Sleep Calculator
Find the best bedtime or wake-up time based on 90-minute sleep cycles.
Go to bed at one of these times
Times assume about 14 minutes to fall asleep. The more cycles the better — 6 cycles (9 hours) is ideal for most adults, and even the shortest option lets you wake at the end of a cycle rather than mid-sleep.
Use on any page (bookmarklet)
Want Sleep Calculator without leaving the page you're on? Drag the button below to your bookmarks bar, then click it on any website to open Sleep Calculator right there — it runs entirely in your browser.
Use responsibly: a bookmarklet runs on whatever page you click it on. Avoid sensitive sites such as online banking, payment, or healthcare pages — you run it at your own risk. Everything is processed locally and no data is sent anywhere. See our Terms.
- Show your bookmarks bar if it's hidden — Ctrl+Shift+B (⌘+Shift+B on Mac).
- Drag the button above onto the bookmarks bar.
- Open any website and click the bookmark — the Sleep Calculator panel appears in the top-right corner. Use ✛ to move it between corners, or ›/‹ to tuck it against the edge and pull it back out.
- Click the bookmark again (or the ✕) to close it.
Note: a few sites with strict security policies may block bookmarklets.
Examples
Wake up at 7:00 AM
Input
Wake time 07:00
Output
Bedtimes: 9:46 PM · 11:16 PM · 12:46 AM · 2:16 AM
Going to sleep at 11:00 PM
Input
Bedtime 23:00
Output
Wake times: 6:14 AM · 7:44 AM · 5:14 AM (by cycle)
Short night
Input
3 cycles
Output
4.5 hours — still wakes you at the end of a cycle
About this tool
This free sleep calculator helps you find the best time to go to bed or wake up so you rise at the end of a sleep cycle instead of mid-sleep — the reason you sometimes feel groggy even after a long night. Enter the time you need to wake up and it works out ideal bedtimes; or enter when you're heading to bed and it shows the best wake-up times.
How to use
- Choose whether you're planning around a wake-up time or a bedtime.
- Pick the time (or tap Now if you're going to sleep right away).
- Pick one of the suggested times — each lands at the end of a 90-minute cycle.
Why sleep cycles matter
Through the night you move between light sleep, deep sleep, and REM in cycles of roughly 90 minutes. Waking during deep sleep is what leaves you groggy; waking at the boundary of a cycle feels much more natural. Aim for 5–6 complete cycles a night whenever you can.
Frequently asked questions
How does the sleep calculator work?
Sleep happens in cycles of about 90 minutes. Waking up at the end of a cycle — rather than in the middle of deep sleep — feels far more refreshing. The calculator counts backward from your wake time (or forward from your bedtime) in 90-minute cycles and adds roughly 14 minutes to fall asleep, then lists the best times.
How many hours of sleep do I need?
Most adults do best with 7–9 hours, which is 5 to 6 full cycles. The calculator shows 6, 5, 4, and 3 cycles (9h, 7.5h, 6h, 4.5h) so you can pick based on how much time you have. More cycles is generally better.
Why add 14 minutes to fall asleep?
On average people take 10–20 minutes to actually drift off after getting into bed. Adding ~14 minutes makes the suggested times more realistic so you finish a cycle right at your alarm.
Should I wake up at the earliest or latest time?
Either works because every suggestion lands at the end of a cycle. If you can, choose the option with the most cycles (longest sleep). The shortest option is there for nights when time is tight — it still beats waking mid-cycle.
Is the 90-minute cycle exact for everyone?
No — real cycles vary from about 80 to 110 minutes and change through the night. 90 minutes is a widely used average, so treat the times as a helpful guide rather than a precise alarm setting.
Does it store my times or work offline?
Everything is calculated in your browser with no upload and no account. Once the page has loaded it keeps working offline, and nothing you enter is saved or sent anywhere.
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