Why 90 minutes per cycle?
Research shows complete sleep cycles (light, deep, REM) average 90 minutes in most adults.
Health Fitness
Circadian Rhythm Calculator
Calculate circadian rhythm with your details.
Personalized estimate
Use realistic inputs for the most useful results.
What this calculator does
Circadian rhythm is your 24-hour biological clock regulating sleep, wakefulness, and hormones. Sleep occurs in ~90-minute cycles. Aligning bedtime with cycles improves waking alertness.
How it works
Work backward from wake time. Each cycle is 90 minutes plus 15 minutes to fall asleep. Calculate bedtimes for 4, 5, or 6 cycles.
Formula
Bedtime = Wake Time - (Cycles × 90 min) - 15 min. 4 cycles = 6 hr, 5 cycles = 7.5 hr, 6 cycles = 9 hr.
Tips for using this calculator
- Use 4-cycle (6 hr) only occasionally—most need 7-9 hours
- Maintain consistent wake times even on weekends
- Bright light within 30 minutes of waking reinforces rhythm
- Dim lights 1-2 hours before bed for melatonin
- Avoid caffeine after 2 PM and alcohol before bed
Frequently asked questions
Is 6 hours enough?
For most adults, no. 6 hours (4 cycles) is insufficient for long-term health. 7-9 hours recommended.
What if I can't fall asleep at calculated time?
Calculator assumes 15 min to fall asleep. Go to bed earlier if you typically take longer.