Circadian Modulation of GH Secretion

Although sleep is clearly a major determinant of the 24-h profile of GH secretion in man, there is also evidence for the existence of a circadian modulation, i.e., an intrinsic effect of the time of day. A careful re-analysis of an early study of normal subjects submitted to a 3-h sleep-wake cycle for a prolonged period of time indicated that the elevation of GH concentration following sleep onset was largest when the sleep episode occurred in the late evening, i.e., around the usual bedtime (26). The late evening and early part of the night appear to represent a period of increased propensity for GH secretion, as studies involving abrupt delays of the sleep period in very young adults have shown modest increases in GH pulsatility at this time of day (despite enforced wakefulness), suggesting the existence of a weak circadian modulation of GH secretion (14,15,27,28). A recent study using repeated injections of GHRH at 3-h intervals demonstrated the existence of a diurnal variation in peak GH response that was interpreted as reflecting a diurnal rhythm in somatostatinergic tone (29). Interestingly, elevated GH responses were already apparent in the early evening, prior to sleep onset.

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