Sleep cycle calculator
When should you go to bed?
Sleep runs in roughly 90-minute cycles. Wake at the end of one and you feel rested; wake in the middle and you feel groggy. Pick the right time below.
If you fall asleep right now… We'll find your best wake-up times.
Popular wake-up times
Sleep, in three terms
- Sleep cycle
- About 90 minutes of light to deep to REM sleep; most people complete 4-6 a night.
- Sleep inertia
- The grogginess from waking mid-cycle, in deep sleep - exactly what cycle timing avoids.
- Sleep latency
- The time it takes to fall asleep, about 15 minutes on average - set it with the slider.
How it works
How long is one sleep cycle?
A full cycle averages about 90 minutes and moves through light sleep, deep sleep and REM. Most people complete 4–6 cycles a night, which is why this tool counts in whole cycles rather than fixed hours.
Why do I wake up groggy even after 8 hours?
If your alarm goes off in the middle of deep sleep, you get "sleep inertia" — that heavy, foggy feeling. Waking at the end of a cycle, in light sleep, usually feels far easier even with slightly less total sleep.
What does the fall-asleep slider do?
Most people take around 15 minutes to actually fall asleep. Drag the slider to match how long it takes you, and the calculator shifts every suggested time to match your real sleep, not just lights-out.
References & method · 参考来源与方法
Calculations use the widely cited ~90-minute average sleep cycle and an adjustable ~15-minute sleep latency. This tool is for general use and is not medically reviewed; for ongoing sleep problems, consult a qualified professional.
- National Sleep Foundation (NSF)
- American Academy of Sleep Medicine (AASM)
- Centers for Disease Control and Prevention (CDC) — sleep
- Ohayon M. et al. (2017), Sleep Health
- Van Dongen H. et al. (2003), Sleep