The Cortisol Clock
Cortisol is often called the "stress hormone," but its most fundamental role is as the body's internal clock signal. In healthy individuals, cortisol follows a predictable 24-hour rhythm orchestrated by the suprachiasmatic nucleus (SCN) — the brain's master circadian pacemaker. Cortisol peaks in the early morning and reaches its lowest point around midnight, a pattern conserved across virtually all diurnal mammals.
Cosinor Model of Circadian Rhythm
The classic cosinor model describes circadian cortisol as C(t) = mesor + amplitude * cos(2*pi*(t - acrophase)/24), where mesor is the 24-hour mean level, amplitude is the half-range of oscillation, and acrophase is the time of peak. This simulation adds ultradian pulsatility — the rapid 60-90 minute pulses that create cortisol's characteristic sawtooth waveform when measured at high temporal resolution.
Clinical Significance of Rhythm Disruption
A flattened cortisol rhythm (low amplitude, reduced peak-to-nadir ratio) is an independent predictor of mortality in breast cancer patients and is associated with chronic fatigue syndrome, major depression, and post-traumatic stress disorder. The cortisol awakening response (CAR) — the sharp rise in the first 30-45 minutes after waking — is used as a biomarker of HPA axis integrity and psychosocial stress.
Experiment with Disruption
Reduce the amplitude to model rhythm flattening seen in chronic illness. Shift the acrophase later to simulate delayed circadian phase (common in adolescents and shift workers). Increase ultradian pulse frequency to model acute stress responses. The visualization shows the full 24-hour cortisol profile with real-time clock position, making it easy to understand how the hormone level changes throughout the day.