Heart Rate Variability as a Predictor of Daytime Sleepiness during Prolonged Isolation

The 120-day isolation experiment (SIRIUS-19 program) within the SIRIUS project ( http://sirius.imbp.ru/ ) involved six volunteers aged 28 to 44 years (three men and three women). Research methods included daily ECG recording to assess heart rate variability, blood pressure measurement, and self-assessment of sleep quality over the past night using a visual analog scale. The studies were carried out in the morning after waking up. Once a week in the evening (17:00–19:00), volunteers filled out a scale of clinical daytime sleepiness self-assessment, which allows the level of sleepiness during the day to be evaluated. During regression analysis, the possibility of predicting the appearance of daytime sleepiness in terms of cardiac activity parameters was assessed. It was found that an increase in heart rate, an increase in PNN50, and a decrease in the Robinson index in the morning predetermine the appearance of subjective daytime sleepiness. It may be suggested that the increased activity of the parasympathetic division of the autonomic nervous system in the morning characterizes both the previous lack of sleep and the risk of developing sleep inertia and daytime sleepiness in general.