The short answer has been published for years and still surprises people by how inflexible it is.
The figure, by age
| Age | Hours per night |
|---|---|
| 13 to 18 years | 8 to 10 |
| 18 to 60 years | 7 or more |
| 61 to 64 years | 7 to 9 |
| 65 years and over | 7 to 8 |
Notice that the recommendation for adults is a floor, not a target: seven or more. The belief that someone «only needs five hours» almost never describes a person who functions on five hours; it describes a person who has been tired for so long that they have forgotten what not being tired feels like.
The debt: what comes back and what does not
Sleeping little from Monday to Friday and stretching out at the weekend recovers part of what was lost, not all of it, and it has a price: sleeping three extra hours on Saturday pushes the internal clock back and means sleep does not come on Sunday night. Saturday is paid for with Monday.
What recovers worst is what is noticed worst: sustained attention degrades progressively and the perception of being degraded does not keep pace with it. Someone a week into six-hour nights performs a good deal worse than they think.
The countdown of the last few hours
Three substances with three different timings, and they are worth treating separately:
- Caffeine. It takes on the order of five hours to clear half the dose, which means a four o'clock coffee still has a quarter in circulation at midnight. Sensitivity varies a great deal from person to person; a margin of six to eight hours before bed is the one that works for most people.
- Alcohol. It is the most deceptive, because it helps you fall asleep and ruins what comes after: it fragments the second half of the night, exactly where the restorative sleep is. Falling asleep sooner and resting worse.
- Nicotine. It is a stimulant and, on top of that, overnight withdrawal wakes you.
The room: three figures and no more
Of everything said about the bedroom, what is actually measured comes down to three things: cool, dark and quiet. Temperature below daytime comfort; light as little as possible, phone included; and constant noise disturbs less than intermittent noise.
The rest — pillows, scents, apps — you may like, but it is not on the same level of evidence and it does not make up for an irregular schedule.
The intervention that pays most is the dullest
Above everything above: a fixed time to get up, every day, weekend included. The internal clock anchors to the waking hour and to morning light, not to bedtime. It is free, it requires buying nothing, and it beats any supplement.
The nap, done properly
Ten to twenty minutes, and early. Longer than that enters deep sleep and you wake up worse than before; later than that, it steals sleep pressure from the night.
When to stop reading advice
All of the above is sleep hygiene for healthy people. There are two situations where it does not apply and where carrying on reading articles is wasting time:
- You have been sleeping badly most nights for more than three months, with consequences during the day. That has a clinical name and a treatment, and the treatment that works is not advice from a blog.
- Someone has told you that you stop breathing while you sleep, or you snore and wake up tired with no explanation. That gets investigated, not corrected with the room temperature.
In both cases, the next step is a doctor.
Where each figure comes from
- CDC — Recommended sleep hours by age group.
- NHLBI · NIH — Sleep debt, circadian rhythm and sleep hygiene.
- NIOSH — Shift work and sleep.
United States Government agencies publish in the open and their works are not subject to copyright under 17 U.S.C. § 105. Every figure can be checked one by one; the writing of this article is our own work.
This is a summary. The architecture of a night phase by phase, what to do when sleep does not come and in what order to try it, and shifts and travel when the schedule is not yours to choose, are worked through separately. It is in the Codex of Rest, a 53-page PDF in Spanish, English and French.
Published by VILLUMINATIONS. This text is informational and does not replace the care of a licensed health professional.
