Sleep Calculator: Find Your Best Bedtime
Enter the time you want to wake up, or the time you plan to go to bed. You get cycle-aligned times straight away, so your alarm lands at the end of a sleep cycle instead of the middle of deep sleep.
Calculate your sleep times
Enter either time. Results update instantly and line your alarm up with the end of a 90-minute sleep cycle.
- Optimal9:45 PM9 hours of sleep · 6 cycles
- Good11:15 PM7.5 hours of sleep · 5 cycles
- Minimum12:45 AM6 hours of sleep · 4 cycles
- Short night2:15 AM4.5 hours of sleep · 3 cycles
- Each bedtime already includes ~15 minutes to fall asleep.
- Optimal8:15 AM9 hours of sleep · 6 cycles
- Good6:45 AM7.5 hours of sleep · 5 cycles
- Minimum5:15 AM6 hours of sleep · 4 cycles
- Short night3:45 AM4.5 hours of sleep · 3 cycles
- Assumes you fall asleep about 15 minutes after getting into bed.
If you go to bed right now
Reading your device clock…
Based on 90-minute cycles plus ~15 minutes to fall asleep. Real cycles run 70–120 minutes, so treat these as targets, not stopwatch times.
What the calculator is doing
Sleep runs in cycles of roughly 90 minutes. Each one climbs from light sleep into deep sleep and then into REM before surfacing again. Waking during deep sleep is what produces the heavy, disoriented feeling called sleep inertia, and it is why eight hours can leave you worse off than seven and a half.
The calculator counts whole cycles forward from a bedtime or backward from an alarm, and adds about 15 minutes for the time it takes to fall asleep. The result is a short list of times that land you at a cycle boundary rather than in the middle of one.
Want a different angle on the same night? Use the bedtime calculator if you know your alarm, the sleep cycle calculator if you would rather pick the number of cycles, the hours of sleep calculator for the total between two times, or the sleep schedule calculator to build a routine you can keep every day.
Where the 90-minute rule breaks down
Cycle length is an average, not a constant. It varies between people (roughly 70 to 120 minutes) and lengthens across the night, with more deep sleep early and more REM toward morning. A schedule that is perfectly cycle-aligned on paper can still put your alarm mid-cycle by the fifth cycle.
Treat the output as a starting point. If you wake groggy on a cycle-aligned time, move your alarm in 15-minute steps for a few days and keep whichever version leaves you clearest 20 minutes after getting up. Consistency beats precision: the same wake time every day does more for how you feel than any single night's arithmetic.
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Frequently Asked Questions
Common questions about sleep cycles, bedtimes, and using the calculator.
Pick your wake-up time, then count back in 90-minute sleep cycles. Aim to wake at the end of a cycle rather than the middle. For a 7:00 AM alarm, good bedtimes are around 9:45 PM (6 cycles) or 11:15 PM (5 cycles), which also allows ~15 minutes to fall asleep. The calculator above does this math for you.
A full sleep cycle averages about 90 minutes and moves through light sleep, deep sleep, and REM sleep. The exact length varies from roughly 70 to 120 minutes per person and changes across the night, so treat 90 minutes as a planning estimate rather than a precise number.
Most adults need 7 to 9 hours per night, which is about 5 to 6 complete sleep cycles. Teenagers need 8 to 10 hours and school-age children need 9 to 12 hours. Waking at the end of a cycle often feels better than sleeping longer but waking mid-cycle.
Grogginess (sleep inertia) usually means your alarm interrupted deep sleep mid-cycle. Aligning your wake time with the end of a 90-minute cycle reduces this, which is exactly what this calculator helps you do.
Yes. The calculator adds about 15 minutes of time to fall asleep (sleep latency) before counting cycles, so the suggested times reflect when to actually get into bed.
It is an average, not a law. Cycle length varies between people and lengthens across the night, so a cycle-aligned alarm can still land mid-cycle. It is a better default than picking a round number of hours, but if you consistently wake groggy on a cycle-aligned schedule, shift your alarm in 15-minute steps and keep whatever works.