Calculate optimal sleep and wake times based on 90-minute sleep cycles for better rest quality.
Sleep runs in cycles that average about 90 minutes, and an alarm that fires mid-cycle can land in deep sleep, which is why some mornings feel worse than others. This calculator takes the time you want to wake up, or the bedtime you are planning, and works backward or forward in whole 90-minute cycles, returning bedtimes and alarm times that end on a cycle boundary. The surprise is that every option is really a choice of total sleep: one wake time can produce both a 10 PM and a 1 AM suggestion, and the 90-minute figure is an average, not a law. Results are targets, not guarantees.
A concrete input and expected output from the current implementation.
Input
Wake at 7:00 AM
Expected output
Suggested bedtimes, each a whole number of 90-minute cycles before 7:00 AM: 10:00 PM (6 cycles, 9 hours), 11:30 PM (5 cycles, 7.5 hours), 1:00 AM (4 cycles, 6 hours), 2:30 AM (3 cycles, 4.5 hours).
The calculator steps backward from 7:00 AM in whole cycles: 07:00 minus six cycles (9 hours) is 10:00 PM, minus five cycles (7.5 hours) is 11:30 PM, and so on. Each option ends the final cycle exactly at the wake time, so the alarm rings at a cycle boundary; the hours shown are the total sleep each bedtime provides.
Sleep alternates between light, deep, and REM stages in cycles that average about 90 minutes, and waking at the end of a cycle, when sleep is lightest, causes less grogginess than waking from deep sleep. From a wake time the calculator subtracts 90 minutes repeatedly to list candidate bedtimes, and from a bedtime it adds the same step to list candidate wake times, so every result ends exactly at a boundary.
Each suggestion is one cycle away from the next, so the gap between adjacent options is exactly 90 minutes of sleep: four cycles give about 6 hours, five give 7.5, and six give 9. Choosing between a 10 PM and an 11:30 PM bedtime is therefore choosing between 9 and 7.5 hours of sleep, and the commonly cited adult target of 7-9 hours points to the five- and six-cycle options.
Not guaranteed. The 90-minute figure is a population average, your cycles vary from night to night and person to person, and alcohol, caffeine, stress, and sleep debt all reshape sleep architecture. Hitting a cycle boundary can reduce morning grogginess, but consistent timing and enough total sleep matter more than the exact minute.
No, it only maps times onto cycle boundaries and leaves the duration to you: each bedtime option carries an implied length, roughly 4.5, 6, 7.5, or 9 hours. The commonly cited adult target is 7-9 hours per night, so the five- and six-cycle options usually fit. Judge by how rested you feel over a week or two.
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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