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Plain arithmetic · nothing measured, nothing stored

What time should I go to bed?

Waking mid-cycle is what makes an eight-hour night feel terrible. Pick your alarm time and this counts whole cycles backwards — with every assumption on screen and editable.

Bedtime or wake timeAdjustable cycle lengthAdjustable fall-asleep delayNo account

Count backwards from your alarm

Change any assumption; the times update immediately.

These are averages, not a measurement of your sleep. Highlighted rows are 5 and 6 cycles, roughly 7.5–9 hours.

How it works

  1. One cycle defaults to 90 minutes, the usual textbook average. Yours may be anywhere from about 80 to 110 minutes, so the slider changes it.
  2. In wake mode: bedtime = alarm − (cycles × cycle length) − fall-asleep delay. In bedtime mode the same sum runs forwards.
  3. Five and six cycles (about 7.5–9 hours) are highlighted because that is where most adult sleep-need estimates sit.
  4. Everything is computed in the page. No date, no alarm and no habit is saved anywhere.

What it will never do

  • It does not measure your sleep. Nothing here detects a real cycle — it is arithmetic on an average, not a reading from your body.
  • It gives no medical advice. Persistent trouble sleeping is a question for a doctor, not a calculator.
  • It never claims a “best” bedtime. It shows options; which one you use is your call.

Questions

Is a sleep cycle really 90 minutes?

90 minutes is the common average, but individual cycles run roughly 80–110 minutes and change through the night. That is exactly why the cycle length here is a slider rather than a fixed number.

Why subtract time for falling asleep?

Because you do not fall asleep the instant you lie down. Sleep-onset latency of 10–20 minutes is typical, so the calculation subtracts it before counting cycles.

Should I always aim for six cycles?

No. Total sleep need varies from person to person. Six cycles is nine hours, which is more than many adults need. Compare the options and see which wake-up actually feels better.

Trouble settling once you are in bed?

Sereno is optional — this free tool runs entirely in your browser and works without it. Check the app’s current App Store listing for exact features, pricing and availability before downloading.

View Sereno on the App Store