Individual differences in circadian rhythm parameters and short-term tolerance to shiftwork: a follow-up study.

The relationship between individual differences in the phase and amplitude of circadian rhythms and tolerance to shiftwork has been the subject of several studies. Those studies recorded circadian rhythms and shiftwork tolerance at approximately the same time. The present study aimed to examine the predictive relationships between the amplitude, phase, and mesor of 24 h rhythms obtained before exposure to shiftwork, and subsequent indices of tolerance measured after one and three years of shiftwork. The results revealed some stable relations between the various rhythm parameters and subsequent tolerance measures. Workers who had a higher mesor of positive moods, and a lower mesor of negative moods and fatigue, before entering shiftwork tended to tolerate shiftwork better. Further, those whose heart rate rhythm showed an earlier acrophase had better subsequent sleep quality scores, while those with a smaller amplitude of their temperature, negative mood and fatigue rhythms showed better night-shift tolerance.