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Sleep Cycle & Bedtime Calculator
Best sleep and wake times on 90-minute REM cycles.
How it works
Sleep is organised in cycles of roughly ninety minutes, moving from light sleep through deep sleep to dreaming REM sleep and back. Waking in the middle of a deep phase is what produces that heavy, groggy feeling, while waking at a cycle boundary tends to feel lighter. This calculator counts whole cycles backwards from your alarm, or forwards from your bedtime, and adds fifteen minutes for falling asleep, so the times it suggests land on cycle boundaries rather than in the middle of one.
How to read the result
Five cycles, or seven and a half hours of sleep, suits most adults, and six cycles suits those who need longer sleep. Four cycles is a short-night fallback, not a routine to aim for. If you consistently need an alarm and still feel unrefreshed, the issue is often total sleep time or regularity, not which exact cycle you chose, and a regular bedtime matters as much as the arithmetic.
One practical tip
Keep the same wake time, even at weekends, as far as life allows. A steady wake time anchors your body clock, makes falling asleep at the suggested bedtime easier within a week or two, and reduces the Monday-morning crash that follows a weekend lie-in.
Cycle length varies between people and across the night, so treat ninety minutes as a useful average rather than an exact clock; if you wake groggy despite using cycle times, shifting bedtime by fifteen to thirty minutes is a sensible experiment.
If your schedule forces a short night, a brief afternoon nap of about twenty minutes can reduce sleepiness without the grogginess of a longer nap, though it is no substitute for regular full nights.