Clockwise and counterclockwise rotating shifts: effects on temperature and neuroendocrine measures.

Introduction: Most researchers suggest that shift rotation in a clockwise (CW) direction produces less disruption of circadian rhythms than rotations in a counterclockwise (CCW) direction. This is based on extrapolation from quasi-experimental studies of shift workers and research on the effects of jet lag that indicate that westward travel results in less disruption of circadian rhythms. Methods: The effect of direction of rotation on cortisol, melatonin, and rectal temperature was examined in participants randomly assigned to either a CW (n = 14) or CCW (n = 14) shift rotation. Results: Results revealed lower amplitude [F(1,21) = 14.6, p < 0.05] and a delay of the acrophase [F(1,21) = 4.4, p < 0.05) in temperature for the CCW group. Sample time effects for cortisol and melatonin revealed normal circadian variation for both rotations, although melatonin levels on the midnight shift were clearly suppressed relative to baseline melatonin levels. Discussion: The changes observed in the temperature rhythm for the CCW rotation may be related to adaptation or shift-work hardiness; however, it is unclear if these differences indicate beneficial or detrimental changes for the individual. The fact that there were no significant effects of rotation condition for cortisol or melatonin argues against a detrimental effect. These findings are bolstered by performance and sleep data, reported elsewhere from this study, indicating a lack of significant differences as a function of rotation condition.