Plan sleep around your body's actual rhythm, not a guess.
Chrona times your bedtime or wake-up around complete 90-minute sleep cycles, so you're far less likely to wake up mid-cycle feeling groggy. Current time: —
Calculate my bedtime →Suggested times
Recent calculations
Every tool, for every part of the night
Chrona is built around the three phases of a real night's sleep, not just one calculator.
Bedtime & Wake Calculator
Work backward or forward from a target time in full 90-minute cycles.
Before sleepCaffeine Cutoff
Find the latest sensible time for your last coffee based on your bedtime.
During sleepHow Sleep Cycles Work
The stages your brain actually moves through each night, explained simply.
RecoveryNap Calculator
Power nap, NASA nap, or full-cycle nap — see the wake time for each.
RecoverySleep Debt Tracker
Log a week of sleep and see how much you're carrying — and how to catch up.
During sleepAll Guides
Chronotypes, caffeine science, napping research, and more.
Why full cycles matter more than total hours
Sleep isn't a single flat state — your brain moves through a repeating sequence of light sleep, deep sleep, and REM roughly every 90 minutes. Waking up in the middle of a deep-sleep stage tends to produce grogginess (sleep inertia), even if you technically got "enough" hours. Waking up near the boundary between cycles tends to feel noticeably easier.
That's the whole idea behind Chrona's calculator: instead of just adding up hours, it works in whole cycles from whichever time actually matters to you — your alarm, or the moment you expect to fall asleep.
Cycle length varies a bit from person to person and night to night, which is why the settings panel lets you adjust it instead of locking you into a single number.
Frequently asked questions
Is every sleep cycle exactly 90 minutes?
No. Ninety minutes is a widely used planning average, but individual cycles commonly range from about 80 to 110 minutes and can shift with age, stress, and sleep debt.
Does this tool give medical advice?
No. Chrona is an informational planning tool, not a substitute for professional medical or sleep-disorder guidance. If sleep problems persist, a clinician can help.
Is my data stored anywhere?
No account or server-side storage is used. Recent calculations are kept only in your browser's local storage, and you can clear them at any time.
Why does the calculator suggest a "recommended" option?
Five cycles (about 7.5 hours) sits close to the middle of commonly recommended adult sleep duration, so it's flagged as a sensible default — not a hard rule for every body.