The Change in Core Temperature and Sweating Response during Exercise Are Unaffected by Time of Day within the Wake Period

Introduction Exercise thermoregulation studies typically control for time of day. The present study assessed whether circadian rhythm independently alters time-dependent changes in core temperature and sweating during exercise at a fixed rate of metabolic heat production (H-prod) during the wake period. Methods Ten men (26 +/- 2 yr, 76.6 +/- 6.3 kg, 1.95 +/- 0.10 m(2)) cycled for 60 min in three combinations of ambient temperature and H-prod (23 degrees C-7.5 W center dot kg(-1), 33 degrees C-5.5 W center dot kg(-1), and 33 degrees C-7.5 W center dot kg(-1)) at two times of day (a.m.: 0800 h, p.m.: 1600 h). Rectal temperature (T-re), local sweat rate, and whole-body sweat losses were measured. Results Absolute T-re was lower at baseline in a.m. versus p.m. for all three conditions (a.m.: 36.8 degrees C +/- 0.2 degrees C, p.m.: 37.0 degrees C +/- 0.2 degrees C, P < 0.01). The Delta T-re was not altered by time of day (P > 0.22) and not different at 60 min between a.m. and p.m. for 23 degrees C-7.5 W center dot kg(-1) (a.m.: 0.83 degrees C +/- 0.14 degrees C, p.m.: 0.75 degrees C +/- 0.20 degrees C; P = 0.20), 33 degrees C-5.5 W center dot kg(-1) (a.m.: 0.51 degrees C +/- 0.14 degrees C, p.m.: 0.47 degrees C +/- 0.14 degrees C; P = 0.22), and 33 degrees C-7.5 W center dot kg(-1) (a.m.: 0.77 degrees C +/- 0.20 degrees C, p.m.: 0.73 degrees C +/- 0.21 degrees C; P = 0.80). The change in local sweat rate was unaffected by time of day (P > 0.16) and not different at 60 min in 23 degrees C-7.5 W center dot kg(-1) (a.m.: 0.67 +/- 0.20 mg center dot cm(-2)center dot min(-1), p.m.: 0.62 +/- 0.21 mg center dot cm(-2)center dot min(-1); P = 0.55), 33 degrees C-5.5 W center dot kg(-1) (a.m.: 0.59 +/- 0.13 mg center dot cm(-2)center dot min(-1), p.m.: 0.57 +/- 0.12 mg center dot cm(-2)center dot min(-1); P = 0.65), and 33 degrees C-7.5 W center dot kg(-1) (a.m.: 0.91 +/- 0.19 mg center dot cm(-2)center dot min(-1), p.m.: 0.84 +/- 0.15 mg center dot cm(-2)center dot min(-1); P = 0.33). Whole-body sweat loss was not different between a.m. and p.m. for 23 degrees C-7.5 W center dot kg(-1) (a.m.: 579 +/- 72 g, p.m.: 579 +/- 96 g; P = 0.99), 33 degrees C-5.5 W center dot kg(-1) (a.m.: 558 +/- 48 g, p.m.: 555 +/- 83 g; P = 0.89), and 33 degrees C-7.5 W center dot kg(-1) (a.m.: 796 +/- 72 g, p.m.: 783 +/- 75 g; P = 0.31). Conclusions The change in core temperature and sweating throughout a 60-min exercise bout in 23 degrees C and 33 degrees C were unaffected by circadian rhythm during the wake period when exercise intensity was prescribed to elicit comparable rates of H-prod, suggesting that scheduling thermoregulatory exercise trials for the same time of day is unnecessary.

Automated summary

This study found that the effect of circadian rhythm on core temperature and sweating during 60 minutes of exercise at 23 degrees Celsius and 33 degrees Celsius was the same when exercise intensity was prescribed to elicit comparable rates of metabolic heat production (H-prod), suggesting that scheduling exercise trials at the same time of day for thermoregulatory studies is unnecessary.