Sleep Calculator
The best times to fall asleep or wake, counted in sleep cycles.
Using the calculator
Tell it when you must wake and it lists bedtimes that fit whole sleep cycles; tell it when you are going to bed — or press "I'm going to bed now" — and it lists alarm times. Times include the average 15 minutes to fall asleep, and the highlighted options are five to six cycles: 7½ to 9 hours of actual sleep, the adult sweet spot.
The idea, and its honest limits
Sleep runs in roughly 90-minute cycles through light, deep and REM stages. Waking from deep sleep mid-cycle produces sleep inertia — the leaden, disoriented feeling that coffee doesn't touch — while waking near a cycle boundary tends to feel cleaner. Timing the alarm to whole cycles is a rough but genuinely useful trick.
The honest caveats: real cycles run 80–110 minutes, vary through the night and between people, and the 15-minute drift-off is an average — so treat the suggested times as good guesses, not clockwork, and prioritise total hours over perfect boundaries. Consistency moves the needle most: the same times daily, weekends included. This is a rule of thumb, not medicine — chronic exhaustion despite adequate hours is a conversation for a doctor. The alarm clock pairs naturally, and if mornings need convincing, sunrise maths lives with the time zone converter.
Frequently asked questions
What time should I go to bed to wake at 7?
Working back in 90-minute cycles plus 15 minutes to fall asleep: 9:45 pm for six cycles (9 hours) or 11:15 pm for five (7½ hours). The calculator lists all the options.
Why 90-minute cycles?
A full pass through light, deep and REM sleep averages about 90 minutes. Waking at a boundary rather than mid-cycle tends to feel easier — though real cycles vary between 80 and 110 minutes.
Is 6 hours of sleep enough?
Four cycles is workable occasionally, but adults consistently need 7–9 hours. Regular short sleep accumulates as measurable deficit in mood, memory and health.
Why do I feel groggy even after a long sleep?
You likely woke from deep sleep mid-cycle — sleep inertia — or the sleep itself was fragmented. If it happens routinely despite adequate hours, it is worth mentioning to a doctor.