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Sleep cycle calculator

Find the best bedtimes to wake up refreshed, based on 90-minute sleep cycles.

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Need Bedtime (6 cycles, 9h), Bedtime (5 cycles, 7.5h), Bedtime (4 cycles, 6h)? The Sleep cycle calculator derives it from Wake-up — hour, Wake-up — minute in one step. For instance, with Wake-up — hour = 7 and Wake-up — minute = 0 it returns Bedtime (6 cycles, 9h) = 21:46, Bedtime (5 cycles, 7.5h) = 23:16 and Bedtime (4 cycles, 6h) = 00:46.

How to use it

  1. Enter your values: Wake-up — hour, Wake-up — minute.
  2. Read the result instantly: Bedtime (6 cycles, 9h), Bedtime (5 cycles, 7.5h), Bedtime (4 cycles, 6h).

Frequently asked questions

What does the Sleep cycle calculator actually compute?

It takes Wake-up — hour and Wake-up — minute and derives Bedtime (6 cycles, 9h), Bedtime (5 cycles, 7.5h) and Bedtime (4 cycles, 6h) from them. The calculation is live as you type, so the result updates on every change.

What information do I need to provide?

2 values: Wake-up — hour and Wake-up — minute. Nothing else is required — no account, no file upload.

Can you show a worked example?

With Wake-up — hour = 7 and Wake-up — minute = 0, the calculator returns Bedtime (6 cycles, 9h) = 21:46, Bedtime (5 cycles, 7.5h) = 23:16 and Bedtime (4 cycles, 6h) = 00:46. Those figures come from running this exact tool, so you can reproduce them by entering the same values.

When would I actually use this?

Working backwards from the hour you have to be up: when to go to bed so an alarm does not land in the middle of deep sleep, how long a nap can run before it starts costing more than it gives, and how late a coffee still counts.

What is the most common mistake?

Treating the cycle length as a fact about you. Ninety minutes is an average, and a real cycle runs anywhere from about seventy to a hundred and ten minutes and changes across the night — so a bedtime computed here is a better guess than the round hour, not a wake-up guarantee. The time it takes to fall asleep is the other half, and it is the part people leave out.

How accurate is it, and what are the limits?

For general guidance only — not medical advice.

What is the difference between the Sleep cycle calculator and the Coffee vs. Sleep Calculator?

This one returns Bedtime (6 cycles, 9h) and Bedtime (5 cycles, 7.5h); the Coffee vs. Sleep Calculator returns Result. That is the whole difference — open the one whose figure you need.

Is there a tool for the next step?

Menstrual Cycle Calculator is the closest one after this: Predict your next periods, ovulation day, fertile window and current cycle phase from your last period.

Where do the figures come from, and how current are they?

Sleep architecture — the ninety-minute cycle, the share of deep sleep early in the night and of REM towards morning — is textbook sleep medicine, and the tools apply it as arithmetic on the hour you give them. Caffeine is modelled on its half-life, around five to six hours in a typical adult, which is why an afternoon cup is still measurably present at bedtime; that half-life varies with genetics, pregnancy, liver function and some medicines, by a factor no calculator can guess. None of this diagnoses insomnia or any other sleep disorder — persistent trouble sleeping is worth a clinician, not an arithmetic.

Further reading

All guides →
How-toBest Time to Wake Up: Using Sleep CyclesSleep runs in cycles of about 90 minutes. Waking at the end of a cycle can help you feel more refreshed. Here's how to count back from your wake time.ExplainerHow Much Sleep Do You Need?Adults need 7–9 hours, and sleep runs in ~90-minute cycles. Here's how much by age, how to time your bedtime, and why quality counts too.ExplainerCaffeine Against Sleep Pressure: What It Blocks, and What It CannotSleep is regulated by two processes: a pressure that builds with every waking hour, and a clock. Caffeine acts on neither directly — it blocks the receptors that report the pressure. The pressure keeps building behind the block, which is why the crash arrives and why an afternoon dose costs that night's sleep even when you do not feel it.ExplainerCaffeine Half-Life: Why Your Afternoon Coffee Is Still There at MidnightCaffeine leaves by first-order kinetics, so what remains is the dose times one half to the power of elapsed time over half-life. The half-life is not a constant — it varies by multiples between people, and that is the whole story.ExplainerNap Length Decides What You Get — Not How Tired You WereA nap's effect depends on which sleep stage the alarm interrupts, and length is the only part of that you control. What 10, 20, 30 and 90 minutes actually do, with the arithmetic for the alarm.GuideBra Sizing Is Two Measurements and Four Incompatible SystemsThe band and the cup are computed differently in the UK, the US, the metric EU, the French and Spanish markets and Italy. One body, its four labels, and the arithmetic behind each.