Free tool

Circadian rhythm calculator: your energy, drawn on a clock.

Your energy windows

Morning peak

8:00am to 10:30am

Good for deep work. Body clock high, sleep pressure still low: the sharpest stretch of your day.

Put here: deep work, writing, the hard problem, the decision you keep deferring.

Afternoon dip

1:00pm to 3:30pm

Lighter tasks, a break. The dip is built into the rhythm itself: it arrives with or without lunch.

Put here: email, admin, errands, routine calls. Or a walk, or a nap under 20 minutes.

Evening peak

5:00pm to 7:30pm

A second window for focus, a little lower than the morning: pressure has been building all day.

Put here: the side project, creative work, tomorrow's plan. One hard thing fits here.

Groggy 6:30am to 7:45am Sleep inertia. Ease in, go light: coffee, a plan for the day, nothing that needs your best.

Wind down from 8:30pm Slow down, screens off. Hard work this late borrows the energy from tomorrow.

Mid-sleep point 2:45am · chronotype leans intermediate

These windows are the average for your sleep times. Reassign draws the curve on your actual day: your logged sleep, your coffee, your blocks, on the same dial you plan on. That calibrated curve is part of Pro; the 7-day trial needs no card.

How the calculator works

It runs the two-process model of sleep regulation (Borbély, 1982), the same frame sleep researchers use. Process C is your body clock: a wave anchored to your mid-sleep point, the clock time halfway through a normal night. Process S is sleep pressure: it builds for every hour you are awake and clears while you sleep. Your energy at any hour is the wave minus the pressure, minus a short stretch of sleep inertia after you wake.

That is why the curve has the shape it does. The wave is biphasic, so two peaks flank a real dip about 11 hours after mid-sleep. Pressure tilts the evening down, which is why the second peak is lower than the first. And the whole thing moves with your sleep: a night owl's dip lands hours after an early riser's.

Find your peak productivity hours

The practical output is two windows. The morning peak lands roughly 2 to 4 hours after you wake: that is the slot for the work that needs your full head. The evening peak is the one most people schedule against by accident, filling it with television because the afternoon dip convinced them the day was done. Put one hard thing in it and see.

The pattern matches how most working days feel from the inside: analytic work goes better in the late morning, and a demanding meeting costs less before mid-afternoon than after it. The calculator moves those windows to your sleep times; the shape itself is still the population estimate.

Your chronotype, without the twenty questions

Chronotype quizzes ask twenty questions to estimate one number: your mid-sleep point. The Munich Chronotype Questionnaire, the standard research instrument, anchors on the same marker, and its core questions are just four times of day: when you sleep on work days, and when you sleep on free days. Tick Different on weekends in the calculator above and you have answered them. It then computes MSFsc, your free-day mid-sleep corrected for the sleep you catch up on, which is the number chronotype research actually uses. Before about 2:00 leans early, after about 4:00 leans late, and the wide middle is intermediate.

The same two schedules give you your social jet lag: the gap between your work-day and free-day mid-sleep. An hour or more means your work week runs against your body clock, and Monday morning behaves like a flight one time zone over.

A later chronotype is not a fault to fix. It is timing to plan with. The curve does not tell you to get up earlier: it tells you where your good hours actually are, wherever they are.

A prediction, not a measurement

This is a population estimate from two times of day, not a reading taken off your body. It knows nothing about your caffeine, your workouts, your actual sleep last month, or how you feel right now, and it can be wrong. It is a starting shape to argue with when you choose an hour, not a verdict.

The energy curve in Reassign is the calibrated version: built from your actual logged sleep, bent by coffee and meals as you track them, tracking sleep debt across 14 nights, and tuned by how you say you feel. This page is the free population model. The app draws yours.

What is a circadian rhythm calculator?
A tool that estimates your alertness across the day from your sleep timing. This one uses the two-process model of sleep regulation: a body-clock wave set by your mid-sleep point, minus the sleep pressure that builds while you are awake. You get a morning peak, an afternoon dip, and an evening peak, placed for your schedule.
When am I most productive?
For most sleep schedules the model places two peaks: one about 2 to 4 hours after waking, and a second one in the early evening. The exact hours shift with your sleep timing. Enter your times above and read the peaks off the dial.
How do I find my chronotype without a quiz?
Use your mid-sleep point: the clock time halfway through a normal night. It is the standard chronotype marker in research instruments like the Munich Chronotype Questionnaire. Before about 2:00 it leans early, after about 4:00 it leans late, and the wide middle is intermediate. For the marker chronotype research uses, tick Different on weekends in the calculator: it then computes MSFsc, your free-day mid-sleep corrected for catch-up sleep, from the same four times of day the MCTQ asks about.
Why do I crash in the afternoon?
Human alertness is biphasic. The post-lunch dip is built into the circadian rhythm itself, and it shows up whether or not you eat lunch. The useful response is scheduling: put lighter tasks in the dip and protect the peaks for the work that needs your head.
What is social jet lag?
The shift between your sleep timing on work days and on free days, like living one time zone away from your own body clock. Tick Different on weekends in the calculator and it computes yours: the gap between your work-day and free-day mid-sleep points, in hours. An hour or more is common and worth planning around.
Does one short night change the curve?
It would, but this calculator does not ask about last night: its curve comes from your usual times only. In the model, a short night carries extra sleep pressure into today, so the whole curve sits lower and the peaks flatten a touch. Real sleep debt builds over many nights, which is what the Reassign app tracks across a 14-night window.
Do I need a wearable or an account?
No. Two times of day are enough for the population model. Everything is computed in your browser: nothing is stored, nothing is sent anywhere.
Is this the same as the energy curve in Reassign?
Same model shape, different depth. This page draws the population estimate for your sleep times. The app draws the curve from your actual logged sleep, bends it with caffeine and meals, tracks sleep debt, and calibrates to how you say you feel. That version is part of Pro.