How does the sleep cycle calculation work?

Sleep runs in roughly 90-minute cycles. The calculator counts back or forward in 90-minute blocks from your target time and adds about 14 minutes to fall asleep, so each suggested time lands at the end of a complete cycle.

How many cycles should I aim for?

Most adults do well with five or six cycles (about 7.5 to 9 hours). The tool lists options from three to six cycles so you can pick what fits your night.

Is my chosen time sent anywhere?

No. The calculation runs entirely in your browser and nothing is uploaded or stored.

What time should I go to sleep to wake up at 7am?

Counting back in 90-minute cycles plus 14 minutes to fall asleep, the best bedtimes to wake at 7:00 are roughly 21:46 (6 cycles), 23:16 (5 cycles) or 00:46 (4 cycles). Pick the one that gives you enough total sleep.

Why does waking mid-cycle feel so groggy?

A waking from deep (slow-wave) sleep, which dominates the middle of a cycle, causes "sleep inertia" — grogginess that can last 15–30 minutes. Waking at the end of a cycle, in light sleep, avoids most of that fog.

Is every sleep cycle exactly 90 minutes?

No — real cycles range from about 70 to 120 minutes and lengthen through the night. The 90-minute average is a useful planning rule, so treat the suggested times as good targets rather than exact guarantees.

Why add 14 minutes to fall asleep?

Most healthy adults take 10–20 minutes to drift off (sleep-onset latency). The calculator adds 14 minutes so that the cycles are counted from when you actually fall asleep, not when your head hits the pillow.

Sleep Cycle Calculator

Sleep cycle calculator

You feel most refreshed when you wake at the end of a sleep cycle rather than in the middle of one, when deep sleep makes you groggy. Sleep runs in roughly 90-minute cycles, so this tool finds bedtimes (or wake-up times) that land you at a clean cycle boundary. It is handy for setting an alarm that leaves you alert, or for working out when to turn in tonight to hit a fixed wake-up.

How it works

From your target time the calculator steps in 90-minute blocks and adds a fixed 14 minutes to fall asleep. In “wake up at” mode it counts backward (so each result is a bedtime); in “go to sleep at” mode it counts forward (each result is a wake-up time). It lists 6, 5, 4 and 3 full cycles — 9, 7.5, 6 and 4.5 hours of sleep.

bedtime = wake time − (cycles × 90 min + 14 min)

Example

To wake at 07:00:

Cycles	Sleep	Go to sleep at
6	9.0 h	21:46
5	7.5 h	23:16
4	6.0 h	00:46
3	4.5 h	02:16

Most adults feel best on 5–6 cycles. Pick a direction, enter a time as HH:MM, and the list appears instantly — all in your browser, nothing uploaded.