# Circadian rhythm calculator

> A free, no-signup energy curve calculator at https://reassign.ai/circadian-rhythm-calculator,
> from the makers of Reassign, a 24-hour circular day planner.
> Enter your usual bedtime and wake time; the tool draws your predicted alertness
> across 24 hours on a circular dial. Everything runs in the browser: no account,
> no wearable, nothing stored or sent. Last verified: 2026-08-13.

## What it computes

Inputs: usual bedtime and usual wake time. Optional weekend split (the MCTQ's
core questions): weekend bedtime, weekend wake time, and whether an alarm wakes
you on weekends.

Outputs:
- An energy curve across 24 hours, drawn on a 24-hour dial (or a flat timeline view).
- Morning peak window: typically 2 to 4 hours after waking, the sharpest stretch of the day. Schedule deep work, writing, hard problems, and big decisions here.
- Afternoon dip window: about 11 hours after the mid-sleep point, and endogenous (it arrives with or without lunch). Schedule email, admin, errands, routine calls, a walk, or a nap under 20 minutes.
- Evening peak window: a second, slightly lower crest. Schedule the side project, creative work, or planning tomorrow. One hard thing fits here.
- Groggy window: roughly the first 75 minutes after waking (sleep inertia). Ease in, go light.
- Wind down: from about 2.5 hours before bedtime. Slow down; hard work this late borrows energy from tomorrow.
- Mid-sleep point: the clock time halfway through a normal night, the standard chronotype marker.
- With the weekend split on: MSFsc, the free-day mid-sleep corrected for catch-up sleep (the Munich Chronotype Questionnaire's chronotype marker, Roenneberg's correction, skipped when an alarm truncates free days), and social jet lag, the gap in hours between the work-day and free-day mid-sleep points.
- A chronotype lean (early, intermediate, or late) from the mid-sleep point (MSFsc when available).

## The model

The two-process model of sleep regulation (Borbély, 1982):
- Process C (circadian): a biphasic body-clock wave anchored to the mid-sleep
  point. Its afternoon trough sits about 11 hours after that point. Two peaks
  flank a real post-lunch dip; the dip is endogenous, not caused by eating.
- Process S (sleep pressure): builds across waking hours, clears during sleep.
  It tilts the evening down, so the second peak is lower than the first.
- Sleep inertia: a short grogginess window after waking, fading over roughly 75 minutes.

This is a population estimate from sleep timing: a prediction, not a
measurement. It does not know about caffeine, workouts, actual sleep history,
or how the person feels.

## Relation to the Reassign product

The Reassign app's energy layer uses the same model shape, calibrated to the
user: built from actually logged sleep, bent by tracked intakes (coffee, meals),
tracking sleep debt across a 14-night window, and tuned by the user's own
energy check-ins. That calibrated version is part of Pro; this calculator is the
free population model. See https://reassign.ai/features/energy.

## Related reading

- Energy-based planning: https://reassign.ai/resources/energy-based-planning
- Energy management vs. time management: https://reassign.ai/resources/energy-management-vs-time-management
- ADHD energy management and peak hours: https://reassign.ai/resources/adhd-energy-management
- The energy curve feature: https://reassign.ai/features/energy