Sleep Calculator
Find the best bedtime or wake-up time based on 90-minute sleep cycles.
What is the Sleep Calculator?
Sleep happens in cycles of roughly 90 minutes each, moving through light sleep, deep sleep and REM sleep in sequence. Waking up in the middle of a cycle (especially during deep sleep) tends to feel much groggier than waking up at the natural end of one — this tool works backward or forward in whole 90-minute cycles to suggest times aligned with that rhythm.
The math
Each suggested time is 90 minutes × number of cycles, plus an extra 15 minutes to account for the time it typically takes to actually fall asleep after getting into bed. Most adults need 5–6 full cycles (7.5–9 hours) for adequate rest.
How to use it
- Choose whether you're planning a wake-up time, or going to bed right now.
- For a wake-up time, enter it and see several bedtime options, each ending on a full cycle.
- For "go to bed now," see several wake-up options instead.
- Pick the option closest to how many hours of sleep you know you need.
90 minutes is an average cycle length — real cycle length varies by person, and by age, stress and alcohol use on a given night, so this is a useful planning heuristic rather than a guarantee of exactly when you'll naturally wake up.
Want the "why" behind this tool? Read Why Waking Up Mid-Cycle Feels Worse Than Sleeping Less.