Cognitive performance during exposure to moderate normobaric hypoxia after sleep restriction: relationship to physiological and stress biomarkers.

INTRODUCTION: Exposure to moderate levels of simulated hypoxia has subtle cognitive effects relative to ground level, in healthy individuals. However, there are few data on the cognitive consequences of the combination of hypoxia and partial sleep deprivation, which is a classic military or civilian operational context. In this study, we tested the hypothesis that exposure to moderate hypoxia while sleep-restricted impairs several domains of cognition, and we also assessed physiological parameters and salivary concentrations of cortisol and alpha-amylase. METHOD: Seventeen healthy males completed two sessions of cognitive tests (sustained attention using the PVT psychomotor vigilance task and executive functions using the Go-NoGo inhibition task and N-Back working memory task) after 30 minutes (T+30') and 4 hours (T+240') of exposure in a normobaric hypoxic tent (FIO2 = 13.6%, ≃ 3,500 m) (HY). This was completed after one night of sleep restriction (3 a.m. to 6 a.m. bedtime, SRHY) and one night of habitual sleep (10 p.m. to 6 a.m. bedtime, HSHY) (with cross-over randomization). The two nights sleep architecture and physiological parameters (oxygen saturation (SpO2) and heart rate (HR) during T+30' and T+240'sessions were analyzed. Salivary cortisol and alpha-amylase (sAA) concentrations were analyzed before hypoxia, after the T+30' and T+240' cognitive sessions, and after leaving the hypoxic tent. RESULTS: Sustained attention (RT and number of lapses in the PVT) and executive functions (Go-NoGo and 1-Back and 2-Back parameters, as inhibition and working memory signatures) were impaired in the SRHY condition compared to HSHY. SpO2 and HR were higher after 4 hours compared with 30 minutes of hypoxia in the HSHY condition, while only HR was statistically higher in the SRHY condition. In SRHY, salivary AA concentration was lower and cortisol was higher than in HSHY. A significant increase in sAA concentration is observed after the cognitive session at 4 hours of hypoxia exposure compared to that at 30 minutes, only in the SRHY condition. There are significant positive correlations between reaction time and the corresponding heart rate (a non-invasive marker of physiological stress) for the executive tasks in the two sleep conditions. This was not observed for salivary levels of sAA and cortisol, respective reliable indicators of the sympathoadrenomedullary system and the hypothalamic-pituitary adrenocortical system. CONCLUSION: Exposure to moderate normobaric hypoxia (≃ 3,500 m / ≃ 11,500 ft simulated) after a single night of 3-hour sleep impairs cognitive performance after 30 minutes and 4 hours of exposure. The key determinants and/or mechanism(s) responsible for cognitive impairment when exposed to moderate hypoxia with sleep restriction, particularly on the executive function, have yet to be elucidated.