Autonomic and endocrine consequences of a daytime napping at simulated altitude on healthy lowlanders

Altitudinal hypoxia is a problem of growing interest, given the increased population that voluntarily or involuntarily is exposed to this condition. Despite the fact that a hypoxic context could have marked the dawn of humanity, most humans currently live below 2500 m asl in the so-called lowlands. Although physiological adaptive mechanisms have allowed the incursion of population groups above that level, non-acclimatized lowlanders may suffer maladaptive changes. Among these changes, sleep disturbances stand out, leading to impaired physical and mental performance. Hence, implementing countermeasures is desirable to improve the performance and well-being of these vulnerable populations. Napping has been recognized as an effective strategy against sleep disturbances under normoxic conditions; yet, its use under altitudinal hypoxia lacks experimental support. This gap is due, among other things, to a poor understanding of the physiological nature of a daytime nap under hypoxic condition. Thus, here our goal was to analyze cognitive, endocrine, and autonomic changes produced around a daytime nap under normobaric hypoxia. Our results show a sleep architecture alteration in individuals when napping in normobaric hypoxia, including less REM and N3 sleep and a concomitant increase in N2, relative to individuals in the normoxic condition. As expected, there was a classic dose-dependent endocrine response with an elevation of EPO and cortisol. These changes were strongly correlated with indirect markers of autonomic activity, namely, Heart Rate Variability parameters. There was an increase in cardiovagal tone while napping, suggested by larger RR intervals and RMSSD; this increase was less in hypoxia. Similarly, sympathetic activity after a 90-min nap increased. These drastic neuroendocrine changes did not have significant cognitive repercussions. Regarding respiratory activity, a notable difference between men and women was evident. Men exhibited high peripheral oxygen desaturation and dose-dependent periodic breathing. The significant findings in men (and one woman with polycystic ovary) point to a potential role of testosterone in modifying the ventilatory threshold and the CO2 reserve. Considering our results, the prescription of a nap as a countermeasure against night sleep disturbances at altitude should be limited, given the little evidence favoring its implementation. Additional strategies, such as oxygen administration, may be necessary.