Laboratoire de Physiologie et de Psychologie Environnementales
21 rue Becquerel
67087 Strasbourg Cedex, France
ulsatile hormone secretion appears to be a common characteristic of numerous endocrine systems, including the hypothalamo-pituitary-adrenal system, the renin-angiotensin system, gonads and pancreas. The temporal organization of the pulses within the 24-hr period may be primarily controlled by an internal circadian clock, or may be primarily dependent on the sleep-wake cycle or may reflect both of these influences. The circadian clock and sleep interact in modulating the amplitude, the frequency and the distribution of hormonal pulses over the 24-hr period.
The pituitary-adrenal hormones show widely differing relationships with sleep. Early studies have reported that GH levels increase during SWS episodes just after sleep onset, irrespective of the time of the day. Sleep exerts major inhibitory effects on nocturnal TSH secretion. This effect is due to a decrease in pulse amplitude but not to an alteration in the frequency or temporal distribution of TSH pulses. Prolactin pulses occur throughout the 24-hr period without significant variation in frequency, but with enhanced pulse amplitude during the sleep period. The modulation of LH pulsatility by sleep varies according sexual maturation. In contrast, ACTH and cortisol are little influenced by sleep but are rather controlled by an internal oscillator generating the well-known circadian rhythm which may be accounted by amplitude modulation of the pulses. Cortisol also exhibits a temporal association with slow-wave sleep which occurs during the declining phases of the secretory pulses and an association between nocturnal awakenings and increased cortisol release has been reported.
Other endocrine systems also are influenced by sleep processes. Renin, the key enzyme of the renin-angiotensin system, displays nocturnal oscillations which are strongly associated with the NREM-REM sleep cycles with levels that increase during NREM and decrease during REM sleep. The 24-hr renin variations are not circadian in nature but are related to sleep processes, which create the nycthemeral rhythm by increasing the frequency and the amplitude of the oscillations. Also it has been shown that insulin and parathyroid hormone pulses are amplified during sleep without any changes in frequency.
Taken together, these results indicate that sleep exerts profound modulatory effects on endocrine function. The underlying regulatory mechanisms and the physiological incidences of these effects remain to be identified.
