ABSTRACT
Human neuroendocrine and metabolic physiology is often influenced by behavioral states of sleep and wakefulness. Extensive research in this area has flourished due to the development of better assays of endocrine function, paralleling the growth of sleep research stimulated by polysomnography and noninvasive breathing measurement. Data have accumulated on the relative influences of circadian phase or sleep state on hormone secretion, energy expenditure, or end-organ targets of neuroendocrine output. Plasma levels of pituitary and other hormones also spontaneously fluctuate across the 24-h period. These endocrine rhythms have often been labeled either sleep related (when the predominant change in fluctuation is nocturnal) or circadian (when the rhythm appears to be regulated by an internal clock rather than periodic changes in the external environment). The predominant influences are intrinsic circadian rhythmicity and sleep, which interact to varying degrees to produce the characteristic 24-h rhythm of each hormone. Other factors such as meals and exercise may also cause some changes in hormone level (1).
