Chronobiology international
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Intolerance to shift work may result from individual susceptibility to an internal desynchronization. Some shift workers (SW) who show desynchronization of their circadian rhythms (e.g., sleep-wake, body temperature, and grip strength of both hands) exhibit symptoms of SW intolerance, such as sleep alteration, persistent fatigue, sleep medication dependence, and mood disturbances, including depression. Existing time series data previously collected from 48 male Caucasian French SW were reanalyzed specifically to test the hypothesis that internal synchronization of circadian rhythms is associated with SW intolerance and symptoms. ⋯ The number of desynchronized circadian rhythms (tau differing from 24 h) was greater in non-tolerant than in tolerant SW (chi square=38.9, p<.0001). In Former SW (i.e., 15 individuals assessed in follow-up studies done 1.5 to 20 yrs after return to day work), both symptoms of intolerance and internal desynchronization were reduced or absent. The results suggest that non-tolerant SW are particularly sensitive to the internal desynchronization of their circadian time organization.