Sleep Calculator
Pick a wake-up time to see the best bedtimes, or start from now to see when to set your alarm — based on 90-minute sleep cycles.
Go to bed at one of these times (includes 15 min to fall asleep):
Set a wake-up time to see suggested bedtimes.
Based on average 90-minute sleep cycles — individual cycles vary, so these times are estimates, not medical advice.
How it works
- Choose a mode: “Wake up at” if you know when your alarm rings, or “Go to bed now” if you are heading to bed.
- In “Wake up at” mode, set the wake-up time — the calculator counts back in 90-minute cycles plus 15 minutes to fall asleep.
- Pick a highlighted time: 5 or 6 full cycles (7.5 or 9 hours of sleep) gives most adults the best chance of waking refreshed.
Frequently asked questions
- Why does the calculator count in 90-minute cycles instead of total hours?
- Sleep moves through repeating stages — light sleep, deep sleep, and REM — in cycles that average about 90 minutes. Waking at the end of a cycle, when sleep is lightest, tends to feel far easier than being pulled out of deep sleep mid-cycle, even if the total time asleep is the same.
- Why is 15 minutes added before the first cycle?
- Most people do not fall asleep the moment they get into bed; the average time to drift off, called sleep latency, is around 10 to 20 minutes. The calculator adds 15 minutes so the cycle count starts from when you are actually asleep, not from when the lights go out.
- How many sleep cycles do adults actually need?
- Most adults do best with 5 to 6 full cycles, which works out to 7.5 to 9 hours of sleep — matching the 7-to-9-hour range recommended for adults by sleep researchers. Fewer cycles can work occasionally, but 4 or fewer on a regular basis builds up sleep debt for most people.
- Are the 90-minute cycles accurate for everyone?
- No — real cycles range from roughly 80 to 110 minutes and shift through the night, with more deep sleep early and more REM toward morning. Treat the suggested times as good starting points, then adjust by 15 to 30 minutes over a week or two based on how you feel when the alarm rings.
- Does this calculator send my data anywhere?
- No. The times are computed with plain JavaScript running entirely in your browser, and nothing is uploaded, logged, or stored on a server. You can load the page, go offline, and it will keep working.
About this tool
This sleep calculator answers a simple question with two angles: if my alarm rings at a certain time, when should I go to bed — and if I go to bed right now, when should I wake up? Instead of just subtracting eight hours, it works in 90-minute sleep cycles, the natural rhythm your brain follows as it moves through light sleep, deep sleep, and REM. Waking at the boundary between cycles, when sleep is at its lightest, is the difference between getting up easily and hitting snooze in a fog.
In “Wake up at” mode, enter your alarm time and the calculator counts backward: 15 minutes to fall asleep, then whole cycles of 90 minutes, producing a list of bedtimes from one to six cycles. Options with 5 or 6 cycles — 7.5 or 9 hours of actual sleep — are highlighted, because that range matches what sleep research recommends for most adults. In “Go to bed now” mode the same math runs forward from the current clock time, giving you a set of alarm times to choose from. Everything is calculated locally in your browser; no times, habits, or personal data ever leave your device.
The tool is most useful in everyday situations where the default advice of “get eight hours” does not fit: an early flight where you must choose between 6 hours and 7.5, a late night when you want the least-bad alarm time, or shift work where bedtime moves around the clock. It also helps night owls see concretely why an 11:00 p.m. bedtime pairs badly with a 6:00 a.m. alarm.
A practical tip: cycle length varies from person to person, roughly 80 to 110 minutes, so use the suggested times as a starting point. If you consistently wake groggy, shift your alarm or bedtime by 15-minute steps for a few nights until mornings feel easier — that offset is your personal correction to the 90-minute average.