Changes in autonomic nervous system activity and in electro-cortical activity during sleep onset

This study is aimed at investigating the behavior of the autonomic nervous system (ANS) at sleep onset (SO) and to correlate it with higher brain activity captured as surface electroencephalogram (EEG). The study was performed on data obtained from 16 healthy subjects who underwent standard polysomnography (PSG). The instantaneous heart rate variability (HRV) provided a measure of ANS activity, and EEG was quantified by time-frequency analysis measuring power in alpha and delta frequency bands. The results revealed a power decrease in the VLF component of the inter-beat (RR) interval to a minimal level around SO, similar changes, though to a lesser degree in the LF component, and almost no change in the HF component. These changes correlated with a drop in alpha and a surge in delta EEG power. The sympathovagal balance, as reflected by LF/HF ratio, pointed towards an increase in the parasympathetic activity after SO.