ultradian rhythm

An ultradian rhythm is a biological cycle that repeats more than once a day — its period is shorter than 24 hours, anywhere from minutes to a few hours. If a circadian rhythm is the slow daily tide, ultradian rhythms are the faster waves riding on top of it. The name literally means beyond a day in frequency: more cycles per day, not fewer.

Hormone pulses are the prime endocrine example. GnRH is released roughly hourly, ACTH and cortisol arrive in pulses every hour or two, and growth hormone and insulin also show ultradian bursts. Sleep itself is organized into about 90-minute cycles, and several hormones track that rhythm during the night.

Ultradian patterns matter because the body often reads the frequency or amplitude of the pulses, not just the average level. Two people with the same daily total of a hormone can have very different biology if one delivers it in crisp pulses and the other in a flattened pattern — a reason that smooth, continuous hormone delivery does not always reproduce normal physiology.