Sleep Calculator
Find bedtimes and wake-up times by 90-minute sleep cycles
Sleep Cycle Calculator
Sleep runs in roughly 90-minute cycles from light to deep and back. Waking at the end of a cycle feels refreshing; an alarm that lands mid-cycle is what makes eight hours feel worse than six and a half. This calculator counts backward (or forward) in whole cycles, plus about fifteen minutes to fall asleep.
Most adults do best on five to six cycles — 7.5 to 9 hours. If you must cut sleep short, aim to cut a whole cycle, not half of one.
Frequently Asked Questions
Why 90-minute cycles?
It's the average adult sleep cycle length; individuals range roughly 80-110 minutes, so treat results as a starting point.
What if I take longer to fall asleep?
The calculator assumes 15 minutes; if you typically need 30, shift bedtimes 15 minutes earlier.
Are naps covered?
For naps, 20 minutes (no deep sleep) or a full 90-minute cycle both avoid grogginess.
Is it free?
Completely free, no signup.
Related Tools
Pair it with the Online Timer, Pomodoro Timer or Age Calculator.
睡眠周期计算器
睡眠以约 90 分钟为一个周期，在浅睡与深睡之间循环。在周期末醒来神清气爽；闹钟落在周期中段，就是「睡了八小时反而更累」的原因。本工具按整周期倒推（或顺推），并预留约十五分钟入睡时间。
多数成年人适合 5–6 个周期（7.5–9 小时）。实在要压缩睡眠，宁可砍掉整个周期，也别砍半个。
常见问题
为什么是 90 分钟？
这是成年人睡眠周期的平均值；个体在 80–110 分钟之间浮动，结果当作起点即可。
我入睡慢怎么办？
工具默认预留 15 分钟；若你通常需要 30 分钟，把就寝时间再提前 15 分钟。
小睡怎么算？
小睡选 20 分钟（不进入深睡）或完整 90 分钟，都能避免起床昏沉。
免费吗？
完全免费，无需注册。