What happens when you fall asleep

What happens when you fall asleep?

A series of small changes, in the body and in the brain, spread over several minutes and overlapping rather than following one another in a neat line. A 2001 review of the research concluded that sleep does not begin “in a moment”: entry into sleep is a continuous, interwoven series of changes that starts in relaxed drowsiness, runs through the lightest stage of sleep and often into the first minutes of the next (Ogilvie, 2001). A 2024 review describes the same stretch as a period with dynamic, complex changes of its own, long overlooked as a mere prelude to sleep (Lacaux and colleagues, 2024).

Even the moment of falling asleep is partly a convention. Sleep laboratories score a night in 30-second slices, and the scoring rules drawn up for the American Academy of Sleep Medicine in 2007 take the fading of the alpha rhythm — the brain’s rhythm of relaxed wakefulness — as the most valid marker of sleep onset. Studies of the older rules had found agreement between trained scorers low for the lightest stage of sleep (Silber and colleagues, 2007).

What are the stages of falling asleep?

Roughly three, and they blur into each other: drowsy wakefulness, stage 1 of sleep (N1) and the beginning of stage 2 (N2). Each has a signature in the brain, a set of changes in the body and a feel from the inside.

After stage 2 the night carries on down, into deep sleep and later REM sleep, which the last sections of this page outline.

What happens to your body when you fall asleep?

Four things change, each in the direction of rest, and none of them all at once.

Your heart slows

Heart rate comes down in steps. In a 1999 laboratory study that followed six people through repeated sleep onsets, heart rate fell at each transition into sleep and rose after each brief arousal back towards waking, settling again within about 12 beats; by stable stage 2 sleep, heart and breathing rates were both lower than when awake (Burgess, Kleiman and Trinder, 1999).

Behind the steps is a change in the nervous system. In a 2001 study of 14 healthy young adults over three nights, the balance in non-REM sleep moved away from the sympathetic branch, which speeds the body up, towards the parasympathetic branch, which slows it — mostly because parasympathetic activity rose — and much of that shift happened abruptly at sleep onset. Heart rate also drifted down across the night with the body clock, not only with the stage of sleep (Trinder and colleagues, 2001). Heart rate when falling asleep has the detail, including what it means when the heart races instead.

Your breathing changes

You breathe more slowly and take in less air, and on the way down the breath is uneven. In a 1992 study of 21 young adults, each followed through about 11 sleep onsets, the amount of air breathed fell every time the brain slipped from alpha into theta and rose every time it came back (Trinder and colleagues, 1992) — so the flicker between drowsy and asleep shows up in the breath as well as in the brain waves.

Heat leaves through your hands and feet

Body temperature falls as you fall asleep, and keeps falling in stage 2 (NINDS). The heat goes out through the skin, much of it through the hands and feet, whose blood vessels open to let it out: in a 1999 study, how far they opened was the best physiological predictor of how quickly people fell asleep (Kräuchi and colleagues, 1999). You have to cool down first explains the temperature curve and why a warm bath helps; does a cooling mattress help? looks at what the bed can add.

Your muscles let go

Muscles relax in stage 1, sometimes with small twitches, and relax further in stage 2. Later in the night, in REM sleep, the muscles of the arms and legs are briefly paralysed, which stops you acting out dreams (NINDS).

The sudden jolt some people feel on the edge of sleep is a hypnic jerk, also called a sleep start. A 2015 review of the research describes it as a phenomenon of the transition between waking and sleep, generally considered benign (Cuellar, Whisenant and Stanton, 2015). The popular explanation — a reflex left over from ancestors who slept in trees — is a guess: we could not find a study that tests it (searched October 2026).

What happens in the brain when you fall asleep?

Its electrical rhythm slows and changes character, in an order the 2001 review traces in three steps: first the fading of alpha; then a phase of theta waves, with sharp waves over the top of the head; and finally sleep spindles, which go with the end of wakefulness (Ogilvie, 2001).

Alpha is the steady rhythm of a relaxed, waking brain, strongest over the back of the head, and its fading is the marker sleep laboratories use for sleep onset. Not everyone makes much alpha, which is why the scoring rules have alternatives for those who do not (Silber and colleagues, 2007).

Stage 2 has two signatures. Sleep spindles are brief bursts of electrical activity (NINDS). K-complexes are large, slow waves that can be set off by a sound, and a 2005 review of 70 years of research on them describes them as part of the sleeping brain’s defence against being woken — a way of registering a noise without surfacing for it (Halász, 2005).

The world fades with them. In a 1982 study, 12 young adults slept in a laboratory for five nights and were woken five times a night at different points: the loudness of sound needed to rouse them rose steeply within a minute of the first spindle (Bonnet and Moore, 1982).

What happens to your thoughts?

They loosen. Sentences give way to images and fragments, one thing stops following from another, and plans for tomorrow thin out. That stretch has a name, hypnagogia, and its own guide; why the sentences get shorter explains why Sheeps’ stories are written to resemble it.

When are you actually asleep?

Later than your brain is, by your own account. In the same 1982 study, the point at which people woken from sleep said they had been asleep half the time came 2 to 4 minutes after the first spindle — although their hearing had already dulled within a minute of it. The authors concluded that, from the inside, falling asleep is a relatively lengthy period in which the sense of being awake or asleep is blurred and uncertain (Bonnet and Moore, 1982).

So if you were sure you were still awake when your partner says you were snoring, you may both be right. How long should it take to fall asleep? looks at what this blur does to your sense of the wait.

What happens after you fall asleep?

You keep going down, and then you cycle. After stage 2 comes stage 3, deep sleep, when heartbeat and breathing reach their lowest levels of the night. About 90 minutes after falling asleep comes the first REM sleep: brain activity closer to wakefulness, faster and irregular breathing, heart rate and blood pressure near waking levels, and the arms and legs briefly paralysed (NINDS).

The sequence repeats roughly 4 to 6 times a night, averaging about 90 minutes a cycle (Patel and colleagues, 2024), with longer stretches of deep sleep in the first half of the night and longer REM periods later (NINDS). The figures are averages, not a timetable: cycles differ from one person, one night and one cycle to the next.

Can you help the descent along?

Not by trying: sleep is one of the few things that effort makes harder (trying to sleep is what keeps you awake). What you can do is set up the conditions and then get out of the way.

• Be sleepy at the right time. Sleep comes most easily when the pressure built up by a long day meets a body clock that has turned towards night (two processes decide when you are sleepy).
• Let the heat out. A cool bedroom and warm hands and feet help the temperature fall (you have to cool down first); whether a cooling mattress adds anything is another question.
• Give a busy mind something harmless to do. How to fall asleep sets out what has evidence behind it, from writing tomorrow down to cognitive shuffling.

When is it worth seeing a doctor?

Most of what happens on the way into sleep — a twitch, a jolt, odd images, an uneven breath — is part of the ordinary process. Some things are worth taking to a doctor:

• If falling asleep has been hard most nights for months and it is affecting your days. For chronic insomnia, cognitive behavioural therapy for insomnia (CBT-I) is the recommended first treatment, by the American College of Physicians (Qaseem and colleagues, 2016) and the American Academy of Sleep Medicine (Edinger and colleagues, 2021).
• If your heart races or pounds and it does not settle, or comes with chest pain, shortness of breath or feeling faint. The NHS advises calling 999 or going to A&E; palpitations that keep coming back or last more than a few minutes are a reason to see a GP (NHS).
• If the jolts come so often that they keep you from sleeping. The 2015 review notes that hypnic jerks may go with other sleep disorders (Cuellar, Whisenant and Stanton, 2015).
• If you fall asleep suddenly during the day, or often have frightening experiences as you drop off, such as being unable to move.