How Sleep Works

Sleep series · Part 2
You're reading How Sleep Works. Before: Why Sleep Decides Everything. Next: In Sync with the Sun — how your day builds your sleep, Good sleep starts two hours before bed and Food and Sleep.

Most people picture sleep like a light switch: you lie down, the light goes off, and a few hours later it comes back on. In fact sleep is the opposite of standstill — it's a highly ordered sequence that plays out in a particular order every night. Once you understand that sequence, you stop leaving sleep to chance.

1. Sleep runs in waves

The moment you fall asleep, your brain doesn't enter one uniform state but a succession of phases. Roughly, they fall into two families: non-REM sleep (from light drifting-off to deep sleep) and REM sleep, where most vivid dreams arise. One complete run — from light sleep into deep sleep and back up into REM — takes about 90 minutes. In a normal night this cycle repeats four to six times.

This order is no accident but a division of labour. Deep sleep sits mostly in the first half of the night — and that's the phase of physical repair. Here your body releases the bulk of its growth hormone.

REM sleep increases toward morning — and it's mainly responsible for head and heart: here your brain processes experience, links what you've learned and sorts emotions. Both phases consolidate memory, but in different ways.

From this follows something practical: a night cut too short doesn't trim "a little of everything". If you have to get up early, you mainly cut off the REM-rich morning sleep. If you fall asleep late and restless, you lose the early deep sleep. The architecture explains why seven unbroken hours are worth more than eight fragmented ones.

2. Two forces decide when you get tired

Why do you get tired in the evening at all — and awake again in the morning? Behind it stand two systems working together. Sleep research calls this the two-process model.

The first force is sleep pressure. Picture it as a bucket that fills over the day: the longer you're awake, the fuller it gets, the stronger the need to sleep. The substance filling that bucket is adenosine — a by-product of your brain cells burning energy all day. The more of it accumulates, the sleepier you become. During sleep, the bucket empties again.

This, by the way, is exactly where caffeine acts: it blocks the docking sites for adenosine — the bucket is full, but your brain no longer gets the message. That's why coffee works, and why it keeps you awake so long.

The second force is your body clock. It makes sure sleep pressure doesn't simply floor you in the afternoon, but that tiredness and alertness follow the day-night cycle. This clock is set by light — and that's exactly the focus of the next part of the series, In Sync with the Sun. For here it's enough: good sleep happens when both forces line up — high sleep pressure and a body clock that says "now it's night".

3. The hormones that set the beat

The body clock talks to the body through hormones. Two of them are worth knowing, because they carry the back-and-forth of being awake and asleep.

Melatonin is the signal "night is coming". It's released in darkness and tunes the body for sleep — it's not a sleeping pill but a timekeeper.

Cortisol is the counterpart — the wake signal. It rises in the early morning hours, makes you awake and capable, and ebbs across the day. Evening stress keeps cortisol high and so works against sleep.

The precise daily course of both hormones — and why the morning light sets it — you'll see in In Sync with the Sun. What matters here is the principle: melatonin and cortisol are two sides of the same beat.

4. The nightly clean-up

There's a task your brain does almost only during sleep — and that was long overlooked: tidying up. During the day, thinking produces metabolic waste products. At night, they get flushed away.

An important note on interpretation: this is a finding from an animal model. That exactly the same happens in humans at the same strength is not yet finally proven — but the direction is convincing and fits everything else we know about sleep's protective function. It's one of the reasons good sleep pays into your brain over the years.

And that's exactly the bridge to the next part: if two forces — sleep pressure and body clock — decide when you get tired, then you can influence both across the day. How your behaviour in daylight builds your night's sleep, you'll read in In Sync with the Sun. And how to shape the evening in concrete terms, in Good sleep starts two hours before bed.