2010Sleep
Cappuccio et al.
16 studies, 1.38 million people
Short sleepers (under 6 hours) had 12% higher risk of death; long sleepers (over 9 hours) had 30% higher, often because long sleep signals illness.
Find on PubMed ↗Research library · Recovery & repair
Sleep: when your body repairs
Sleep is when your brain clears waste, your memories get filed, your muscles rebuild and your hormones reset. How regularly you sleep turns out to matter as much as how long.
Good evidence+12%risk of death sleeping under 6 hours a nightCappuccio 2010, Sleep, 1.3M+ people
−20–48%death risk for the most regular sleepers vs the leastWindred 2024, Sleep, 60,977 people
4.2×more likely to catch a cold when sleeping under 6 hoursPrather 2015, Sleep, 164 adults
01
The basics
What happens while you sleep
A night's sleep runs in cycles of about 90 minutes, usually 4–6 a night. Each cycle moves from light sleep into deep sleep and then REM (dream) sleep. Deep sleep is concentrated early in the night; REM sleep gets longer towards morning.
In deep sleep, growth hormone peaks, tissues repair and the brain's 'glymphatic' system flushes out waste proteins, including the amyloid linked to Alzheimer's. In REM sleep, the brain processes emotions and strengthens learning.
Your body clock (circadian rhythm) times all of this. It's set mainly by light hitting your eyes in the morning. Going to bed and waking at different times every day confuses it, a bit like constant jet lag.
A typical night's sleep
Deep sleep comes mostly in the first half of the night. REM (dream) sleep gets longer towards morning.
02
How it works
What short sleep does to your body
1
Too little or irregular sleep
Fewer deep and REM cycles, and a body clock out of sync.
2
Blood sugar
Insulin works worse the next day; hunger hormones rise and cravings follow.
3
Immunity
Fewer immune cells respond, so infections take hold more easily.
4
Brain
Less waste clearance, worse memory, focus and mood.
03
Measure it
How to check your sleep
- Duration: time actually asleep, not time in bed. Wearables give a fair estimate; a simple diary works too.
- Regularity: how much your bedtime and wake time vary across the week, including weekends. Aim for less than 30–60 minutes of difference.
- How you feel: needing an alarm every day, dozing off in the afternoon or relying on caffeine are signs you're short.
- Warning signs: loud snoring, gasping or pauses in breathing can mean sleep apnea. Ask your doctor about a sleep test.
Try your number
Very shortShortA bit shortHealthyLong
410
Adults need 7–9 hours (7–8 over 65). Regularly sleeping 9+ hours can be a sign of illness worth checking.
04
What the research says
The key studies
2024Sleep
Windred et al.
60,977 UK Biobank adults
The most regular sleepers had 20–48% lower risk of death than the least regular. Regularity predicted death better than sleep length.
Find on PubMed ↗2013Science
Xie et al.
Mice
During sleep, the space between brain cells grew by about 60%, letting fluid wash out waste proteins much faster than when awake.
Find on PubMed ↗1999Lancet
Spiegel et al.
11 young men
After six nights of 4 hours' sleep, healthy young men handled sugar like people on the way to diabetes. It reversed with recovery sleep.
Find on PubMed ↗2015Sleep
Prather et al.
164 adults given a cold virus
People who slept under 6 hours were 4.2 times more likely to catch the cold than those sleeping over 7.
Find on PubMed ↗2003Sleep
Van Dongen et al.
48 adults
Two weeks of 6 hours a night impaired attention as much as one to two nights of no sleep at all, yet people felt only slightly sleepy.
Find on PubMed ↗05
Take action
How to sleep better
Start with the body clock: fixed wake time and morning light are the strongest levers.
- 1
Same wake time every day
Including weekends. Your bedtime will follow.
- 2
Morning light
10–30 minutes outside within an hour of waking, even on cloudy days.
- 3
Caffeine cut-off
None after early afternoon. Caffeine 6 hours before bed cut sleep by over an hour in one study (Drake 2013).
- 4
Cool, dark, quiet
Around 18 °C, blackout curtains or an eye mask, earplugs if needed.
- 5
Wind down
Dim lights and put screens away 30–60 minutes before bed. Keep work out of the bedroom.
- 6
Alcohol early or not at all
It helps you fall asleep but breaks up sleep and cuts REM later in the night.
- 7
Insomnia? Try CBT-I
Cognitive behavioural therapy for insomnia is the first-choice treatment, ahead of sleeping pills.
06
Myth or fact
Common myths, checked
✕ MythI can catch up at weekends.
✓ FactLie-ins help a little, but shifting your sleep times creates 'social jet lag' that has its own costs.
✕ MythA nightcap helps me sleep.
✓ FactAlcohol knocks you out but fragments sleep and suppresses REM.
✕ MythOlder people need less sleep.
✓ FactNeeds drop only slightly. Older adults still need about 7–8 hours, though sleep gets lighter.
✕ MythI'm fine on 5 hours.
✓ FactIn studies, people who were short of sleep felt fine while their performance kept getting worse.
07
Questions
Frequently asked
Are naps good or bad?
A 10–20 minute nap before 3 pm can help. Long or late naps make night sleep harder.
Should I use melatonin?
It helps with jet lag and shift work. For everyday insomnia, habits and CBT-I work better.
What about sleep trackers?
Good for duration and regularity; less accurate for sleep stages. Don't let the score stress you out.
08
Sources
References and related topics
- Cappuccio et al. (2010). Sleep. 16 studies, 1.38 million people. PubMed ↗
- Windred et al. (2024). Sleep. 60,977 UK Biobank adults. PubMed ↗
- Xie et al. (2013). Science. Mice. PubMed ↗
- Spiegel et al. (1999). Lancet. 11 young men. PubMed ↗
- Prather et al. (2015). Sleep. 164 adults given a cold virus. PubMed ↗
- Van Dongen et al. (2003). Sleep. 48 adults. PubMed ↗