[Biologic rhythms. Nyctemeral variation in man]

CORTICOTROPIC AXIS: The nycthemeral pattern of cortisol is a good marker of the circadian clock. Cortisol levels fluctuate between a peak level, observed in the early hours of the morning, and a minimal level around midnight. This variability is considerably reduced or even abolished in Cushing s syndrome. THYREOTROPIC AXIS: The nycthemeral pattern of TSH secretion is dependent on both the circadian clock and sleep (which inhibits hormone secretion). The moment of the evening rise is a reliable marker of the circadian rhythmicity. SOMATOTROPIC AXIS: Growth hormone is essentially pulsatile. GH levels are often undetectable between pulses. The circadian rhythmicity plays only a minor role in the regulation of growth hormone secretion. LACTOTROPIC AXIS: Nycthemeral variations in prolactin secretion are mainly regulated by wake-sleep cycles; peak levels occur in the middle of the night. Prolactin secretion is also modulated by the circadian rhythmicity. GONADOTROPIC AXIS: Gonadotropins are secreted in pulses, following the pulses of GnRH secretion. In adult women, nycthemeral variations in LH are strongly modulated by the menstrual cycle. MELATONIN: The nychtemeral pattern of melatonin is an excellent marker of the circadian clock. Diurnal concentrations are low and vary little whereas peak levels are observed in the middle of the night. Melatonin rhythmicity is not influenced by sleep, but is dependent on exposure to light and darkness.