Report No: DOT/FAA/AM-02/20

Title and Subtitle: A laboratory comparison of clockwise and counter-clockwise rapidly rotating shift schedule, Part III: Effects on core body temperatures and neuroendocrine measures

Report Date: November 2002

Authors: Boquet A, Cruz C, Nesthus TE, Detwiler C, Knecht W, and Holcomb K

Abstract: Most researchers suggest that shift rotation in a forward or clockwise direction produces less disruption of circadian rhythms than those that rotate in a backward or counter-clockwise direction. This is based upon extrapolation from quasi-experimental studies of shift-workers and research on the effects of jet lag, which indicate that westward travel results in less disruption of circadian rhythms. The effect of direction of rotation on cortisol, melatonin, and core body temperature was examined in participants randomly assigned to either a clockwise or counter-clockwise shift rotation.

Twenty-eight participants worked a day shift (0800-1600) for one week followed by either a clockwise (n=14) or counter-clockwise (n=14) shift rotation for two weeks. Participants wore a flexible rectal temperature sensor for the three weeks of the study and were allowed to remove the sensor for 90 minutes each day. Saliva samples were collected at the end of the baseline week for later assay for melatonin and cortisol, and were time-locked to collection times during the two "shiftwork" weeks. No group differences were found for cortisol for either of the workweeks. The clockwise group, however, had a significantly greater increase in melatonin during the early morning shift, compared with the counter-clockwise group. Finally, the analyses of core body temperature revealed a significantly lower amplitude and a delay of the acrophase for the counter-clockwise group during both weeks of testing. While cortisol levels did not appear to differ based on direction of shift rotation, there were inherent differences with melatonin secretion and core body temperature.

Precisely why such differences exist remains unclear but may be similar to that seen with circadian resynchronization during westward travel. Furthermore, it is not clear if these differences would persist in individuals exposed to longer periods of shift rotations.

Key Words: Shiftwork, Cortisol, Melatonin, Core Body Temperature, Circadian Rhythms, Clockwise, Counter-Clockwise

No. of Pages: 19