Calculate optimal bedtimes and wake times based on 90-minute sleep cycles.
Enter the time your alarm must go off and this tool works backward in 90-minute sleep cycles to suggest bedtimes that place the alarm near the end of a cycle, the point where waking feels easiest. Enter a bedtime instead and it returns wake times the same way. The model assumes an average 90-minute cycle plus roughly 15 minutes to fall asleep, so every suggestion is an estimate, not a promise. The surprise for most users: it never tells you how many cycles to sleep. You pick the option that fits your schedule, and your own sleep need decides how much of it to use.
A concrete input and expected output from the current implementation.
Input
Wake at 7:00 AM
Expected output
Suggested bedtimes: 9:45 PM (6 cycles, in bed 9h 15m), 11:15 PM (5 cycles, 7h 45m), 12:45 AM (4 cycles, 6h 15m), 2:15 AM (3 cycles, 4h 45m)
Counting back from 7:00 AM in 90-minute steps gives cycle endings at 5:30, 4:00, 2:30, 1:00, 11:30 PM and 10:00 PM. Subtract the tool's assumed 15-minute fall-asleep delay and those endings become the suggested bedtimes, so 7:00 AM lands on a cycle boundary: 11:15 PM plus 15 minutes to fall asleep plus 7.5 hours of sleep (5 cycles) equals 7:00 AM.
Sleep is modeled as a repeating unit of about 90 minutes: roughly an hour of non-REM sleep followed by REM. The tool anchors on the time you set, treats it as a cycle boundary, and adds or subtracts whole cycles, so every suggestion differs from the next by exactly 90 minutes. In the bedtime direction it also subtracts a fixed sleep latency, typically about 15 minutes, because the clock starts in bed, not asleep.
Give a wake time and it lists bedtimes for different cycle counts; give a bedtime and it lists wake times after the same counts. Both lists are the same table read forward or backward. Some versions let you change the cycle length or the fall-asleep allowance, but the defaults of 90 minutes and 15 minutes reflect the commonly cited sleep-cycle average.
Each option is a different number of cycles, and the calculator cannot know how much sleep you need. Most adults function best on seven to nine hours, which is five to six cycles, so those two are usually the sensible options. Pick the bedtime closest to the one your schedule allows rather than the earliest or latest one.
That can happen, and it is not a failure of the tool. The 90-minute model is a population average, so your own cycles may end elsewhere, and how you feel on waking depends mostly on sleep debt, consistency and total sleep time. Use the suggestions as a starting point, then adjust toward the times where you actually wake easiest.
The example and behavioral notes were checked against the browser implementation. Standards and primary references below define the relevant format, formula, or platform behavior.
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