Sleep Cycle Calculator

How it is calculated

Sleep is modelled as repeating 90-minute cycles, plus 15 minutes to fall asleep. For every option the tool uses offset = 15 + cycles × 90 minutes and offers 6, 5, 4 and 3 cycles (9, 7.5, 6 and 4.5 hours of sleep). If you choose a wake-up time, it subtracts the offset to give bedtimes; if you choose a bedtime, it adds the offset to give wake-up times. Times wrap past midnight on a 24-hour clock.

The 5-cycle option is highlighted as the default. The 90-minute length and the 15-minute delay are fixed averages. Real cycles vary between people and through the night, and falling asleep may take you more or less time, so treat the times as a guide and adjust to how you feel.

Common mistakes

• Going to bed at the listed time instead of getting into bed with the lights off; the 15 minutes to fall asleep are already included, so do not add it twice.
• Choosing the wrong direction: I want to wake up at returns bedtimes, while I'm going to bed at returns wake-up times.
• Reading the 02:15 option as healthy; 3 cycles is only 4.5 hours.
• Expecting exactly 90 minutes per cycle. It is an average and varies from person to person.