Effect of 24-Hour Sleep Deprivation on Whole-Body Heat Loss in Older Males during Exercise in the Heat

ABSTRACT Background Older adults commonly experience sleep deprivation, which has been observed to alter thermoregulatory function in young adults. This may exacerbate risks of heat-related injury and illness during heat stress, due to age-related reductions in heat loss capacity. However, whether sleep deprivation modulates thermoregulatory function in older adults remains unclear. Thus, we aimed to assess the impact 24 h of total sleep deprivation has on whole-body heat loss during exercise-heat stress in older adults. Methods Following either a night of normal sleep or 24-h sleep deprivation, 10 older males (mean (SD); age 61 (4) years) performed three 30-min exercise bouts at incrementally increasing rates of metabolic heat production (150, 200, 250 W·m -2 ), each separated by 15-min rest in hot, dry conditions (40 °C, ~13% relative humidity). Whole-body total (dry + evaporative) heat loss and metabolic heat production were measured continuously using direct and indirect calorimetry, respectively, and used to calculate body heat storage. Results Peak whole-body total loss (mean difference [95% confidence interval]: -12 [-20, -3] W·m -2 ; P = 0.010) was reduced and body heat storage (0.60 [0.32, 0.89] kJ·min -1 ; P = 0.002) increased during high-intensity exercise following sleep deprivation compared to normal sleep. However, evaporative heat loss (-10 [-20, 1] W·m -2 ; P = 0.062) and dry heat gain (-2 [-9, 5] W·m -2 ; P = 0.512) were not different between conditions. Conclusions Sleep deprivation impaired whole-body total heat loss in older males, leading to greater heat storage during exercise-heat stress. Thus, sleep-deprived older adults performing prolonged high-intensity exercise in hot conditions may be at a heightened risk of heat-related illness or injury.