which have no brain). Dolphins and some birds sleep with one half of the brain at a time. Something about sleep must be essential.

Memory and learning

Sleep supports memory consolidation. The hippocampus replays recent experiences during slow-wave sleep, and sleep spindles and slow oscillations coordinate transfer to the cortex. People generally remember better after sleep than after equal time awake, and sleep also helps the brain extract patterns and insights.

Synaptic rebalancing

The synaptic homeostasis hypothesis, proposed by Giulio Tononi and Chiara Cirelli, suggests that learning during the day strengthens many synapses, which is costly in energy and space, and that slow-wave sleep scales them back down, keeping the important changes while restoring capacity. Studies in flies and mice have found synapses tend to be larger after waking and smaller after sleep.

Cleaning and maintenance

Research in mice suggested waste clearance through the glymphatic system increases during sleep — though this is debated. Sleep supports immune function, hormone regulation and metabolic health.

What sleep loss does

Short-term sleep deprivation impairs attention, reaction times, working memory, mood regulation and judgement. After about 17–19 hours awake, performance on some tasks can be comparable to having a blood alcohol level around legal driving limits in some countries. Sleep-deprived people often underestimate how impaired they are. Chronic insufficient sleep is associated with higher risks of obesity, diabetes, cardiovascular disease, depression and accidents. In 1964, 17-year-old Randy Gardner stayed awake for about 11 days in a school science project, suffering mood changes, memory lapses and hallucination-like experiences. The extremely rare inherited prion disease fatal familial insomnia destroys the ability to sleep and is fatal.

How much?

Expert bodies such as the American Academy of Sleep Medicine recommend 7 or more hours for most adults, with more for children and teenagers — around 8–10 hours for teens. Needs vary between individuals; a few people carry rare gene variants that let them thrive on less.