Sleep Cycle Calculator

Features

• Both directions: sleep now → wake time, or wake time → bedtime
• Lists options for 4, 5 and 6 cycles of 90 minutes
• Adds the time it takes to fall asleep (15 min by default, adjustable)
• Set a target time and the closest row is highlighted
• Wraps correctly past midnight, so late and early times are not swapped
• Type now to use the current time
• Results are rows, each copyable on its own
• Computed locally; the times you enter are never uploaded

How to use

1. Choose a direction (sleep now, or wake at a time)
2. Type a bedtime, a wake-up time, or simply now
3. Adjust the suggested number of cycles or time to fall asleep if needed
4. Results appear at once, with the suggested or closest row marked

FAQ

Why 90 minutes?

A sleep cycle (REM plus non-REM) is usually estimated at 90 minutes, but individuals range from 80 to 110. The table assumes 90 and is meant to help you wake at the end of a cycle rather than be pulled out of deep sleep, which usually feels better.

Why add the time to fall asleep?

Because the time you type is when you get into bed, not when you fall asleep. Dropping off usually takes 10–20 minutes (15 is the default). Skip it and the whole table shifts by 15 minutes — and every row shifts by the same amount, so it still looks perfectly normal.

Can I use this as an alarm?

As a convenient target, yes, but do not trust it to the minute. Cycle length varies between people and bedtimes vary night to night. “About five cycles”, with the alarm near that time, is the sensible use.

Does it handle midnight correctly?

Yes. Everything is computed in minutes-of-day and wrapped, so 23:30 plus 7.5 hours lands on 07:00 the next day, marked “next day”.