What happens during the night

A night in cycles

Falling asleep is not like switching off a light. Through the night, sleep moves through phases that repeat. According to the NHLBI, a US health institute, each cycle lasts 80 to 100 minutes, and there are usually four to six cycles a night.

There are two broad types of sleep: non-REM, which has three stages, and REM, short for rapid eye movement. Each shows up in its own way across the night, and no two nights are the same.

The three stages of non-REM sleep

Stage 1. This is the transition between being awake and sleeping. It is short, and you may not even notice you were dozing.

Stage 2. Here you are asleep. It is lighter than the next stage and usually takes up a good part of the night.

Stage 3. This is deep sleep, also called slow-wave sleep, after a pattern that appears in measurements of brain activity. You usually spend more time in it early in the night. It tends to decrease through adult life, and older people may have very little of this stage.

REM sleep

In REM sleep your eyes move, and your brain activity looks similar to when you are awake. This is when dreaming usually happens. Your muscles normally go limp, which stops you from acting out your dreams.

There is usually more REM sleep late in the night. That is why waking up much earlier than usual tends to cut this part short.

Waking up a little is part of it

You may wake briefly between one cycle and the next. Often you will not remember it later. A night with brief awakenings is not, in itself, a bad night.

What deserves more attention is taking a long time to fall back asleep, often waking up tired, or noticing it is affecting your day. In your sleep log, “Woke up several times” and “Woke up too early” help you mark this kind of night and see whether it repeats.

Why you do not see stages here

Stages are defined by measurements of brain activity and eye movement, recorded with sensors in sleep studies. Watches and bands estimate sleep from other signals, such as movement and heart rate. A 2019 scientific review points out that these devices use proprietary algorithms and that a standard for assessing their performance is still missing.

Here you enter the time you fell asleep, the time you woke up and how the night felt. Without sensors, showing a chart of stages would mean inventing data. These simple records already give you a good basis for looking at your times and for talking to a health professional.

If you use a watch, it is worth treating its numbers as clues, not a diagnosis. A night with little deep sleep on a chart does not, by itself, mean something is wrong. How you feel during the day is still an important sign.

What to do with this

You do not need to control your stages, and trying to usually just adds tension at bedtime. What is within reach is your habits: waking at a similar time every day, setting aside time to wind down, and keeping your bedroom dark and quiet. The piece “Sleep and mood” explains these habits and the treatments with the strongest evidence.

If sleep stays difficult for weeks, or if it is weighing on your days, see a health professional. Insomnia and other sleep problems can be treated.