Deep sleep

What it's like

The part of the night you never notice — until someone wakes you from it.

You don't experience deep sleep so much as disappear into it. It dominates the early part of the night, while REM and its vivid dreams take over toward morning.

What you might notice

• Hard to wake: Woken from slow-wave sleep, people are groggier than from REM or light sleep.
• Zombie mornings: That grogginess rarely lasts more than about 30 minutes without serious sleep loss.
• Dreams, quieter: Dreams can happen in non-REM sleep too, but the vivid ones cluster in REM.

What's happening in the brain

Waves, replay and a rinse. The model lights each one as you read.

Slow waves

Each slow wave travels across the cortex, usually starting at the front and moving back, and they come almost once a second as sleep deepens.

Powered down — selectively

The brainstem, thalamus and basal forebrain shut down profoundly, and so do the frontal and parietal association areas — while primary sensory areas keep going. It isn't a simple switch-off.

Filing the day

Slow oscillations, spindles and ripples coordinate the replay of the hippocampus's memories and their move into the cortex for longer keeping.

Sleep also seems to clean. In sleeping mice, the space between brain cells grew by 60%, fluid flushed through faster, and beta-amyloid — the protein that builds up in Alzheimer's — was cleared more quickly.

How much you need

By age, per 24 hours, from the American Academy of Sleep Medicine.

Recommended sleep

• 4–12 months: 12–16 hours, including naps.
• 1–2 years: 11–14 hours, including naps.
• 3–5 years: 10–13 hours, including naps.
• 6–12 years: 9–12 hours.
• 13–18 years: 8–10 hours.
• Adults: 7 or more hours, regularly.

Sleeping better

What actually works when sleep won't come.

• CBT for insomnia (A short talking therapy that retrains habits and thoughts around sleep.): The recommended first treatment for every adult with chronic insomnia.
• Sleeping pills (Short-term medication.): Something to discuss, short-term, only if CBT for insomnia alone hasn't worked.

Small things worth trying

• Keep a regular bedtime and wake-up time.
• Give yourself enough time in bed for the hours your age needs.
• If you wake groggy, give it half an hour before judging the day.

Check what you know

Three questions about deep sleep.

• You cram until 3am, sleep four hours, and can't remember much of what you learned. Which part of sleep did you lose? Deep sleep and its replay — the brain's time for moving memories from the hippocampus to the cortex. Slow-wave sleep coordinates the replay of hippocampal memories into the cortex.
• Your alarm goes off an hour after you fell asleep for a nap, and you wake feeling like a zombie. Why so groggy? You were woken from deep sleep, which leaves you groggiest. Grogginess is worst when woken from slow-wave sleep, and usually lifts within 30 minutes.
• A friend has had insomnia for months and asks what to try first. What does the evidence say? CBT for insomnia. It's the recommended first treatment for chronic insomnia in adults.