Physical Activity, Sedentary Behaviour and Melatonin Among Rotating Shift Nurses
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Background: Shift work is associated with increased risk of cardiovascular disease and cancer, where decreased melatonin has been proposed as an intermediate in the causal pathway. The influence of physical activity on melatonin has rarely been studied in an observational setting, and it may be important in mediating the effects of shift work. We aimed to assess the influence of energy expended during physical activity of different intensities on melatonin among rotating shift nurses. We hypothesized that physical activity before the night shift would lessen the decrease in melatonin production that occurs with exposure to light at night. Methods: 123 female rotating shift nurses working at Kingston General Hospital were recruited over a one-year period. Physical activity and sedentary behaviours for each participant were recorded during both a day and a night shift using activity diaries, and analysis was restricted to activities between 3 p.m. and 7 a.m. Concentrations of urinary 6-sulfatoxymelatonin, a melatonin metabolite, in morning void urine samples were analyzed for each shift. Results: The average age of participants was 41 years, and 60% were overweight or obese (body mass index ≥ 25 kg/m2). An average of 6.9 and 5.2 hours of sleep were reported after the day shift and night shift, respectively. Sedentary behaviours such as standing and television watching accounted for over half of the total reported energy expenditure. During the day shift, energy expended in moderate and vigorous intensity physical activity between 3 p.m. and 7 a.m. was negatively associated with melatonin levels (p=0.024, R2 = 0.09). During the night shift, energy expended in sedentary behaviours was negatively associated with melatonin levels (p=0.008, R2 = 0.03). Conclusions: Physical activity energy expenditure explains only a small amount of melatonin variation, suggesting that other factors are influencing melatonin production, or that melatonin production is minimally effected by these patterns of physical activity.