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

2020; Lippincott Williams & Wilkins; Volume: 53; Issue: 6 Linguagem: Inglês

10.1249/mss.0000000000002575

1530-0315

Nicholas Ravanelli, Ollie Jay,

Climate Change and Health Impacts

ABSTRACT 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°C–7.5 W·kg −1 , 33°C–5.5 W·kg −1 , and 33°C–7.5 W·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°C ± 0.2°C, p.m.: 37.0°C ± 0.2°C, P < 0.01). The Δ 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°C–7.5 W·kg −1 (a.m.: 0.83°C ± 0.14°C, p.m.: 0.75°C ± 0.20°C; P = 0.20), 33°C–5.5 W·kg −1 (a.m.: 0.51°C ± 0.14°C, p.m.: 0.47°C ± 0.14°C; P = 0.22), and 33°C–7.5 W·kg −1 (a.m.: 0.77°C ± 0.20°C, p.m.: 0.73°C ± 0.21°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°C–7.5 W·kg −1 (a.m.: 0.67 ± 0.20 mg·cm −2 ·min −1 , p.m.: 0.62 ± 0.21 mg·cm −2 ·min −1 ; P = 0.55), 33°C–5.5 W·kg −1 (a.m.: 0.59 ± 0.13 mg·cm −2 ·min −1 , p.m.: 0.57 ± 0.12 mg·cm −2 ·min −1 ; P = 0.65), and 33°C–7.5 W·kg −1 (a.m.: 0.91 ± 0.19 mg·cm −2 ·min −1 , p.m.: 0.84 ± 0.15 mg·cm −2 ·min −1 ; P = 0.33). Whole-body sweat loss was not different between a.m. and p.m. for 23°C–7.5 W·kg −1 (a.m.: 579 ± 72 g, p.m.: 579 ± 96 g; P = 0.99), 33°C–5.5 W·kg −1 (a.m.: 558 ± 48 g, p.m.: 555 ± 83 g; P = 0.89), and 33°C–7.5 W·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°C and 33°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.