Secretion of GH coincides with the sleep onset, reaching peak levels within 1 h (10). Further, secretion of GH is associated with slow-wave sleep and is not influenced by rapid-eye movement sleep (11). Sleep appears to not only facilitate, but also to augment GH secretion, with GH pulse frequency nearly twofold greater than during awake hours (12). GHRH-stimulated GH secretion is inhibited following awakening when compared with the undisturbed sleep state, a possible role of SRIF. Sleep-onset, then, appears to promote pituitary GH release via alterations in somatostatin tone (13).
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