Calculate the best time to go to bed or wake up based on 90-minute sleep cycles.
A sleep cycle calculator works out the optimal bedtime or wake time so that you wake at the end of a 90-minute sleep cycle rather than in the middle of one. Sleep occurs in cycles of approximately 90 minutes — light sleep, deep sleep, REM sleep — and waking mid-cycle (especially mid-deep-sleep) produces grogginess that can last hours.
Most adults need 5–6 complete sleep cycles per night (7.5–9 hours). Aim for cycles, not just total hours. A 6-hour sleep (4 cycles) often feels better than a 7-hour sleep (4 cycles + 60 minutes of an incomplete fifth).
\text{Wake time} = \text{Bedtime} + n \times 90 \text{ minutes} + 15 \text{ min sleep onset}
Inputs: You want to wake at 7:00 AM. When should you go to bed?
No — typical range is 80–110 minutes, with 90 minutes as the population average. Children and teenagers have slightly shorter cycles; older adults have slightly longer ones. A few weeks of using a sleep tracker watch will reveal your personal average. Once known, set wake alarms at multiples of your cycle length for maximum freshness on waking.
For most people, yes — waking at the end of a REM phase feels dramatically better than waking from deep sleep. The bigger gain is consistency: going to bed and waking at the same time every day (yes, including weekends) trains your circadian rhythm and outweighs any single-night cycle optimisation.
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