Why CRH and ACTH come in bursts
The HPA axis does not secrete steadily. The hypothalamus releases corticotropin-releasing hormone (CRH) in discrete pulses, which drive matching pulses of adrenocorticotropic hormone (ACTH) from the pituitary. These bursts recur on the order of roughly every 60 to 90 minutes, an ultradian rhythm sitting beneath the larger 24-hour circadian curve.
Pulsatile signaling is common in endocrine systems because target tissues can become desensitized to a constant signal. Delivering hormone in bursts keeps receptors responsive and lets the body encode information in pulse frequency and amplitude, not just average concentration.
How cortisol shapes its own rhythm
Cortisol, the downstream product of these pulses, feeds back on the hypothalamus and pituitary. This feedback both restrains the average level and helps sculpt the timing and size of subsequent pulses, so the rhythm is self-organizing rather than externally clocked alone.
The frequency and amplitude shift across the day, peaking in the morning. This is general educational information, not medical advice; clinical assessment of the HPA axis should be done by a qualified professional.