Internal dissociation of the circadian markers of the cortisol rhythm in night workers

To determine whether the circadian system of night workers is adapted to a night-active schedule, we submitted 11 night workers and 11 day-active subjects to a 10-min blood sampling procedure during their usual sleep-wake cycle, permitting a precise determination of circadian and ultradian cortisol variations. In night workers, the usual shift of 8 h in the sleep period was associated with a distortion of the normal 24-h cortisol rhythm. The acrophase exhibited a shift of similar to 6.5 h, whereas the quiescent period, abruptly interrupted by a large peak, underwent a shift of only 3 h and lasted for similar to 5 h, as in day-active subjects. Slow-wave sleep and sleep onset occurred during periods of low or decreasing cortisol secretory rates, whereas awakenings were associated with an increase in cortisol secretory rates. These results revealed that the circadian system of night workers only partially adapts to night work and that adaptation processes rely on an internal dissociation of the markers of the cortisol pattern, without disturbing the processes that couple cortisol release and specific sleep stages.