Exposure to total 36‐hr sleep deprivation reduces physiological and psychological thermal strain to whole‐body uncompensable passive heat stress in young adult men

Total sleep deprivation (TSD) is associated with endothelial dysfunction and a consequent decrease in vascular reactivity and increase in peripheral vascular resistance. These effectors compromise the body's ability to thermoregulate in hot and cold stress conditions. We investigated heat‐unacclimated young adult men (26 ± 2 years) to determine whether 36 hr of TSD compared to an 8 or 4‐hr sleep condition, would suppress the responses of the autonomic system (body rectal temperature [ T re ], heart rate [HR], root mean square of successive interbeat intervals, physiological strain, blood pressure [BP], circulating blood catecholamines, sweating rate and subjective sensations) to whole‐body uncompensable passive heat stress in traditional Finnish sauna heat (T air = 80–90°C, rh = 30%). Sauna bathing that induced whole‐body hyperthermia had a residual effect on reducing BP in the 8‐hr and 4‐hr sleep per night conditions according to BP measurements. By contrast, 36 hr of total wakefulness led to an increase in BP. These observed sleep deprivation‐dependent differences in BP modifications were not accompanied by changes in the blood plasma epinephrine and norepinephrine concentrations. However, during sauna bathing, an increase in BP following 36 hr of TSD was accompanied by significant decreases in body T re , HR and physiological strain, together with a diminished sweating rate, enhanced vagus‐mediated autonomic control of HR variability, and improved thermal perception by the subjects. Our results suggest the impaired ability of the body to accumulate external heat in the body's core under uncompensable passive heat conditions following 36 hr of TSD, because of the TSD‐attenuated autonomic system response to acute heat stress.