Long-term adjustment of circadian rhythms to a rotating shiftwork schedule.

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Abstract
The long-term effects of a weekly rotating day-night shift schedule on circadian rhythms were studied. It was performed 3 yr after the introduction of an increase in night work. Six male typesetters were observed in connection with the beginning and end of the nightshift week and in connection with their free days in the day week. Measurements were taken of body temperature during the awake and sleep periods and of self-rated activation. For the same subjects a comparison was also made of variations in awake temperatures and self-rated activation after 1 mo., 1 yr and 3 yr on the new shift schedule. Results showed that after 3 yr of experience the awake temperature curves had a flattened shape in connection with the beginning and end of the nightshift week and also during the free days. During sleep there was a short-term adjustment of the temperature curves within the nightshift week. Comparisons over years revealed significant changes in the course of variation in body temperature during night and day work, while no significant changes were found for self-rated activation. The possibility that experience had a differential influence on the phase adjustment of temperature maxima compared to minima was discussed.