EEG markers confirm the arousing effect of moderate hyperthermia in healthy subjects as opposed to sleep restriction

Objective This study aimed to examine the evolution of vigilance, sleepiness and electrophysiological markers of arousal in healthy subjects exposed to moderate hyperthermia, during habitual or restricted sleep conditions.Methods Twelve healthy males (30.4 ± 7.3 yr) completed two experimental crossover sessions in a bioclimatic chamber, consisting of sequential exposure to a thermoneutral condition (TCORE = 37.0 ± 0.2 °C) then an hyperthermic condition (TCORE = 38.3 ± 0.2 °C). Sessions followed either an habitual night of sleep (>6h time in bed, TIB) or sleep restriction (<3h TIB). A 10-minute psychomotor vigilance task (PVT) and a sleepiness scale were administered under thermoneutrality and hyperthermia conditions, immediately after recording a one-minute eyes-closed resting state electroencephalogram (EEG). This allowed for the calculation of individual alpha frequency (IAF), relative spectral powers (alpha, theta and beta bands) and theta-to-alpha ratio in frontal and parieto-occipital territories.Results Moderate hyperthermia induced an increase in PVT speed and a decrease in sleepiness. This arousal response was associated with an increase of IAF, a reduction in frontal theta power and an increase in frontal alpha power, leading to a decrease in the theta/alpha ratio. In contrast, sleep restriction induced the opposite effect on PVT performances and sleepiness, as well as for EEG parameters (without influence on IAF). No significant interaction was observed for all parameters.Conclusion Sleep restriction and moderate hyperthermia induced opposite effects in our model with limited time exposure to heat. This confirms that heat can help with arousing under certain conditions, although this needs to be confirmed by further studies.