Sleep stages and REM sleep: what happens through the night

Sleep stages: a night with a structure

Sleep comes in two broad types: NREM sleep, without rapid eye movements, and REM sleep, short for rapid eye movement. NREM sleep is split into three stages, from lightest to deepest, called N1, N2 and N3.

We know about these stages thanks to polysomnography, the overnight test that records the brain's electrical activity, eye movements and muscle tone. Older textbooks describe four NREM stages; since 2007, the American Academy of Sleep Medicine (AASM) scoring manual has merged the two deepest into one, N3.

You don't run through the stages just once a night. They come in repeating cycles: each one goes from light to deep sleep, climbs back up, and ends with a stretch of REM.

N1: drifting off

Stage N1 is the handoff between waking and sleep, and it usually lasts only a few minutes. Your breathing and heart rate start to slow, your muscles relax, and a small noise can easily wake you. Many people don't even realize they were asleep.

N1 is when hypnagogic imagery shows up: flashes, voices or brief scenes that appear as you drift off, halfway between thinking and dreaming. It's also when many people get a hypnic jerk, that sudden twitch, sometimes with a feeling of falling, that snaps you awake for a moment. It's normal and not a sign of anything wrong.

N2: the light sleep that fills much of the night

Stage N2 is light but settled sleep, and it's where we spend the most time overall. Body temperature drops, eye movements stop, and brain activity slows down, with short characteristic bursts on the recording known as sleep spindles.

Light sleep isn't wasted time. Researchers study how N2 relates to memory and to learning new skills. Waking from it also tends to feel easier than waking from deep sleep.

N3: deep sleep

Stage N3 is deep sleep, also called slow-wave sleep after the kind of brain activity it shows. It's the hardest stage to break: if someone wakes you in the middle of N3, you'll probably feel foggy and clumsy for a few minutes. According to the National Institutes of Health (NIH), it's the sleep you need to wake up feeling refreshed.

Deep sleep is concentrated in the first half of the night and shrinks in later cycles. That's why sleepwalking and night terrors, which arise from this stage, tend to happen in the first few hours after bedtime, and why the people who have them rarely remember them.

REM sleep: an active brain in a still body

REM sleep is the stage of rapid eye movements, and the one most tied to vivid dreams. Your eyes dart under your lids, your breathing and pulse turn irregular, and your brain activity looks a lot like waking. At the same time, most of the muscles in your arms and legs go temporarily limp, which keeps you from acting out your dreams.

REM sleep was first described in 1953, when Eugene Aserinsky and Nathaniel Kleitman at the University of Chicago noticed those eye movements in sleeping volunteers. When they woke people during these episodes, most described a detailed dream. Soon after, William Dement and Kleitman showed that these periods come back several times a night and that waking people during them yielded dream reports far more often than waking them at other times.

Sleep cycles of about 90 minutes

A full sleep cycle, from N1 to the end of a REM period, lasts about 90 minutes on average, though it varies from person to person and across the night. A night of seven or eight hours usually holds four to six cycles.

The cycles aren't identical, though. According to the NIH, the first REM period begins about 90 minutes after you fall asleep and is short. Later ones grow longer while deep sleep shrinks. Early in the night N3 dominates; toward morning it's mostly N2 and REM, with REM periods that can stretch from a few minutes to half an hour or more.

• The first few hours: more deep sleep, short REM periods.
• The middle of the night: a mix of N2, N3 and REM.
• The last few hours: little or no N3, long REM periods, and brief awakenings you often don't remember.

Why dreams are most vivid in REM

The longest, strangest and most emotional dreams tend to come from REM sleep. During REM, brain regions linked to imagery, emotion and memory are highly active, while areas that help you reason and keep track of time work differently. The result is intense scenes you accept without question.

But REM isn't the only time we dream. When people are woken from NREM sleep, some of them also recall something, though it's usually shorter and more like a passing thought. A 2017 study in Nature Neuroscience led by Francesca Siclari found that in both REM and NREM, dreaming tracked activity in an area toward the back of the brain. In other words, REM favors vivid dreams, but it isn't the only way in.

Why we remember the dreams from near morning

The dreams you remember on waking are usually from the last few hours of the night, and there are good reasons for that. Toward morning, REM periods run longer, so you're more likely to wake up during a dream or right after one. In the early lab studies, waking someone in the middle of REM produced a dream report in most cases.

Dream memories are fragile. If you jump out of bed, check your phone or start planning your day, the dream often fades within minutes. So if you keep a dream journal, try to lie still for a moment with your eyes closed and write down what you remember before doing anything else. The dictionary can then help you explore the symbols you jotted down.

When the stages overlap

The borders between stages aren't always clean, and that explains some experiences that feel frightening but are usually harmless. In sleep paralysis, you wake up while REM's stillness lingers for a few seconds or minutes. In a false awakening, you dream that you've already gotten up. In a lucid dream, you realize you're dreaming without leaving REM.

Some situations are worth bringing to a doctor: if you thrash, shout or strike out during dreams, if sleepwalking puts you at risk, if sleep paralysis happens often and distresses you, or if you're very sleepy during the day even after enough sleep. A sleep evaluation can help sort out what's going on.