How sleep works

Sleep occupies something like a third of a life, and for most of the twentieth century it was assumed to be a passive state — the body switched off, waiting for morning. It is not. The brain during sleep is intensely active, running processes that cannot be run while awake, and the last few decades of research have been largely about working out what those processes are.

What sleep is for

There is no single answer, and anyone offering one is overstating the case. Several things are now reasonably well established:

Memory. Sleep consolidates what was learned during the day, moving material from short-term to long-term storage and strengthening the connections that hold it. Deep sleep appears to do most of the work on facts and events; dreaming sleep on skills, patterns and emotional processing. This is why revising late into the night at the cost of sleep tends to be counter-productive.

Clearance. The brain has a drainage system that flushes metabolic waste out of brain tissue, and it operates far more vigorously during sleep than during waking. This is one of the newer findings and one of the more interesting, because the substances cleared include proteins implicated in dementia.

Repair and growth. Growth hormone is released mainly during deep sleep, tissue repair is concentrated there, and this is why children and adolescents need so much of it.

Immune function. Sleep deprivation measurably reduces the response to vaccination and raises susceptibility to infection.

Metabolic and hormonal regulation. Short sleep disturbs appetite hormones, pushing hunger up and satiety down, and reduces insulin sensitivity. The link between short sleep and weight gain is consistent across a great many studies.

Emotional regulation. A night of poor sleep makes the brain more reactive to unpleasant things and less able to moderate the response, which is the mechanism behind the ordinary experience of everything feeling worse when tired.

What is not established, despite being asserted confidently in a great deal of writing, is that sleep exists for any one of these. It most likely evolved to do several things at once.

Two things decide when sleep comes

This is the single most useful idea about sleep, and it is missing from most popular accounts. Sleep is not governed by one mechanism but by two, working independently, and most sleep problems are a disagreement between them.

Sleep pressure builds steadily from the moment of waking. A substance called adenosine accumulates in the brain throughout the day, and the longer the waking, the stronger the drive to sleep. Sleep discharges it. A nap discharges some of it, which is why a long afternoon nap makes the night harder. Caffeine works by blocking adenosine's effect — it does not remove the pressure, it masks it, which is why the tiredness arrives all at once when the caffeine wears off.

The internal clock runs on roughly a twenty-four-hour cycle regardless of how long anyone has been awake, and it determines when the body expects to sleep. It is why someone exhausted at three in the afternoon can be wide awake at eleven at night, and why a night worker lying in a dark room at ten in the morning cannot sleep despite being desperate for it.

Good sleep happens when the two line up: high sleep pressure arriving at the point in the clock cycle when the body expects to sleep. Jet lag, shift work and the teenage morning are all versions of the same problem — the pressure is there and the clock disagrees.

The clock, and what sets it

The master clock is a small cluster of cells in the hypothalamus called the suprachiasmatic nucleus. Almost every tissue in the body has a clock of its own, and the one in the brain keeps them synchronised. It governs not only sleep but body temperature, blood pressure, digestion, hormone release, alertness and mood, all of which rise and fall on a daily pattern.

Light is overwhelmingly the main signal that sets it, through a dedicated pathway from the eye. Nothing else comes close — not temperature, not meal times, not activity, though all of those have some influence. This is the practical heart of the matter: morning light advances the clock towards earlier sleep, evening light delays it, and that is the lever available to anyone whose timing has drifted.

Melatonin is a timing signal, not a sedative. It rises in the evening as light fades, peaks in the middle of the night, and falls before waking, and its function is to tell the body's tissues what time it is. It does not knock anyone out, and expecting it to is the commonest misunderstanding about it. Cortisol runs the opposite way, rising sharply in the hour or two before waking in anticipation of the day rather than in response to the first daylight.

The clock also varies between people, and this is genuine rather than a matter of discipline. Some people are reliably at their best early and some late — a chronotype, substantially inherited. It shifts across a lifetime: children are early, adolescents swing markedly late, and the tendency drifts earlier again through adulthood and old age. The teenage inability to get up is a real change in biology rather than laziness, which is why school start times have become a subject of serious argument.

The stages, and the shape of a night

Sleep is not uniform. It moves through distinct stages in cycles of roughly ninety minutes, four to six times a night.

The current classification has four stages, not five. Older writing describes stages one to four of non-dreaming sleep plus dreaming sleep; stages three and four were merged in 2007, and accounts still using the old scheme are describing a system that has been retired.

N1 is the brief transition into sleep — a minute or several, easily interrupted, often not experienced as sleep at all. The sudden jerk that can happen here, sometimes with a sensation of falling, is ordinary and harmless.

N2 is light sleep and makes up about half of a night. Brain activity slows, with characteristic bursts that appear to be involved in memory consolidation. Body temperature falls, the heart slows.

N3 is deep, or slow-wave, sleep. This is the sleep that restores — hardest to be woken from, and the stage most of the physical repair and growth hormone release happens in. Someone roused from it is confused and thick-headed for several minutes, which is the familiar awfulness of being woken at the wrong moment.

REM sleep, named for the rapid eye movements that accompany it, is where most vivid dreaming occurs. The brain is almost as active as when awake, breathing and heart rate become irregular, and the muscles of the body are temporarily paralysed — which is what stops dreams being acted out.

Where these fall in the night matters more than most accounts convey. Deep sleep is front-loaded, concentrated in the first few hours; REM sleep is back-loaded, with the longest stretches towards morning. Two consequences follow. Cutting a night short at the far end removes mostly dreaming sleep. And the first REM period does not arrive at the end of the first light stage — it comes about seventy to ninety minutes after falling asleep, after a full descent into deep sleep and back.

The stages also explain why certain things happen when they do. Sleepwalking, night terrors and sleep talking arise out of deep sleep, early in the night, in someone who is not dreaming and will not remember it. Nightmares arise out of REM sleep, later, and are remembered. They are different phenomena with the same reputation.

How much is needed

The honest answer is a range rather than a number, and it narrows mainly by age:

• Newborns — around fourteen to seventeen hours in twenty-four, in short stretches.
• Babies from about four months — twelve to fifteen hours, including naps.
• Toddlers — eleven to fourteen hours.
• Pre-school children — ten to thirteen hours.
• School-age children — nine to eleven hours.
• Teenagers — eight to ten hours, which almost none of them get.
• Adults — seven to nine hours.
• Older adults — seven to eight hours.

Several things about that list are worth saying out loud.

Eight hours is the middle of a range, not a requirement. Plenty of people function perfectly on seven, some need nine, and treating eight as a target to be hit is a reliable way of becoming anxious about sleep — which is itself a cause of sleeping badly.

Older adults do not need less sleep. They need about the same and tend to get less, because sleep becomes lighter and more broken with age and deep sleep declines. That is a change in the quality of sleep, not a reduction in the need for it, and the common belief otherwise means genuine sleep problems in later life are dismissed as normal ageing.

The test is how the days go, not what a tracker reports. Someone who wakes reasonably refreshed and gets through the day without fighting sleep is getting enough, whatever the hours. Someone who nods off when inactive, falls asleep within a minute or two of lying down, or needs an alarm and several attempts to get up is probably not.

Sleeping unusually long is also worth attention. Regularly needing well over nine or ten hours and still feeling unrefreshed is a symptom rather than a trait, and points at something — a sleep disorder, depression, an underactive thyroid, anaemia — rather than at a large sleep requirement.

What losing sleep does

A single bad night costs irritability, poorer concentration and a short temper, and is recovered from easily. Beyond that the effects compound, and they are more serious than the folklore around pulling an all-nighter suggests.

Attention, reaction time, judgement and hand-eye coordination all degrade. The degradation is comparable to that produced by alcohol, and the sinister part is that the person affected is a poor judge of how affected they are — self-assessed alertness recovers while performance does not. Language, memory and the ability to plan suffer. Microsleeps of a few seconds intrude without being noticed. Over several days without sleep, hallucinations and disordered thinking follow.

The road safety consequence is the one that matters most in practice. Sleep-related collisions cluster in the early hours and in the mid-afternoon, which are the two low points of the clock, and they tend to be severe because nobody brakes.

Chronic partial sleep loss — six hours a night for weeks rather than none for one night — is the common version and the one least likely to be recognised, because the person adapts to feeling that way and stops noticing. Its established associations are with raised blood pressure, weight gain and type 2 diabetes, impaired immune response, and depression and anxiety.

One widely held belief does not survive contact with the evidence: that lost sleep can be made up at the weekend. Some recovery happens, and it is partial. A long Sunday lie-in also pushes the clock later, which makes Monday morning harder, so the strategy tends to perpetuate the problem it is meant to solve.

Where to go from here

Difficulty falling or staying asleep, where the days are affected, is covered under insomnia — including why the sensible-seeming response of spending longer in bed makes it worse. Unrefreshing sleep with daytime sleepiness, snoring or witnessed pauses in breathing is covered under sleep apnoea. Sleep that is fine in quality but at the wrong time of day, because of a rota, is covered under shift work and sleep. And leg sensations that make falling asleep difficult are under restless legs and limb movements.