How Sleep Works

You spend about one third of your life asleep. It can feel like doing nothing. But your brain and body are very busy.

Sleep has stages. First you sleep lightly. Then you go into deep sleep. In deep sleep, your body repairs muscles and grows. After that comes REM sleep. REM means "rapid eye movement" because your eyes move quickly under your eyelids. Most dreams happen in REM. In this stage, your brain organises memories from the day.

One night has several cycles of these stages, about 90 minutes each. Most adults need seven to nine hours to finish enough cycles.

When you do not sleep enough, you feel tired, of course. But you also think slowly, feel more emotional, and get sick more easily. Over many years, poor sleep is linked to serious health problems. Sleep is not a luxury. It is basic care for your body.

Sleep looks like an off switch, but it is closer to a maintenance shift. Across a night the brain moves repeatedly through a set of stages, each with a distinct job.

Light sleep is the transition in and out. Deep sleep, which dominates the first half of the night, is when the body does most of its physical repair — releasing growth hormone, rebuilding tissue and strengthening the immune system. It is also when the brain appears to "clear out" waste products that build up during waking hours. REM sleep, named after the rapid eye movements that accompany it, becomes longer towards morning. This is the stage of vivid dreaming, and it seems central to how the brain processes emotion and consolidates what was learned during the day.

These stages repeat in cycles of roughly 90 minutes. Cut sleep short and you lose whole cycles, and disproportionately the REM-heavy ones near waking — which is one reason a short night affects mood and memory so noticeably.

The consequences of sleeping too little are wide-ranging. In the short term: slower reactions, poorer judgement, stronger emotional swings and a weaker immune response. Chronic short sleep is associated with higher risks of heart disease, type 2 diabetes, weight gain and cognitive decline. Sleep is not time subtracted from a productive day; it is part of what makes the productive day possible.

For an activity that consumes roughly a third of the human lifespan, sleep was until recently strikingly under-studied. That has changed. A generation of research has replaced the image of sleep as passive shutdown with something far more active and, in evolutionary terms, expensive: an organism that switches off awareness and mobility for hours every day is taking a considerable risk, which implies the benefit must be substantial.

The architecture of a night is now reasonably well mapped. Non-REM sleep, and deep slow-wave sleep in particular, front-loads the night and handles the somatic side of the ledger: tissue repair, immune consolidation, hormonal regulation and — a more recent finding — the clearance of metabolic by-products from brain tissue via the so-called glymphatic system, a process that appears far less efficient during waking. REM sleep, which lengthens towards morning, is more cognitive and affective in its function, implicated in emotional processing, creative recombination of ideas and the integration of new memories with existing knowledge.

Because these functions are distributed unevenly across the night, the cost of curtailing sleep is not linear. Losing the final ninety minutes removes a disproportionate share of REM, which helps explain why chronically shortened sleep produces such pronounced effects on mood regulation, emotional reactivity and the consolidation of learning.

The public-health picture is now fairly settled. Habitual short sleep is associated with elevated risk across cardiovascular disease, metabolic disorders, immune dysfunction and long-term cognitive decline, and with impairments in attention and decision-making that individuals consistently underestimate in themselves. The cultural framing of sleep as discretionary — a resource to be borrowed against when ambition demands — sits awkwardly against that evidence.