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To determine whether genetic factors control the expression of human circadian rhythmicity, we analyzed the 24-h profile of plasma cortisol in 11 monozygotic and 10 dizygotic pairs of normal male twins. Blood was sampled every 15 min, and sleep was monitored. Circadian rhythmicity was characterized by measures of amplitude, phase, and overall waveshape. Pulsatility was quantified by pulse frequency, pulse amplitude, and relative contribution of pulsatile vs. circadian variations. Data were analyzed by a procedure specifically developed for twin studies. Genetic control was demonstrated for the timing of the nocturnal nadir and for the proportion of overall temporal variability associated with pulsatility. Environmental effects were detected for the 24-h mean and the timing of the morning acrophase. The timing of the cortisol nadir is a robust marker of human circadian phase and is dependent, under entrained conditions, on the length of the endogenous period. Animal studies have shown that the endogenous period and the pattern of entrainment to exogenous 24-h periodicities are genetically controlled. Our results indicate that, despite the increased impact of social inputs, genetic factors also control human circadian rhythmicity.
Linkowski et al. (Mon,) studied this question.