Orpheus

Sleep Cycle Calculator

Bedtimes that land between cycles, not in the middle of one.

Inputs
0–23
15

Fifteen is the usual estimate for an adult without sleep problems.

Go to bed at
23:15for 5 cycles
Too littleShortRecommendedLong
6 cycles · 9.0 h
21:45
5 cycles · 7.5 h
23:15
4 cycles · 6.0 h
00:45
3 cycles · 4.5 h
02:15
Waking at
07:00
bedtime = wake time − (cycles × 90 min) − time to fall asleep

Each cycle is about 90 minutes, and 15 minutes to fall asleep is added on top. Five or six cycles — 7.5 to 9.0 hours — is the range most adults need.

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Sleep runs in cycles of roughly 90 minutes. Count back from your wake-up time in 90-minute blocks and add about 15 minutes to fall asleep — to wake at 07:00, go to bed at 23:15 for five cycles.

How to use Sleep Calculator

  1. Enter your wake-up time. Hour and minute, on a 24-hour clock.
  2. Adjust the time to fall asleep. Fifteen minutes is typical; increase it if you know you take longer.
  3. Pick a bedtime. Five or six cycles suits most adults; four is a short night rather than a plan.

About calculating sleep and wake times

Sleep is not uniform. It moves through repeating cycles that pass from light sleep into deep slow-wave sleep and then into REM, and each full pass takes roughly ninety minutes. The composition changes across the night: the early cycles are weighted toward deep sleep, which is when most physical recovery happens, while the later ones carry progressively more REM, which is associated with memory consolidation and dreaming. Waking is far more pleasant at the end of a cycle than in the middle of one. Being pulled out of deep sleep produces sleep inertia — a period of grogginess and impaired reaction time that can persist for twenty to thirty minutes and occasionally much longer. Timing a wake-up to land in light sleep is what a cycle calculation is for, and it is why six and a half hours can leave you feeling worse than six. The ninety-minute figure is an average and should be treated as one. Real cycles vary between about seventy and a hundred and twenty minutes, differ between people, and shift with age, accumulated sleep debt, alcohol and stress. If the calculated bedtimes reliably feel wrong, that is informative rather than a failure of the method. It is also worth being clear that cycle alignment is a comfort optimisation, not a substitute for duration. Four cycles is six hours, and no arrangement of them makes six hours sufficient for an adult who needs eight.

Frequently asked questions

How long is a sleep cycle?
About 90 minutes on average, though real cycles range from roughly 70 to 120 minutes and change through the night — early cycles carry more deep sleep, later ones more REM. Ninety is a useful planning figure rather than a measured constant.
Why does waking mid-cycle feel worse?
Because being woken out of deep sleep produces sleep inertia: grogginess, slow reaction time and impaired thinking that can last twenty to thirty minutes. Waking at the end of a cycle, when sleep is lightest, largely avoids it.
Is six hours enough if it is four full cycles?
For most adults, no. Cycle alignment affects how you feel on waking; total sleep affects memory, immune function and cardiovascular health. Adults generally need seven to nine hours, and consistently sleeping six carries real costs regardless of where the cycles fall.
Does the 90-minute figure apply to everyone?
Only approximately. Individual cycle length varies by twenty minutes or more, and it shifts with age, sleep debt and alcohol. If the calculated times consistently feel wrong, your own cycle is probably longer or shorter than the average.
What matters more than cycle timing?
Consistency. Going to bed and waking at the same times every day, including weekends, does more for sleep quality than optimising any single night. Light exposure in the morning, and avoiding it in the evening, comes second.

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