Sleep Cycle Calculator
Sleep moves through roughly 90-minute cycles. Waking at the end of a cycle — in light sleep — feels far better than being torn out of deep sleep. Pick a direction and find the times that let you wake between cycles, not inside one.
A typical night: deep N3 sleep dominates the first cycles; REM lengthens toward morning. Notice how the line rises near light sleep at the end of each cycle — those crossings are the comfortable moments to wake.
Why 90 minutes
Across a night, the brain repeatedly cycles through stages: light non-REM sleep (N1, N2), deep slow-wave sleep (N3), and REM sleep, where most dreaming happens. One full pass averages about 90 minutes, though it ranges from roughly 70 to 120 minutes and shifts across the night — early cycles are heavy on deep sleep, later ones on REM.
The practical insight is about where in the cycle your alarm lands. Waking during deep N3 sleep produces "sleep inertia" — that groggy, disoriented heaviness that can linger for half an hour. Waking during light sleep or just after REM, near the end of a cycle, tends to feel markedly easier. This calculator simply lines your wake time up with the end of a cycle.
How the times are worked out
The tool adds your sleep-onset latency — the time it takes you to actually fall asleep, defaulting to 15 minutes (the rough population average) — to your bedtime, then counts forward in 90-minute blocks. Five to six cycles (7.5–9 hours) is the range most adults need and is marked Recommended; four cycles (6 hours) is flagged as a workable minimum; three (4.5 hours) as short rest for an unavoidable early start.
Important caveats
This is a helpful approximation, not a sleep study. Real cycle length varies between people and between nights, latency depends on caffeine, stress, and screen exposure, and individual sleep need genuinely differs. Treat the suggested times as good targets to aim near, not exact instants. If you regularly struggle with sleep, persistent insomnia, or daytime exhaustion despite adequate hours, that's worth raising with a doctor — it can signal conditions a timing trick won't fix.
Privacy
All calculation happens in your browser. No times, no data, nothing is sent anywhere.