Circadian Rhythm Calculator: Your Body Clock Timeline

Your circadian rhythm is a roughly 24-hour clock run by the suprachiasmatic nucleus, a cluster of about 20,000 neurons in the hypothalamus. It cannot be read directly at home, but two of its landmarks sit at fairly steady distances from the times you sleep and wake, and everything useful on this page is worked out from those two.

This page is about the clock. If you want bedtimes that line up with 90-minute sleep cycles instead, the sleep cycle calculator linked below does that job.

The two landmarks

Melatonin onset. In dim light, melatonin starts to rise about 2 to 3 hours before your usual bedtime. Sleep labs measure it from saliva and call it DLMO, dim light melatonin onset, and it is the standard way researchers tell what time a person’s body thinks it is. The calculator places it 2 hours before the bedtime you enter.

The body-temperature low. Core temperature bottoms out in the second half of the night, usually 2 to 2.5 hours before you would wake on your own, and about 7 hours after melatonin onset. The calculator uses 2.5 hours before your wake time. This point matters more than it sounds, because it decides which way light pushes you.

Light pushes the clock, and the direction depends on the hour

Light reaching your eyes after the temperature low moves the clock earlier, so morning light is what makes an evening person sleepy sooner. Light before the low, in the evening and the first half of the night, moves it later. That is the whole reason a bright phone at midnight and a walk outside at 8 AM pull in opposite directions. Outdoor daylight is tens of times brighter than a lit room, which is why a morning walk does more than any lamp.

The hardest time to fall asleep is just before bedtime

Researchers call the stretch from about 3 hours to 1 hour before your usual bedtime the wake maintenance zone, or less kindly the forbidden zone (Peretz Lavie named it in the 1980s). The clock pushes hardest for alertness here, to carry you through the end of the day. Going to bed an hour early “to catch up” usually means lying awake in it. Shifting earlier works better in 15 to 30 minute steps, with the morning light doing the work.

Caffeine needs a longer runway than people give it

In Christopher Drake’s 2013 study, 400 mg of caffeine taken 6 hours before bed still cut measured sleep by more than an hour, and the people taking it did not notice. A strong coffee shop drink can carry 300 mg or more. So the cut-off here is 6 hours before your bedtime, and for a large dose that is the minimum rather than a safe margin.

How rough are these times?

Fairly. The offsets are averages from people sleeping on schedules of their own choosing, and they drift in shift workers, after travel, and with age. Treat each time as the middle of a band about an hour wide. The pattern matters more than the minute: bright mornings, dim evenings and a wake time that does not move at weekends will shift a clock that a perfect bedtime never touches.