Seasonal variation in adaptation to shiftwork

This study examined the effects of seasonal changes in daylength on adaptation to shiftwork. Shiftwork can be problematic when it conflicts with our natural daily or circadian rhythms. As synchronization of the internal clock that drives circadian rhythms is achieved primarily through exposure to the light-dark cycle, shiftwork schedules during the winter months, which significantly restrict shiftworkers' exposure to daylight, were expected to have particularly adverse effects on shiftworker health and well-being. A longitudinal study was conducted at the Vancouver International Airport in British Columbia, Canada from December 2001 until January 2003. Eighty-eight shiftworkers completed the Standard Shiftwork Index (SSI) and the 21-item Hamilton Depression Rating Scale in the summer, when daylight hours were longest, and in the winter, when daylight hours were shortest. The SSI provides six measures of shiftwork adaptation and four measures of individual difference factors associated with shiftwork adaptation. As predicted, there was a significant increase in psychological distress and depressed mood during the winter months, while sleep was more disturbed in the summer. Measures of physical health and psychosocial well-being, and relationships among explanatory and outcome variables, showed no seasonal effects. Across both seasons, neuroticism was the strongest predictor of adaptation to shiftwork. Findings provide evidence of a seasonal pattern of shiftwork adaptation and suggest that shiftworkers may be at risk for seasonal-type depression.