ultradian

Ultradian rhythms are recurrent biological cycles with a period shorter than 24 hours and that occur multiple times within a day. They are a key concept in chronobiology and are distinct from circadian rhythms, which complete a roughly 24-hour cycle, and infradian rhythms, which have longer than daily periods. Ultradian cycles can range from minutes to several hours and influence a variety of physiological and behavioral processes.

In humans, well-known ultradian patterns appear in sleep and hormone secretion. Sleep is organized into cycles that alternate between non-rapid eye movement and rapid eye movement stages roughly every 90 to 110 minutes, repeating several times through the night. Hormone release often occurs in pulses with ultradian timing, such as growth hormone surges about every 1.5 to 2 hours during sleep, and pulsatile secretion of hormones like luteinizing hormone and insulin. Cortisol also exhibits ultradian pulses in addition to its circadian rhythm, producing frequent daytime bursts. Other processes that may show ultradian variation include fluctuations in appetite-related signals, body temperature, and certain neuronal activity patterns.

Measurement typically involves longitudinal hormonal sampling, sleep monitoring with polysomnography, and analysis of time-series data to detect rhythmic bursts. Disruptions or misalignment of ultradian rhythms can be associated with sleep disorders, endocrine dysfunction, stress, shift work, and certain psychiatric conditions. Understanding ultradian rhythms helps illuminate the timing and coordination of bodily functions beyond the daily cycle.