The Circadian Rhythm & Light Tracker

How this schedule is built

Everything on this page is arithmetic performed on the two times you enter. There is no model of your physiology behind it and nothing is stored or sent anywhere. Knowing exactly where each number comes from is the difference between using the schedule well and treating it as an oracle, so here is the whole of it.

Two milestones are counted forward from your wake time: the sunlight window opens thirty minutes after you wake, and the eating window opens at one hour. Three are counted backward from your bedtime: the caffeine cutoff at ten hours before, the close of the eating window at three and a half hours before, the screen boundary at ninety minutes before, and the amber-light threshold at forty-five minutes before. Change either input and all of them shift with it.

Why the offsets sit where they do

The morning light anchor reflects the best-established finding in circadian research: light is the dominant signal that sets the master clock in the suprachiasmatic nucleus, acting through melanopsin-containing retinal cells that respond to intensity and timing rather than to what you consciously see. Indoor lighting typically sits between 100 and 500 lux; an overcast morning outdoors is 1,000 to 10,000. That gap, not the duration, is what makes stepping outside worth more than sitting by a lamp.

The ten-hour caffeine cutoff is deliberately conservative. Caffeine’s half-life in a healthy adult is commonly cited at three to six hours, but it varies several-fold between people depending on CYP1A2 activity, hormonal contraceptive use, pregnancy and smoking. Ten hours leaves most of that variation covered. If you sleep well after an evening coffee, this milestone is not for you.

The three-and-a-half-hour eating boundary, the ninety-minute screen boundary and the forty-five-minute amber threshold are practical conventions rather than precise thresholds. Late eating and evening light exposure are both associated with delayed sleep onset and lighter sleep in the literature, but the exact number of minutes at which the effect becomes meaningful is not settled, and it plainly differs between individuals. Treat them as a shape for the evening, not a deadline.

The organ clock alongside it

The phase names draw on the traditional Chinese organ clock, which divides the day into twelve two-hour periods, each associated with an organ system. It is included because it is a genuine and long-standing way of thinking about daily rhythm, and because readers of this site are interested in it. It is not offered as physiology. Some of its divisions rhyme with what chronobiology has since described, most notably the emphasis on morning digestion and late-night rest; many do not. Read it as a tradition worth knowing rather than as a second, parallel science.

What the tool cannot know

It does not know your chronotype, and the spread between a strong morning type and a strong evening type is a matter of hours, not minutes — which is why the presets are a starting point rather than a diagnosis. It does not know whether you work nights, cross time zones, have young children, take medication that affects sleep, or are managing a diagnosed sleep disorder. In any of those situations this schedule is the wrong starting point and a clinician is the right one. See the disclaimer for the reasoning in full.

Used sensibly, the value is not in hitting every time. Pick the one that is furthest from what you currently do — for most people that is either morning light or the evening screen boundary — and hold it for a fortnight before adding another. A schedule you follow loosely for a month is worth more than a perfect one you abandon in three days.