Endocrine Ultradian Rhythms During Sleep and Wakefulness

Abstract

Episodic hormone secretion appears to be a common characteristic of several endocrine systems, including the hypothalamic-pituitary axis, adrenal glands, gonads and the pancreas. Alterations in episodic secretion are associated with the pathophysiology of certain diseases. Correction of these pulsatile disorders with exogenous hormonal replacement therapy frequently cures the defect, the most notable examples of this phenomenon being the hypogonadotrophic disorders. While there is substantial evidence indicating that the central nervous system is at the origin of hormone pulsatility, the organization of the corresponding pacemaker structures remains to be elucidated. In particular, the question of unity versus multiplicity of pacemakers responsible for the ultradian rhythms in the 80–120-min range is debated (Lavie and Kripke 1981). In humans, included in this range is the time of recurrence of the episodic pulses of many hormones as well as a number of rhythms of apparently unrelated physiological and behavioural processes, such as the rate of urine flow, gastric motility and cognitive variables. One of the most prominent ultradian rhythms is the alternation of rapid eye movement (REM) and non(N)REM stages of sleep, which is accompanied by similar cycles of dreaming, penile erection and cardiac variability. Kleitman proposed the concept of a basic rest-activity cycle (BRAC), suggesting that the periodic recurrence of REM sleep reflects a fundamental physiological periodicity which also modulates brain functions during wakefulness (Kleitman 1963, 1982).