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Screens off, teeth, reading. Set it to zero to leave it out.
22:30
the night before, to be asleep by 22:45
Changes the range quoted above. It does not change any of the arithmetic.
Arithmetic, not medical advice. Insomnia, shift work, apnoea and daytime sleepiness that does not shift are all reasons to talk to a doctor rather than to a calculator, and a bedtime worked out here will not fix any of them.
There is no sleep-cycle arithmetic here on purpose. The familiar ninety-minute cycle is a rough average of something that varies between people, between nights and across a single night, and no tool can know when a sleeper enters one — so multiplying it up produces a precise-looking bedtime with nothing behind it. Subtracting the sleep you want, plus the time you take to fall asleep, from the moment you must be up is the whole of the honest calculation.
The 7–9 hour range quoted above is the recommendation for that age group in Hirshkowitz M et al., Sleep Health 2015;1(1):40–43 (National Sleep Foundation), published as a range for a population rather than a target for a person. A separate consensus statement, Watson NF et al., SLEEP 2015;38(6):843–844 (AASM and SRS), puts 7 hours a night as a regular floor for adults. Both describe habitual sleep, not one night.
You know when you have to be up. Work backwards from there: subtract the sleep you are aiming for, then subtract the stretch of lying in the dark before you are actually asleep, and you have the moment you need to be in bed. Add a wind-down period if getting into bed is not the same event as putting the day down, and you have the moment the evening has to end.
That is the whole calculation, and it is worth doing precisely because nobody does it in their head at half past eleven. The arithmetic is trivial; the honesty is not. Most people set a bedtime that assumes they fall asleep instantly and then treat the resulting shortfall as a personal failing rather than a subtraction they never did.
The familiar version of this tool multiplies ninety minutes by four, five or six and offers a list of bedtimes at which you will supposedly wake refreshed. It is a tidy idea and it does not survive contact with the evidence. Cycle length varies between people, varies from night to night in the same person, and lengthens across a single night as the balance of deep and REM sleep shifts. Ninety minutes is a rough average of a moving quantity, not a constant.
The second problem is worse. Even if the number held, a bedtime calculator does not know when you fall asleep, so it cannot know where any cycle begins. Multiplying an average by an unknown produces a time expressed to the minute with nothing behind it, and the precision is what makes it persuasive. Arithmetic you can check is offered here instead.
Sleep latency — the delay between settling down and losing consciousness — is the term that gets dropped, and it is rarely small. Fifteen to twenty minutes is unremarkable for many adults, and it is longer after caffeine late in the day, after a screen-lit evening, after exercise close to bedtime, and on the night before anything that matters.
Because the tool asks for it rather than assuming it, the answer moves with you. Twenty minutes of latency across a week is well over two hours of sleep that a naive bedtime silently spends. If you have no idea what your own figure is, notice it for a few nights before trusting the schedule.
The age ranges quoted alongside the result come from the National Sleep Foundation’s duration recommendations, published in Sleep Health in 2015 by Hirshkowitz and colleagues after a panel review of the literature. A separate consensus statement from the American Academy of Sleep Medicine and the Sleep Research Society, published in SLEEP the same year, puts seven hours a night as a regular floor for adults.
Both are population statements about habitual sleep, and both are ranges rather than prescriptions — the first paper even publishes wider bands either side that it describes as possibly appropriate. Landing outside one is a prompt to look at how you feel during the day, not a diagnosis. This is a calculator, and persistent trouble sleeping is a conversation for a doctor.
There is no good reason to. The figure is an average of something that varies within and between people, and no device on your bedside table knows which stage you are in, so a bedtime built on it is arbitrary precision. Aim for the total duration that leaves you functional and keep the timing consistent.
Your own, ideally measured over several ordinary nights rather than remembered. If you have nothing to go on, something in the region of fifteen minutes is a common starting point, but the value of the field is that it holds your figure rather than a stranger’s.
Not by itself. The ranges describe what suits most people in an age group over the long run, and individuals sit outside them without difficulty. Daytime sleepiness, poor concentration or trouble staying asleep are the signals worth acting on, and they are worth raising with a clinician rather than a calculator.
Only for the plain subtraction. Rotating shifts and crossing time zones involve circadian timing, light exposure and adaptation over days, none of which is arithmetic on a single night, and both are areas where proper guidance exists and is worth seeking out.