Why regularity beats duration, what one short week does to a healthy metabolism, and the four things that actually decide tonight.
Kratos Natural · Domain: Sleep
1. How many hours of sleep do you need?
A meta-analysis of 16 prospective cohorts, covering more than 1.3 million people, found the relationship between sleep and dying is U-shaped. Short sleepers had around 12% higher all-cause mortality than people sleeping seven to eight hours; long sleepers, over nine hours, had around 30% higher[1].
The long-sleep half is where most people misread the evidence. Nobody has shown that sleeping ten hours harms you. Illness makes people sleep more, and in observational data the illness arrives before the sleep does. The short-sleep half has an experiment behind it. The long-sleep half has an association and a plausible explanation running the other way.
2. What matters more than hours?
In a prospective cohort of around 60,000 people wearing accelerometers, how regular somebody's sleep was predicted mortality better than how long they slept. People with the most irregular sleep timing had substantially higher risk than the most regular, and the pattern held after adjusting for duration[2].
That is the single most useful thing in this guide. Going to bed and getting up at roughly the same time is free, it needs no product, and by this data it is worth more than the extra half hour everyone is chasing.
Still observational. A regular sleeper differs from an irregular one in shift work, drinking, children and illness. But it points the same way as everything known about circadian biology, and it costs nothing to act on.
3. What does short sleep do to you?
Eleven healthy young men were restricted to four hours in bed for six nights. Glucose tolerance and thyrotropin concentrations fell, evening cortisol rose, and sympathetic nervous system activity went up. Their glucose handling looked, in the authors' own comparison, like an older adult's[3].
Six nights. In healthy young men. That is a controlled experiment rather than an association, and it is the reason short sleep is treated as a cause here while long sleep is not.
4. What actually changes your sleep?
Light
The circadian clock is set by light, and the response depends entirely on when it lands. A phase-response curve to single bright-light pulses shows light in the morning pulling the clock earlier and light in the evening pushing it later[4]. Ordinary indoor light in the hours before bed is enough to matter: room light before bedtime suppressed melatonin and shortened its duration by about 90 minutes[5].
Do: get outside within an hour of waking. Dim what you can in the last two hours. It is the amount of light in the evening that matters most, not only its colour.
Caffeine
400 mg of caffeine taken six hours before bed still reduced total sleep time by more than an hour, measured objectively, and the people taking it did not notice[6]. That last part matters. "Coffee doesn't affect me" is a statement about perception, not sleep.
Do: last coffee eight hours before bed, or earlier.
Alcohol
It shortens the time to fall asleep and wrecks the second half of the night. Nobody needs a citation to have noticed; the evidence agrees.
Temperature
Your core temperature has to fall for sleep to start. A cool room, and heat loss through hands and feet, are what let it. A warm shower an hour or two before bed works by the same mechanism: blood to the skin, heat out.
5. What do you do if you cannot sleep?
The treatment with the strongest evidence is not a supplement. A meta-analysis in Annals of Internal Medicine found cognitive behavioural therapy for insomnia improved sleep onset latency by about 19 minutes, wake after sleep onset by 26 minutes, and sleep efficiency by around 10 percentage points, with effects sustained at follow-up[7].
Those numbers look small on paper and are large in a life, and unlike a sleeping pill they hold after the treatment stops. CBT-I is available in the Netherlands through a GP referral and as a self-guided programme.
The two rules it rests on, which you can start tonight:
- Get up at the same time every day, including weekends, no matter how the night went. The wake time is the anchor; the bedtime follows it.
- If you are awake more than about 20 minutes, get out of bed. Do something dull in dim light and return when sleepy. Lying awake teaches your body that bed is where you lie awake.
6. What we honestly do not know
Whether making an irregular sleeper regular lowers their risk. The regularity finding is observational[2]. Nobody has randomised people to regularity and followed them for a decade.
Whether long sleep is a cause of anything. See chapter 1.
What your ideal duration is. The cohorts point at seven to eight hours as a population average[1]. They cannot tell you your number, and the only honest test is how you function on a week of a given amount.
Whether tracker sleep stages mean anything. Consumer devices estimate stages from movement and heart rate, and they disagree with polysomnography enough that treating a "deep sleep" figure as a score is a mistake. The hours and the timing are the parts worth reading.
7. When should you see a doctor?
Straight to a doctor if:
- You snore loudly and someone has seen you stop breathing
- You fall asleep during the day without intending to, especially driving
- You act out dreams, or move violently in your sleep
- You have an irresistible urge to move your legs in the evening
- Sleep has been bad for three months or more, most nights
Insomnia lasting months is a condition with a treatment, not a personal failing. This guide is written for a healthy sleeper having a bad run.
8. Sources
- Cappuccio FP, D'Elia L, Strazzullo P, Miller MA. Sleep duration and all-cause mortality: a systematic review and meta-analysis of prospective studies. Sleep, 2010. PMID 20469800
- Windred DP, Burns AC, Lane JM, et al. Sleep regularity is a stronger predictor of mortality risk than sleep duration: a prospective cohort study. Sleep, 2024. PMID 37738616
- Spiegel K, Leproult R, Van Cauter E. Impact of sleep debt on metabolic and endocrine function. Lancet, 1999. PMID 10543671
- Khalsa SBS, Jewett ME, Cajochen C, Czeisler CA. A phase response curve to single bright light pulses in human subjects. Journal of Physiology, 2003. PMID 12717008
- Gooley JJ, Chamberlain K, Smith KA, et al. Exposure to room light before bedtime suppresses melatonin onset and shortens melatonin duration in humans. JCEM, 2011. PMID 21193540
- Drake C, Roehrs T, Shambroom J, Roth T. Caffeine effects on sleep taken 0, 3, or 6 hours before going to bed. Journal of Clinical Sleep Medicine, 2013. PMID 24235903
- Trauer JM, Qian MY, Doyle JS, et al. Cognitive behavioral therapy for chronic insomnia: a systematic review and meta-analysis. Annals of Internal Medicine, 2015. PMID 26054060
- Windred DP, Burns AC, Rutter MK, et al. Brighter nights and darker days predict higher mortality risk: a prospective analysis of personal light exposure in >88,000 individuals. PNAS, 2024. PMID 39405349
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