Positive Relationship between Endogenous Melatonin and Core Temperature Responses to Exercise

At rest, circadian variation in the vaso-active substance, melatonin, is inversely related to that of core body temperature (Tcore). Although ingestion of exogenous melatonin has been reported to lower Tcore by approximately 0.2°C during subsequent exercise (Atkinson et al. 2005, J Pineal Res, 39, 353-9), the relationship between endogenous melatonin and Tcore has not been investigated during and after exercise at different times of the day. PURPOSE: To examine the relationship between exercise-related responses of salivary melatonin and Tcore measured at different sites in the morning and afternoon. METHODS: At 08:00 and 17:00 h, seven male subjects (mean±SD age: 27±5 y) completed 30 min of cycling at 70% peak oxygen uptake followed by 30 min recovery. Salivary melatonin levels were measured at baseline, 15 min into the exercise, immediately post-exercise and following 30 min recovery. Oesophageal (Tes), rectal (Trec), intestinal (Tit), and skin (Tskin) temperature were monitored during trials. Data were analysed using time of day × time general linear models and within-subjects correlation analysis. RESULTS: Melatonin was generally higher at 08:00 than 17:00 h (P=0.059) whilst Tcore was significantly lower during the morning trials compared to the afternoon (P=0.003). This difference was much smaller for Tes compared to Tit and Trec. Melatonin, Tcore and Tskin all increased with exercise but the standardised effects of exercise were smaller for melatonin (0.64-1.03) than for Tcore (2.23-5.26) and Tskin (2.22-4.18). The within-subject correlation between Tes and melatonin was 0.65 and 0.67 in the morning and afternoon respectively (P=0.001). Correlations between Tcore and melatonin were also positive for the other core temperature sites, irrespective of time of day. The correlation between Tskin and melatonin was 0.40 (P=0.065) in the morning and 0.65 (P=0.001) in the afternoon. CONCLUSION: These data indicate that, although circadian rhythms in endogenous melatonin and core temperature are inversely related at rest, exercise mediates a parallel acute increase in both variables. It seems that the effects of exercise are relatively more pronounced on core body temperature than on melatonin, adding weight to the notion that melatonin is a more stable marker of circadian timing.