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CALCULATOR · Health

Sleep Calculator

Plan your bedtime or wake-up time around 90-minute sleep cycles with a 15-minute fall-asleep buffer, in either direction depending on the time of day you can hold fixed.

Show formula
Plan sleep around ~90-minute cycles plus ~15 minutes to fall asleep. Wake time minus (cycles × 90 min + 15 min) gives a bedtime; bedtime plus (cycles × 90 min + 15 min) gives a wake time. This is a planning tool, not a clinical device.

Method reviewed:Feinberg (1974) cyclic model and subsequent consumer-facing variants; sleep latency from Carskadon & Dement (2005). Estimate only — not a substitute for clinical advice.

Inputs

Plans sleep around ~90-minute cycles plus ~15 minutes to fall asleep. This is a planning heuristic, not a medical rule.

Result

Suggested bedtimes

Suggested bedtimes: 23:15

Most adults feel rested with about 5 cycles.

{n} cycles
05:15

1.5 h

{n} cycles
03:45

3 h

{n} cycles
02:15

4.5 h

{n} cycles
00:45

6 h

{n} cycles
23:15

7.5 h (+1)

{n} cycles
21:45

9 h (+1)

Show formula
Bedtime = wake − (cycles · 90 min + 15 min). Wake = bedtime + 15 min + cycles · 90 min.

Not a medical device. Persistent sleep problems — insomnia, daytime sleepiness, snoring — should be discussed with a physician or board-certified sleep specialist.

How the sleep calculator works

  1. Pick a mode: "Wake-up time → bedtimes" works back from when you need to be up; "Bedtime → wake-up times" works forward from when you plan to sleep.
  2. Enter the anchor time in 24-hour HH:MM format and choose 90- or 120-minute cycle length (90 is the convention; 120 is offered for long-cycle sleepers).
  3. Read the cycle-by-cycle options on the result panel. Pick the slot that matches your target sleep duration.

Frequently asked questions

Is the 90-minute sleep cycle exact?

No. Sleep cycles vary by individual, age and sleep stage, typically between 70 and 110 minutes. 90 minutes is the most common average used in consumer sleep planning; treat any result as an estimate rather than a guarantee.

Why add 15 minutes to fall asleep?

Average sleep latency (the time it takes to fall asleep) for healthy adults is 10–20 minutes. The calculator includes a 15-minute buffer so a bed time of 23:00 means "in bed by 23:00", not "asleep at 23:00".

Should I rely on this for medical decisions?

No. SleepCalculator is a planning tool. Persistent sleep problems — insomnia, daytime sleepiness, snoring — should be discussed with a clinician or a board-certified sleep specialist.

Examples

Wake up at 06:30 — what bedtime gives five full cycles?
5 × 90 + 15 = 465 minutes back from 06:30, so 22:45. Six cycles pushes bedtime to 21:15.
Going to bed at 23:30 — when should the alarm go off?
Five 90-minute cycles forward (plus the 15-min latency on the bed side, not the wake side) lands at 07:15; six cycles at 08:45.

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