Circadian Rhythm Calculator

Written by Great Medico

Updated September 25, 2026 •2 min read

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Your circadian rhythm is a roughly 24-hour biological timing system shaped by light, behavior and internal clocks. This calculator does not measure that biological phase; instead, it uses a 90-minute sleep-cycle shortcut to suggest bedtimes from a desired wake time.

What this calculator does

The only active inputs are your target waking time and the number of minutes you expect to take to fall asleep. The calculator returns suggested sleeping times for four, five and six modeled sleep cycles, highlighting the five-cycle option.

How to use it

Choose the wake time you actually need and enter a realistic sleep-onset delay. Use the result as a bedtime-planning aid. If you are trying to shift jet lag, delayed sleep phase or another circadian problem, this tool does not ask for light exposure, chronotype, melatonin timing or time-zone change.

How the calculation works

The calculation works backward from wake time: bedtime = wake time − sleep-onset delay − (number of cycles × 90 minutes). It performs this for four, five and six cycles. No circadian phase marker, core body-temperature minimum or dim-light melatonin onset is calculated.

Example

For a 7:00 AM wake time and 15 minutes to fall asleep, the modeled five-cycle bedtime is 11:15 PM. The six-cycle option is 9:45 PM and the four-cycle option is 12:45 AM.

How to interpret the result

Interpret the output as a sleep-duration schedule built around a simplified cycle length, not as a measurement of your circadian rhythm. Consistent wake time and adequate sleep opportunity are usually more important than hitting a mathematically exact cycle boundary.

Limitations and notes

Real NREM–REM cycles vary across the night and between people, and circadian timing cannot be inferred from wake time alone. If your sleep schedule is persistently shifted, you cannot sleep when desired, or daytime functioning is impaired, a sleep professional can assess true circadian and sleep disorders.