Cardiac autonomic activity during daytime nap in young adults

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During sleep, heart rate decelerates and cardiac vagal activity, as indexed by heart rate variability (HRV) measures, increases from wake to non‐rapid eye movement (NREM) sleep (Bušek et al., 2005; de Zambotti et al., 2012, 2014; Trinder et al., 2001). Also, cardiac sympathetic activity, as measured by means of pre‐ejection period (PEP), shows marked reduction from presleep wakefulness to sleep, but no differences across sleep stages (de Zambotti et al., 2012, 2014; Trinder et al., 2001). These results indicate a sleep‐related modulation of cardiac autonomic activity. Conversely, the pre‐sleep autonomic level may affect subsequent sleep quality (Trinder et al., 2012). For example, insomniacs often show a ‘hyperaroused’ pre‐sleep state characterized by elevated sympathetic activity (Riemann et al., 2010).
Interestingly, vagal activity seems to be modulated by sleep even during a daytime nap (Cellini et al., 2016), supporting the idea that naps may be beneficial for cardiovascular health (Naska et al., 2007). However, to date, how sympathetic activity is modulated during a nap and the impact of pre‐nap arousal level on the subsequent sleep remain unknown.
Here we measured cardiac vagal and sympathetic activity concurrently across sleep stages during an afternoon nap in healthy adults. We aimed to: (1) assess the cardiac sympathetic activity across sleep stages during a daytime nap; (2) explore the impact of the pre‐sleep cardiac activity on the subsequent sleep quality; and (3) reproduce previous findings of heart rate variability (HRV) patterns during naps.
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