Circadian Rhythm Calculator
Circadian rhythm timeline
Landmarks are offsets from your habitual wake-up time, not from the clock.
The temperature minimum is the lever. Bright light after it pulls your clock earlier; bright light before it pushes it later. That one rule explains most successful jet lag and shift plans.
Sleep timing is downstream of the body clock, not the other way round. That is why forcing an earlier bedtime rarely works on its own, and why one well-timed walk outside can do more than an hour of extra time in bed.
The two systems that decide when you sleep
Sleepiness is the sum of two independent things. The first is homeostatic pressure: adenosine builds from the moment you wake and only clears during sleep. The longer you are awake, the heavier it gets. The second is the circadian signal: an alerting drive from your body clock that rises and falls on a roughly 24-hour cycle regardless of how long you have been awake.
You fall asleep easily when pressure is high and the circadian alerting signal has dropped. You lie awake when pressure is high but the clock is still pushing. That mismatch is the entire explanation for the 11 PM second wind and for jet lag.
The temperature minimum is the control knob
Your core body temperature bottoms out about two hours before your habitual wake time, and that point splits the day in two for the purpose of light exposure:
- Bright light after it advances your clock, making you sleepy earlier the following evening. This is the morning light everyone recommends.
- Bright light before it delays your clock, pushing sleep later. This is why the 4 AM phone check is worse than it looks.
One rule, two directions. Almost every practical jet lag or shift-work plan is an application of it, including the night shift plan.
Working with the timeline instead of against it
| Window | Use it for | Avoid |
|---|---|---|
| 2-5h after waking | Hardest cognitive work | Meetings you could move |
| 7-9h after waking | A 20-minute nap, admin, walking | Driving long distances, precision work |
| ~11h after waking | Training, anything physical | Winding down |
| ~13h after waking | Nothing demanding; dim the lights | Bright screens, intense exercise, alcohol |
Chronotype is real but not fixed
Genuine late chronotypes exist and the difference can be two or three hours. What is usually mistaken for chronotype, though, is an unanchored clock: no morning light, bright evenings, and a wake time that moves by three hours at the weekend. Fix those three and most people land closer to the middle than they expected.
Next: turn the timeline into an actual routine with the sleep schedule calculator, or pick the alarm with the best time to wake up guide.