Circadian Rhythm Calculator

The two-process model of sleep regulation

The scientific foundation of this calculator is the two-process model. The model proposes that two independent biological processes interact to determine your level of alertness at any moment:

Process S (Homeostatic Sleep Pressure): Adenosine accumulates in the brain during all waking hours. The longer you have been awake, the higher the sleep pressure.

Process C (Circadian Drive): The suprachiasmatic nucleus (SCN) in the hypothalamus generates a sinusoidal alertness signal on a roughly 24-hour cycle. This signal opposes Process S during waking hours — keeping you alert despite rising sleep pressure — and then drops sharply in the evening, opening the "sleep gate."

Chronotype offsets

Chronotype is the genetically influenced tendency to prefer earlier or later timing. The calculator applies an offset based on your chronotype selection: Early chronotypes shift approximately −1.5 hours earlier; Late chronotypes shift approximately +1.5 hours later.

The role of light in resetting your clock

The SCN pacemaker is entrained primarily by light — specifically blue-wavelength light. Morning light exposure advances the clock; evening light delays it. This is why bright evening light maintains late chronotype, and why morning light exposure is the most efficient behavioral tool for shifting your rhythm earlier.