THE CIRCADIAN THYROTROPIN RHYTHM IS DELAYED IN REGULAR NIGHT WORKERS

In order to determine whether the circadian thyrotropin (TSH) rhythm i s adapted to a night-active schedule, plasma TSH and body temperature were measured for 28 h every 10-min in 8 regular night workers and in 8 day-active subjects. In night workers, the shift of 8-h in the sleep period induced a mean shift of 6 h 30 min of the TSH acrophase which remained located, as in day-active subjects, at about the time of slee p onset. The nadir of the body temperature rhythm was shifted by an eq uivalent amount and occurred systematically during the sleep period, s o that both parameters maintained a fixed phase relationship. TSH and temperature rhythms had similar amplitudes in the two groups. However, mean TSH values in night workers returned more rapidly to basal value s. These results demonstrate that, together with body temperature, TSH acrophase is adapted to regular night work, suggesting that TSH may b e a good index for evaluating the orientation of the endogenous clock.