Sleep Calculator
Find the optimal bedtime to wake up at a target time, or the best wake time given a bedtime — based on 90-minute sleep cycles.
How to use the Sleep Calculator
- Enter your inputs into the Sleep Calculator above.
- Results update instantly as you type — no submit button needed.
- Adjust any value to see how the result changes in real time.
The sleep cycle math
Optimal bedtime = Wake time − (N × 90 min + 14 min to fall asleep)
A sleep cycle is roughly 90 minutes. Waking at the end of a cycle (light sleep) feels more refreshing than waking mid-cycle. Add ~14 minutes to fall asleep when calculating bedtime.
Worked example
To wake at 7:00 AM after 5 cycles (7.5 hours): bedtime = 7:00 AM − 7h 30min − 14min = 11:16 PM. After 6 cycles (9 hours): bedtime = 9:46 PM.
Frequently asked questions
Are 90-minute cycles universal?
They're an average. Individual cycles range from 70–120 minutes. The 90-minute rule is a useful approximation, not a hard rule. Listen to your body.
How much sleep do I need?
Adults: 7–9 hours nightly per CDC. Teens: 8–10 hours. Older adults often function well on 7 hours. Optimal varies — your daytime alertness is the best signal.
Why does waking mid-cycle feel awful?
Mid-cycle wakeups (during deep sleep or REM) interrupt the natural transition to lighter sleep. The end of a cycle (just before the next begins) is when you naturally wake feeling rested.