Mutual interactions between neurohormones, sleep, and the circadian system have been extensively studied. Hormonal secretion is either influenced by sleep and is independent of circadian timing or is closely coupled with the light-dark cycle, although both processes ultimately interact with each other. Sleep has a strong effect on the levels of some hormones (e.g., growth hormone) but little effect on others that are primarily regulated by the circadian system (e.g., melatonin). The exact mechanisms through which sleep affects circulating hormonal levels are not well understood. Much more is known about how the circadian system influences the secretion of hormones. Under normal circumstances, behaviors and the circadian system are synchronized with an optimal phase relationship, and consequently, hormonal systems are exquisitely regulated. Every bit of information constitutes but one small component of a broader, more global neurohormonal picture. In this review, we attempt to divide this analysis into sections including the pineal gland, adenohypophysis, neurohypophysis, describing the reciprocal influence regarding sleep and various neurohormones.
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