Cortisol Circadian Rhythm — HPA Axis (2D)

This simulator computes the hypothalamic-pituitary-adrenal (HPA) axis as a real feedback loop rather than drawing a pre-baked curve: an excitatory pre-dawn drive feeds hypothalamic CRH, CRH drives pituitary ACTH, ACTH drives adrenal cortisol, and cortisol feeds back to suppress CRH and ACTH. Scrub the clock or hit auto-play to watch the 24-hour rhythm unfold, including the cortisol awakening response 30–45 minutes after your chosen wake time, adjust the negative-feedback gain to see the rhythm lose its shape without it, and fire an acute stress event at any time of day to see a spike ride on top of the circadian baseline and get pulled back down by feedback.