Release of human growth hormone during sleep is significantly related to slow, synchronized stages of sleep and therefore would seem to be controlled by related neural mechanisms. When sleep-waking cycles are reversed by 12 hours, the release of growth hormone with sleep is reversed; thus release does not follow an inherent circadian rhythm independent of sleep.

Human prolactin was measured in plasma by radioimmunoassay at 20 minute intervals for a 24-hour period in each of six normal adults, whose sleep-wake cycles were monitored polygraphically. A marked diurnal variation in plasma concentrations was demonstrated, with highest values during sleep; periods of episodic release occurred throughout the 24 hours.

Four normal young adults underwent partial or complete inversion of their sleepwaking cycles to determine the relationship of the nocturnal release of human prolactin to sleep. Prolactin release shifted immediately and completely with shifts of sleep onset of 3, 6 and 12 hr. Thus, the nocturnal rise is dependent on the occurrence of sleep and is not based on an inherent rhythm related to time of day. ( / Clin Endocrinol Metab 37: 436, 1973)

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