Partial reduction of parasympathetic nerve activity during sleep after endurance exercise under hypoxic conditions

The present study compared changes in sympathetic and parasympathetic nervous activities during sleep following endurance exercise under either normoxic or hypoxic condition.

Ten young men (20.5 ± 0.2 years) were recruited for the study. All of them carried out three trials on different days: [1] pedaling exercise in hypoxia (FiO2: 14.5%; HYP), [2] pedaling exercise in normoxia (FiO2: 20.9%; NOR), and [3] rest in normoxia (REST). Each trial was separated at least one week, with randomized orders. The exercise in HYP and NOR trials consisted of 60 min of pedaling exercise at 60% of maximal oxygen uptake. During exercise, heart rate (HR), rating of perceived exertion and arterial oxygen saturation (SpO2) were determined. Also, HRV was continuously monitored until next morning (17:00-8:00) to evaluate frequency domain HRV parameters and time domain HRV parameters. On the following morning, the scores of fatigues, sleepiness, vitality, and quality of sleep were measured by visual analog scale.

During sleep, majority of frequency domain HRV parameters (LF, HF, LF/HF, Total Power) or time domain HRV parameters (SDNN, RMSSD, NN50) did not differ significantly among three trials, although the average of pNN50 was significantly lower in NOR and HYP trials.

Evening endurance exercise under hypoxic conditions did not exacerbate autonomic nerve activity during sleep compared to the same endurance exercise under normoxic conditions. However, despite different inspiratory oxygen levels during exercise (FiO2: 20.9% or 14.5%), evening endurance exercise may partially suppress parasympathetic nerve activity during sleep. These findings would apply to people who are involved in endurance exercise under hypoxic conditions.