Sleep Calculator
Find bedtimes or wake-up times that line up with complete 90-minute sleep cycles, including a 15-minute buffer for falling asleep — because waking at the end of a cycle feels easier than waking in the middle of one.
Mode
Suggested Bedtimes
9:45 PM
9h of sleep
11:15 PM
7.5h of sleep
12:45 AM
6h of sleep
Each bedtime allows ~15 minutes to fall asleep, then 6, 5, or 4 complete 90-minute cycles before your alarm.
How the Math Works
bedtime = wake time − 15 min buffer − (cycles × 90 min)
A full sleep cycle averages about 90 minutes, but that is only an average — individual cycles range from roughly 70 to 120 minutes and shift over the course of the night. Treat these times as a starting point, not a precise prescription. This tool is not medical advice; if you have ongoing sleep problems, talk to a doctor.
Frequently Asked Questions
- How does the sleep calculator work?
- It counts backwards (or forwards) in 90-minute sleep cycles from your wake-up time or bedtime, plus a 15-minute buffer for falling asleep. Waking at the end of a full cycle — after 4, 5, or 6 cycles (6, 7.5, or 9 hours of sleep) — tends to feel easier than waking mid-cycle.
- Why 90-minute sleep cycles?
- A full sleep cycle — moving through light sleep, deep sleep, and REM — averages roughly 90 minutes in adults. It is only an average, though: real cycles range from about 70 to 120 minutes and vary from person to person and across the night.
- How many sleep cycles do I need per night?
- Most adults need 5 to 6 full cycles, which is 7.5 to 9 hours of sleep. Four cycles (6 hours) is a workable minimum for a single night, but consistently sleeping that little leaves most people short of the recommended 7+ hours.
- Why does the calculator subtract 15 minutes before the first cycle?
- Sleep cycles start when you fall asleep, not when you get into bed. The average adult takes around 15 minutes to drift off, so the calculator adds that buffer between bedtime and the start of the first cycle.
- Is this medical advice?
- No. This is a simple arithmetic tool built on an average cycle length — individual cycles vary, and it cannot diagnose or treat sleep problems. If you regularly struggle with sleep, talk to a doctor or sleep specialist.