Calculate sleep cycle

Count backward from a fixed wake time to two bedtimes separated by 90-minute sleep intervals.

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ToolSleep Cycle Calculator
Input
Output
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Preview

How it works

The entered sleep-onset delay is removed first, followed by either 6 × 90 minutes for 9 hours or 5 × 90 minutes for 7.5 hours. The clock wraps through midnight and returns times without dates; 90 minutes is the midpoint of the approximate 80-to-100-minute cycle range, not a measured personal cycle.

Formula
candidate bedtime = wake time − sleep-onset delay − 90 minutes × cycle count
  • Sleep-onset delay is required because assuming 15 minutes would move both candidate times.
  • Only the 7.5- and 9-hour intervals are shown, avoiding the former 4.5- and 6-hour suggestions.

Worked example

Waking at seven
The bedtimes that land on a cycle boundary
Input
											Wake time: 07:00
Minutes to fall asleep: 15
Output
												Bedtimes: 21:45, 23:15
Cycles: 6, 5
Recommended: 21:45 for 6 cycles
Hours if recommended: 9

When to use this

Work and school alarms can be counted backward into candidate lights-out times, sleep diaries compare scheduled time in bed with onset delay, and shift schedules use the previous-day result when the clock wraps past midnight.

Edge cases

  • Actual sleep cycles vary from roughly 80 to 100 minutes and change during the night, but both candidates retain fixed 90-minute blocks.
  • Night-to-night variation in time to fall asleep is compressed into one entered delay that shifts both results equally.
  • A time that wraps before midnight has no date attached, so 23:15 means the previous calendar day for a morning alarm.

References