Bright Light and Thermoregulatory Responses to Exercise

Abstract

The thermoregulatory responses to morning exercise after exposure to different schedules of bright light were examined. At 07:00 h, six males ran on two occasions in an environmental chamber (temperature = 31.4 ± 1.0 °C, humidity = 66 ± 6 %) for 40 min at 60 % of maximal oxygen uptake. Participants were exposed to bright light (10 000 lux) either between 22:00 - 23:00 h (BT_low) or 06:00 - 07:00 h (BT_high). Otherwise, participants remained in dim light (< 50 lux). It was hypothesized that BT_low attenuates core temperature during morning exercise via the phase-delaying properties of evening bright light and by avoiding bright light in the morning. Evening bright light in BT_low suppressed (p = 0.037) the increase in melatonin compared to dim light (1.1 ± 11.4 vs. 15.2 ± 19.7 pg · ml^-1) and delayed (p = 0.034) the core temperature minimum by 1.46 ± 1.24 h. Core temperature was 0.20 ± 0.17 °C lower in BT_low compared to BT_high during the hour before exercise (p = 0.036), with evidence (p = 0.075) that this difference was maintained during exercise. Conversely, mean skin temperature was 1.0 ± 1.7 °C higher during the first 10 min of exercise in BT_low than in BT_high (p = 0.030). There was evidence that the increase in perceived exertion was attenuated in BT_low (p = 0.056). A chronobiologically-based light schedule can lower core temperature before and during morning exercise in hot conditions.

body temperature - physical activity - circadian rhythm - melatonin

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