Sleep Better
Lesson 2: How sleep works
Goal: know the stages of sleep, why the night runs in cycles, and the two forces that decide when you feel sleepy.
Two forces: sleep pressure and the body clock
Sleep researchers describe two systems working together:
- Sleep pressure. From the moment you wake, a chemical called adenosine slowly builds up in the brain. The longer you’re awake, the stronger the pull to sleep. Sleep clears it. Caffeine works by blocking adenosine’s effect, which is why coffee hides tiredness rather than removing it.
- The body clock (circadian rhythm). A master clock in the brain runs on a roughly 24-hour cycle and pushes you towards alertness during the day and sleep at night, regardless of how long you’ve been awake. It’s set mainly by light (Lesson 4).
You sleep best when both line up: plenty of sleep pressure, and your body clock saying “night”. Insomnia, jet lag and shift work are largely what happens when they don’t.
The stages of sleep
| Stage | What happens | Typical length in early cycles |
|---|---|---|
| N1 (light) | The doze between waking and sleep. Easy to wake from. | 1 to 7 minutes |
| N2 (light) | Heart rate and breathing slow, body temperature drops. About half of the night is spent here. | 10 to 25 minutes |
| N3 (deep, slow-wave) | Hardest to wake from. Important for physical recovery and feeling refreshed. | 20 to 40 minutes |
| REM (dreaming) | The brain is very active, most vivid dreams happen, and the body’s muscles are temporarily switched off. About a quarter of adult sleep. | 10 to 60 minutes |
The night runs in cycles
You move through these stages in cycles of about 90 minutes (the first is often 70 to 100 minutes, later ones 90 to 120), and most people have four to six cycles a night.
The mix changes as the night goes on:
- Early in the night: more deep sleep (N3).
- Later in the night and towards morning: REM periods get longer, up to an hour each.
That has a practical consequence. The last hour or two of sleep is REM-rich. If you regularly cut your night short with an early alarm, you lose a much bigger share of REM than of deep sleep. Lesson 3 looks at how to wake up without that cost.
Waking in the night is normal
Brief awakenings between cycles are completely normal, and most are so short you won’t remember them. Waking once or twice and drifting back off is not insomnia. It only becomes a problem when you’re awake long enough to start worrying about being awake (Lesson 9).
Sleep inertia
That groggy, heavy feeling just after waking, especially if an alarm pulled you out of deep sleep, is called sleep inertia. It usually clears within 15 to 30 minutes. Light, water and moving around help (Lessons 4 and 5).
Watch
- A walk through the stages of sleep (TED, Sleeping with Science series)
- Mastering the art of sleep: deep, light and REM (Medical Centric Podcast): how much of each stage to expect
Exercise 2
Look at your usual bedtime and wake time from Lesson 0 and work out roughly how many 90-minute cycles fit in. Do you usually wake feeling groggy (probably cut off mid-cycle or short on sleep) or naturally just before the alarm (a good sign)?
Sources
- Sleep Foundation, Stages of sleep
- Russell Foster, Why do we sleep? (TED talk)
This course is general information, not medical advice. If you have ongoing sleep problems, loud snoring with pauses or gasping, or sleepiness that affects driving or work, see your doctor. In Australia, if you are struggling or feel unsafe, call Lifeline on 13 11 14.