Free tool
Sleep calculator
Work out what time to go to bed — or what time to wake up if you sleep now — so your alarm lands at the end of a 90-minute sleep cycle instead of the middle of one.
Go to bed at
9:45 PM
6 cycles · 9h
Recommended
11:15 PM
5 cycles · 7h 30m
Recommended
12:45 AM
4 cycles · 6h
2:15 AM
3 cycles · 4h 30m
Based on average 90-minute sleep cycles plus ~15 minutes to fall asleep. Real cycles vary from person to person and night to night — treat these as good starting points, not medical guidance.
Why waking mid-cycle feels so bad
A night of sleep isn't one flat block — it's a loop of light sleep, deep sleep, and REM that repeats roughly every 90 minutes. An alarm that lands in light sleep feels like surfacing naturally; the same alarm landing in deep sleep feels like being dragged out of anesthesia. That heavy, disoriented feeling is sleep inertia, and it's a big part of why people sleep through alarms or dismiss them without ever waking up.
You can't control which stage you're in, but you can stack the odds: pick a wake time that's a whole number of cycles after you fall asleep, and keep it consistent — the body clock does the fine-tuning when the schedule is steady.
How to use the results
Pick the bedtime that gives you five or six cycles and treat it as a hard appointment — screens down, lights dimmed a little before. If the recommended bedtime looks impossibly early, fix it gradually: go to bed 15 minutes earlier every few nights rather than attempting a two-hour jump on night one. And hold the same wake time on weekends (within an hour), or Monday's alarm lands in the middle of your biological night.
Then make the wake-up end of the plan as solid as the bedtime end. Set the alarm for your chosen time in the online alarm clock for naps and desk days — and for the morning that actually matters, use an alarm you can't silence from bed. WakeUpBroo rings on your iPhone with no snooze and no dismiss button; to stop it, you read a rotating code from wakeupbroo.com/code on a device across the room. The calculator gets the timing right — the walk gets you vertical.
Start from your bedtime
Each of these shows the wake-up times that end on a full cycle for that bedtime, plus how much sleep every common alarm time gives you:
Start from your alarm
Or the other way round — pick the time you have to be up and get the bedtimes that fit, with the alarm already filled in:
FAQ
Frequently asked questions
How does the sleep calculator work?
Sleep runs in cycles of roughly 90 minutes, and waking at the end of a cycle usually feels easier than being pulled out of the middle of one. The calculator adds about 15 minutes to fall asleep, then counts back (or forward) in 90-minute steps, so your alarm lands near a cycle boundary.
How many sleep cycles do I need?
Five to six cycles — about 7.5 to 9 hours — is the range most adults do best on, which is why the calculator highlights those options. Four cycles (6 hours) works occasionally but is a debt, not a plan, and three is an emergency.
Is the 90-minute sleep cycle exact?
No — it's an average. Real cycles run roughly 80 to 110 minutes and shift through the night and between people, so treat the suggested times as good starting points. If you consistently wake up exhausted despite enough hours, that's worth discussing with a doctor rather than a calculator.
I know when to wake up — how do I actually get up?
That's the half a calculator can't solve. If you dismiss alarms half-asleep, use an alarm your sleeping brain can't defeat: WakeUpBroo for iPhone has no snooze and no dismiss button — the alarm stops only when you enter a rotating code from wakeupbroo.com/code on a device away from your bed.
Recommended next
Keep reading
The bedtime math is the easy half
Knowing the right wake-up time doesn't get you out of bed. WakeUpBroo has no snooze and no dismiss button — the alarm stops only when you read a rotating code from this website on another device. Free to download, 7-day free trial.
Free on iPhone · Android in development