Marrin, K., Drust, B., Gregson, W., Morris, C., Chester, N., & Atkinson, G. (2009). The melatonin responses to exercise are moderated by time of day. A paper presented at the 14th Annual Congress of the European College of Sport Science, Oslo, Norway, June 24-27.

This study compared salivary melatonin levels during and after steady-state exercise between the morning and afternoon. Relationships between exercise-mediated changes in melatonin and body temperatures were explored. At two times of day (08:00 and 17:00 hours), males (N = 7) completed 30 minutes of cycling at 70% peak oxygen uptake followed by 30 minutes of rest. Salivary melatonin levels were measured at baseline, 15 minutes into exercise, immediately post-exercise, and following 30 minutes of recovery. Oesophageal, rectal, intestinal, and skin temperatures were measured.

When averaged over all four time-points, melatonin was 15.6±17.7 pg/ml higher at 08:00 than at 17:00 hours (not statistically significant). Exercise generally increased melatonin during and after exercise compared with baseline. The significant change from baseline to post-exercise was higher in the morning compared to the afternoon. All core temperatures were lower in the morning, with diurnal variation being greatest for the intestines. Skin temperature did not differ between morning and afternoon. At 08:00 hours, changes in melatonin were positively correlated with oesophageal temperature, but not intestinal, rectal, or skin temperatures. At 17:00 hours, melatonin correlated positively with changes in all body temperature measurements.

Implication. Thirty minutes of moderate-intensity exercise mediates an increase in melatonin that is greatest in the morning when resting melatonin is higher and core temperature is lower. Relationships between melatonin and body temperature changes during exercise are positive and appear more robust in the afternoon than in the morning, irrespective of whether core or skin temperature is considered, supporting the notion that melatonin may have both central and peripheral influences during exercise.

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