Sleep Calculator
Work backwards from your alarm (or forwards from bedtime) in 90-minute sleep cycles — so you wake at the end of a cycle instead of the middle of deep sleep.
Recommended: 7–9 hours for adults.
Cycle length varies between 70 and 110 minutes. If these times feel slightly off, adjust by 10–15 minutes — the goal is to wake between cycles, not exactly on a 90-minute grid.
Frequently asked questions
How does a sleep cycle calculator work?
Sleep runs in cycles of roughly 90 minutes, moving through light sleep, deep sleep and REM. Waking in the middle of deep sleep produces grogginess (sleep inertia) even after a long night, while waking at the end of a cycle feels much easier. The calculator counts backwards from your wake-up time in 90-minute blocks and adds 15 minutes to fall asleep, so each result lands at a cycle boundary.
Is 90 minutes accurate for everyone?
It’s an average. Individual cycles run roughly 70–110 minutes and vary through the night — early cycles contain more deep sleep, later ones more REM. Treat the suggested times as a starting point: if you consistently wake up foggy at a “correct” time, your personal cycle is probably a little shorter or longer. Adjust by 10–15 minutes and see what feels better.
How many hours of sleep do I actually need?
The American Academy of Sleep Medicine recommends 7–9 hours for adults 18–64, 7–8 for adults 65+, 8–10 for teenagers and 9–12 for school-age children. Most people need 5–6 complete cycles. Consistently getting under 7 hours is linked to impaired memory, mood and immune function — the calculator marks results below 7 hours so you can see the trade-off.
Why add 15 minutes to fall asleep?
Sleep latency — the time from lights out to actual sleep — averages about 15 minutes for healthy adults, longer if you have caffeine late, use screens in bed, or are stressed. If you typically take 30 minutes, add that instead: the point is to hit the cycle boundary at the moment you wake, not when you lie down.