Sleep calculator
Pick a wake-up time and get bedtimes that land at the end of a sleep cycle, or start from bedtime and get the best alarm times.
To wake at 6:30 AM, get into bed at:
- 9:15 PM6 cycles · 9 h asleeprecommended
- 10:45 PM5 cycles · 7 h 30 min asleeprecommended
- 12:15 AM4 cycles · 6 h asleepbelow 7 h
- 1:45 AM3 cycles · 4 h 30 min asleepbelow 7 h
Includes 15 min to fall asleep. Adults need at least 7 hours (AASM/SRS), so 5 or 6 cycles is the target.
Why cycle timing changes how you wake up
Through the night your brain moves through light sleep, deep slow-wave sleep and REM sleep in repeating cycles. Early cycles hold most of the deep sleep; later cycles hold more REM. If the alarm catches you in deep sleep you get sleep inertia: grogginess that can last half an hour. Try it:
Drag the alarm across a night of sleep
Light (N1) Light sleep. The easiest place to wake: you surface quickly and feel alert sooner.
Shaded bands are ~90-minute cycles. Real nights vary (cycles run roughly 80 to 110 minutes), so treat cycle timing as a nudge, not a precision tool.
Bedtime chart for common wake times
| Wake at | 6 cycles (9 h) | 5 cycles (7.5 h) |
|---|---|---|
| 5:00 AM | 7:45 PM | 9:15 PM |
| 5:30 AM | 8:15 PM | 9:45 PM |
| 6:00 AM | 8:45 PM | 10:15 PM |
| 6:30 AM | 9:15 PM | 10:45 PM |
| 7:00 AM | 9:45 PM | 11:15 PM |
| 7:30 AM | 10:15 PM | 11:45 PM |
| 8:00 AM | 10:45 PM | 12:15 AM |
Assumes 15 minutes to fall asleep.
If you already owe sleep, cycle timing won't fix that. Start with the sleep debt calculator. For daytime rest, the nap calculator uses the same logic on a shorter scale.
Questions people ask
How does a sleep cycle calculator work?
It counts back (or forward) in blocks of about 90 minutes, the average length of one sleep cycle, and adds the time it takes you to fall asleep. Waking at the end of a cycle, when sleep is light, tends to feel easier than waking from deep sleep.
How many sleep cycles do I need?
Most adults need 5 or 6 cycles: 7.5 or 9 hours. Four cycles (6 hours) is below the 7-hour minimum recommended by the American Academy of Sleep Medicine for adults.
What time should I go to bed if I wake up at 6 AM?
For 5 cycles, fall asleep by 10:30 PM; for 6 cycles, by 9:00 PM. Add the time you usually take to fall asleep, so with 15 minutes you would get into bed around 10:15 PM.
Are sleep cycles exactly 90 minutes?
No. They typically range from about 80 to 110 minutes and change across the night, with more deep sleep early and more REM late. The 90-minute rule is a useful average, not a precise timer.
Is it better to sleep 6 or 7.5 hours?
7.5 hours, in almost every case. Hitting the end of a cycle does not make up for sleeping less than you need.
Sources
- Carskadon MA, Dement WC. Normal human sleep: an overview. In: Principles and Practice of Sleep Medicine (Kryger, Roth, Dement, eds.).
- Tassi P, Muzet A (2000). Sleep inertia. Sleep Medicine Reviews, 4(4).
- Watson NF et al. (2015). Recommended amount of sleep for a healthy adult. Sleep, 38(6).