Entrainment of nocturnal pituitary-adrenocortical activity to sleep processes in man--a hypothesis.

The 24 hr patterns of plasma ACTH and cortisol concentrations are characterized by prominent circadian and ultradian oscillations. Usually both, nadir and acrophase of the circadian rhythm occur during sleep. This led us to re-evaluated the temporal relationship between sleep processes and nocturnal plasma ACTH and cortisol levels and the impact of several types of sleep manipulation (sleep delay, sleep disruption, sleep prolongation, sleep deprivation, and reversal of the sleep-wake cycle). Pituitary-adrenocortical activity appeared to be linked to the cyclic process of nocturnal sleep with inhibitory influences present during the first two sleep cycles. After initiation of the third sleep cycle stimulatory effects of sleep prevailed, lasting until awakening. The sleep associated influences appeared to act in concert with influences of circadian oscillators and resulted in an amplification of the circadian rhythm of pituitary-adrenal activity; they were strong enough to entrain the circadian rhythm of the pituitary-adrenal system to the sleep-wake cycle, as long as phase delays were moderate. However, with acute sleep-wake reversals the sleep associated influences were masked by the dominant effects of the circadian clock. In contrast, GH secretion appeared to be controlled primarily be sleep-associated mechanism with only minor circadian influences.