Sleep Time Calculator
Bedtimes that finish on 90-minute cycles before wake-up.
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Formula
Bedtime = wake time − (cycles × 90 min) − fall-asleep buffer
Work backward from your alarm: subtract whole 90-minute sleep cycles, then a few minutes for sleep latency. Waking at cycle boundaries tends to feel lighter than mid-cycle interrupts.
Worked example
To wake at 06:30 after five 90-minute cycles (~7.5 h asleep), be in bed around 22:45 — allowing ~15 minutes to drift off so sleep onset lands on a cycle boundary.
Where this can give the wrong answer
- Shorter cycle counts (3–4) suit naps or constrained schedules; six cycles may require a very early bedtime for a 06:30 alarm.
- The calculator lists alternate bedtimes for fewer cycles — trading sleep length for slightly later nights.
- Shift workers with daytime sleep should pick wake time as 'when you must get up' regardless of sun position.
FAQ
- No — cycles vary and include lighter sleep at the start. Ninety minutes is a planning heuristic popularised by sleep apps, not a clinical polysomnography measurement.