Physiology of growth hormone secretion during sleep.

The temporal relation between the first few hours of sleep and the secretion of growth hormone (GH), which is present in normal persons of both sexes from early childhood until late adulthood, is reviewed. In adults the most reproducible pulse of GH secretion occurs shortly after the onset of sleep in association with the first phase of slow-wave sleep (SWS) (stages III and IV). In men approximately 70% of the GH pulses during sleep coincide with SWS, and the amount of GH secreted during these pulses correlates with the concurrent amount of SWS. Sleep-related secretion of GH appears to be primarily dependent on the release of growth hormone-releasing-hormone. Rodent and human studies have shown that growth hormone-releasing hormone injections decrease wakefulness and increase SWS. During the fourth decade of life (ages 30 to 40 years) the total amount of GH secreted over a 24-hour span decreases by two- to threefold. Similarly, the amount of SWS decreases dramatically over the same narrow age range. Because the sleep-onset GH pulse is often the major secretory output in adults, age-related decrements in sleep-related GH secretion likely play a major role in the hyposomatotropism of senescence.