Why cycles matter for shift work

Planning in 90-minute blocks reduces mid-cycle wake-ups that worsen sleep inertia during demanding schedules.

Shift work challenges circadian alignment. Using full 90-minute cycles helps you wake at lighter sleep stages, improving alertness. Most adults do best with 4-6 cycles per 24 hours (6-9 hours), which can be split around shifts when necessary.

General rules

  • Anchor wake time to the first immovable commitment (shift start, commute).
  • Target 4-6 cycles across 24 hours; split into a main block plus a strategic nap if needed.
  • Keep a 10-20 minute fall-asleep buffer; adjust with lived data.
  • Protect the final 1-2 cycles from caffeine, heavy meals, and bright light.

Fixed early shift (e.g., 05:00-13:00)

Required wake time: 03:50. Choose five cycles (7h30m) plus a 14-minute buffer.

  • Main sleep: 20:06-03:50 (5 cycles).
  • Fallback: four cycles 21:36-03:50 plus a 20-30 minute nap at 14:00.

Evening shift (e.g., 15:00-23:00)

Required wake time: 12:30. Choose five cycles.

  • Main sleep: 04:46-12:30 (5 cycles).
  • On busy weeks: four cycles 06:16-12:30 plus a one-cycle nap 14:15-15:45.

Overnight shift (e.g., 23:00-07:00)

Required wake time: 21:30. Choose five cycles, split 3+2 while adapting.

  • Before shift: three cycles 12:16-16:46.
  • After shift: two cycles 08:04-11:04.
  • Stabilize: move toward one five-cycle block 08:04-15:34 on settled weeks.

Rotating weeks

Transition by shifting bedtime 30-45 minutes per day toward the new schedule for 3-5 days. Use bright morning light and dim evenings to move the body clock.

How to use the calculator

  1. Set the must-hit wake time for your next workday.
  2. Choose 4-6 cycles and set the buffer between 10-20 minutes.
  3. Tap options and pick the one that best fits your routine and commute.
  4. Review alertness 30 minutes after waking; adjust cycles by ±1 as needed.

Research notes

Recommendations draw on circadian and sleep inertia literature; see our References page for source links and commentary.