Work out a bedtime from sleep cycles
A calculator for bedtime and wake time based on the 90-minute cycle, together with a plain account of how far the 90-minute figure is true and where it is not.
Key points
- A sleep cycle lasts roughly 90 minutes, but that figure is a population AVERAGE. In practice it ranges from about 70 to 120 minutes and changes within a single night.
- Most people need 4 to 6 cycles a night.
- Waking at the end of a cycle usually feels better than waking out of deep sleep. That is why this tool is useful, but it is an estimate rather than a measurement.
- ⛔ Do not use this tool if you currently have insomnia: fixing a bedtime by calculation and then lying there waiting for it is a reliable way to increase anxiety about sleep.
How to use it
Choose one of the two directions. If you know what time you have to be awake, the tool works backwards to the times you should go to bed. If you know what time you will go to bed, it works out the times to set an alarm. Both directions add about 15 minutes for falling asleep.
How far the 90-minute figure is true
True in that sleep genuinely runs in cycles, and each cycle passes through light sleep, deep sleep and then REM. Not true in treating 90 minutes as a constant. The first cycle of the night is usually shorter and contains more deep sleep; the cycles towards morning are longer and contain more REM. Cycle length also differs from person to person.
What to prioritise if you can only do one thing
Holding a fixed WAKE time matters more than holding a fixed bedtime. The wake time is the main signal anchoring the body clock; the bedtime is better left to the body to decide, through genuine sleepiness. Lying down before you are sleepy only creates more time to turn over in bed.
About the copyright of this questionnaire
A pure calculator, written by the clinic. It uses no questionnaire or scale.
Frequently asked questions
- Is 6 hours of complete cycles better than 7 hours of broken ones?
- There is no good evidence for that, and it is repeated a great deal online. Total sleep duration is the stronger factor. If you have to choose, choose the hours.
- How many cycles should I get?
- Most adults need 7 to 9 hours in total, which is roughly 5 to 6 cycles. But the requirement is individual: the right measure is whether you are alert during the day, not the number.
References
- Recommended Amount of Sleep for a Healthy Adult: A Joint Consensus Statement - AASM & Sleep Research Society, 2015 · DOI 10.5664/jcsm.4758
- How Sleep Works - Sleep Phases and Stages - National Heart, Lung, and Blood Institute (NIH), 2026