Sleep Calculator

Find bedtimes from a wake-up time, or wake times from a bedtime, using 90-minute sleep cycles plus time to fall asleep.

I want to

Wake or bedtime

Minutes to fall asleep

min

Suggested bedtimes

Sleep at 23:15

Assumed time to fall asleep15 min

Cycle length90 minutes

Best (5 cycles / 7.5 h)23:15

3 cycles (4.5 h)02:15

4 cycles (6 h)00:45

5 cycles (7.5 h)23:15

6 cycles (9 h)21:45

7 cycles (10.5 h)20:15

How to use this sleep calculator

A sleep calculator uses 90-minute cycles because many people wake less groggy at cycle boundaries. It is a schedule helper, not a sleep-medicine diagnosis. Adults often want 7–9 hours; five cycles plus latency is about 7.75 hours in bed.

If you lie awake longer than 15 minutes, raise latency. Teens may need more cycles. Caffeine late in the day delays sleep onset — pair this with the Caffeine Half-Life Calculator if afternoon coffee is the problem.

Formula

Sleep spans = fall-asleep minutes + N × 90 minutes, for N = 3 to 7 cycles. Bedtime = wake − span (or wake = bedtime + span), wrapped on a 24-hour clock. Five cycles (7.5 hours of sleep) is highlighted.

Worked example

Wake at 07:00 with 15 minutes to fall asleep: a 5-cycle night means lights out at 23:15 the evening before.

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Frequently asked questions

What time should I go to bed to wake up at 7?

Count back 15 minutes to fall asleep plus 90-minute cycles. Five cycles lands near 11:15 p.m. for a 7:00 a.m. wake.

Why 90-minute cycles?

Typical adult NREM+REM cycles last about 90 minutes. Waking between cycles often feels easier than waking out of deep sleep.

How much sleep do I need?

Most adults 7–9 hours. This tool shows 4.5 to 10.5 hours of sleep (3–7 cycles) so you can pick a fit.

Does it include time to fall asleep?

Yes. Default 15 minutes. Heavy caffeine or stress may need more.

Is this medical advice?

No. Chronic insomnia, apnea, or shift work belongs with a clinician.

Results are estimates for informational purposes and are not financial, legal, or professional advice. Last reviewed 2026-08-20.