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WHAT HAPPENS AT NIGHT

Sleep comes
in cycles.

A night is not one long sleep. It is four to six rounds.

Every 80 to 100 minutes, you move from light sleep into deep sleep and on to REM, the stage where most dreams happen, then start again. Deep sleep comes early in the night and REM grows later. Here is how a night unfolds, in plain English.

An independent guide, with the sources linked.

Questions this page answers

An empty bed with rumpled white linen in a dark bedroom, moonlight through sheer curtains and a dim lamp glowing on the bedside table
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4–6 cycles a night

  • Light sleep
  • Deep sleep
  • REM sleep
  • Then round again

THE SHORT ANSWER

Two kinds of sleep, four stages.

Non-REM sleep has three stages.
REM sleep is the fourth, and the dreamiest.

STAGES 1 AND 2

Light sleep

The way in. Stage 2 alone takes up about 45% of the night, more than any other stage.

Explore the stages

STAGE 3

Deep sleep

Slow-wave sleep, when breathing and heartbeat are at their lowest. It comes mostly in the first half of the night.

Early versus late

REM

Dreaming sleep

The brain is almost as active as when awake, and most dreams happen here. REM periods lengthen towards morning.

Sleep facts

STAGE EXPLORER

Where does
the night go?

Pick a stage to see roughly how much of an adult’s sleep it takes up and what happens in the body. Shares come from Cleveland Clinic; descriptions from the US National Institute of Neurological Disorders and Stroke.

Stage explorer

Choose a stage of sleep
Stage 1About 5% of sleep

The changeover from waking to sleep. Heartbeat, breathing and eye movements slow and muscles relax, sometimes with small twitches. It usually lasts only a few minutes.

Stage 2About 45% of sleep

Light sleep before deeper sleep. Body temperature drops and eye movements stop. Brain waves slow, with brief bursts of electrical activity. You spend more of the night here than in any other stage.

Stage 3About 25% of sleep

Deep or slow-wave sleep. Heartbeat and breathing reach their lowest levels and it can be hard to wake you. It comes in longer periods in the first half of the night.

REMAbout 25% of sleep

Rapid eye movement sleep. Brain activity looks close to waking, breathing turns faster and irregular, and most dreaming happens here. Arm and leg muscles go temporarily limp. REM periods get longer later in the night.

Bar shows the stage’s share of total sleep, relative to stage 2.

These are typical figures for adults. They shift with age and vary from person to person and night to night.

EARLY NIGHT, LATE NIGHT

Deep first,
dreams later.

The cycles repeat, but they do not
look the same all night.

Early in the night

Where stage 3 sleep is longest

NINDS says deep sleep comes in longer periods during the first half of the night, and NHLBI says you usually spend more time in it early on. Cleveland Clinic notes the body tries to fit in as much of it as early as possible.

FIRST HALF · DEEP SLEEP

~10 to 60 min

Length of a REM period

The first REM period of the night is usually the shortest, at around 10 minutes, according to Cleveland Clinic. Each one after it is longer than the last, up to an hour, so most REM sleep falls towards morning.

SECOND HALF · REM SLEEP

NHLBI sums it up: you usually spend more time in deep sleep early in the night, and have more REM sleep later.

BEFORE TONIGHT

Four things the
sources point out.

  • Caffeine lingers

    The American Academy of Sleep Medicine gives caffeine a half-life of 3 to 5 hours, so half of an afternoon coffee can still be in your body hours later.

  • Light is a wake-up signal

    NINDS calls light perhaps the greatest influence on the sleep-wake cycle. NHLBI says artificial light can give the body a false cue that it is time to be awake.

  • Waking briefly at night is part of the pattern

    NHLBI notes that you may wake up briefly between sleep cycles, as one cycle ends and the next begins.

  • Your clock may not match the wall clock

    NHLBI says most people’s natural body clock runs slightly longer than 24 hours, and that it is natural for many teenagers to prefer later bedtimes.

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SLEEP FACTS

Five things worth
sleeping on.

Soft pale dawn light glowing through linen curtains onto the corner of an unmade bed
AI-generated image, not a photograph of a real person or place.

  1. A cycle lasts about an hour and a half

    NHLBI says the cycle of non-REM and REM sleep starts over every 80 to 100 minutes, usually four to six times a night, and you may wake briefly between cycles.

  2. REM arrives after about 90 minutes

    According to NINDS, the first REM sleep of the night usually begins about 90 minutes after falling asleep.

  3. Your muscles go limp while you dream

    During REM sleep, arm and leg muscles become temporarily paralysed, which NINDS says prevents you from acting out your dreams.

  4. REM sleep runs a little cold

    NHLBI says the body does not regulate its temperature properly during REM sleep, which is why you get less of it in colder temperatures.

  5. Deep sleep fades with age

    A 2004 meta-analysis of 65 studies, covering 3,577 people aged 5 to 102, found that the share of slow-wave and REM sleep fell with age in adults. NHLBI adds that older people may have no slow-wave sleep at all.

QUICK QUESTIONS

Sleep, answered.

Short answers to the questions people ask most.

How long is a sleep cycle?

The US National Heart, Lung, and Blood Institute (NHLBI) says the cycle of non-REM and REM sleep starts over every 80 to 100 minutes, and that there are usually four to six cycles a night. Cleveland Clinic gives about 90 to 120 minutes per cycle.

When do we dream?

Most dreaming happens during REM sleep, although some can also happen in non-REM sleep, according to the US National Institute of Neurological Disorders and Stroke (NINDS). The first REM period usually comes about 90 minutes after falling asleep.

What is deep sleep?

Deep sleep is stage 3 of non-REM sleep, also called slow-wave sleep. Heartbeat and breathing slow to their lowest levels and it can be hard to wake someone. NINDS describes it as the sleep you need to feel refreshed in the morning, and it comes in longer periods in the first half of the night.

Why do I feel groggy if I wake from deep sleep?

Cleveland Clinic says that someone woken from stage 3 sleep will probably have sleep inertia, a state of confusion or mental fog, which lasts about 30 minutes.

How does caffeine affect sleep?

Adenosine is a substance in the body that promotes sleepiness, and caffeine blocks it. The American Academy of Sleep Medicine says caffeine peaks in the blood within 30 to 60 minutes and has a half-life of 3 to 5 hours.

Sources

Where the facts come from

  1. Brain Basics: Understanding SleepNINDS, National Institutes of Health
  2. How Sleep Works: Sleep Phases and StagesNHLBI, National Institutes of Health
  3. How Sleep Works: Your Sleep/Wake CycleNHLBI, National Institutes of Health
  4. Sleep Basics: stages and cyclesCleveland Clinic
  5. Caffeine and sleepAmerican Academy of Sleep Medicine
  6. Ohayon et al.: normative sleep values across the lifespan (meta-analysis)Sleep, 2004

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