The circadian clock is a transcription-translation negative feedback loop: clock proteins activate their own genes, then their own protein product represses that activation, producing a self-sustaining ~24 hour oscillation.
d[PER]/dt = synthesis/(1+[PER/CRY]^n) - degradation
light resets phase toward zeitgeber
- Cycle eccentricity — how much the clock's trajectory deviates from a perfectly even 24h loop.
- Cycle speed (period) — the free-running period length of the molecular oscillator absent external cues.
- Light entrainment strength — how strongly a light/dark zeitgeber pulls the internal clock phase into sync with the environment.
- Feedback repression — how strongly accumulated CLOCK/BMAL1-driven proteins (PER/CRY) repress their own transcription, the core negative feedback.
Jet lag and shift work desynchronize this exact molecular loop from the light-dark cycle, and chronotherapy now times drug delivery to the body's internal clock phase rather than the wall clock.