Higher heart rate variability during napping is associated with decreased perceived sleepiness

Parasympathetic nervous system (PNS) activity increases and sympathetic tone decreases during sleep, particularly during non-REM (NREM) stages. Given the high prevalence of sleep insufficiency, afternoon naps are a common countermeasure for short- and/or long-term sleep loss. While relationships between PNS activity and overnight sleep quality have been documented, less is known about these relationships during naps. The present study examined the associations between heart rate variability (HRV) and the change in sleepiness after an afternoon nap. We hypothesized that greater HRV would be associated with greater reductions in sleepiness after a nap. Nineteen participants (9 male, 10 female; age: 22 ± 1 yrs) rated their perceived sleepiness before and after a 90-minute afternoon nap opportunity. All participants underwent polysomnography sleep assessment, which included continuous measures of electroencephalography, electrooculography, electromyography, electrocardiography, and respiration. Epworth Sleepiness Scale (ESS) was used to record pre- and post-nap subjective sleepiness and HRV was analyzed during NREM sleep (stages 2 and 3) for the duration of the nap. Changes in subjective sleepiness as measured by the ESS were negatively associated with high frequency HRV (ρ = -0.671, p = 0.002), root mean square of successive differences (RMSSD; ρ = -0.675, p = 0.002), and percent of successive RR intervals that differ by at least 50 ms (pNN50; ρ = -0.58, p = 0.009). There was no significant association between the percent change of HRV during NREM sleep from wake (all p > 0.05), nor a significant association between the percent of the nap spent in stage 3 (p > 0.05), with changes in sleepiness. In summary, higher HRV during sleep correlates with decreased perceived sleepiness upon nap awakening, but does not correlate with the percentage of time in slow wave sleep. These results suggest a role for elevated PNS activity in the restorative benefits of napping. This abstract was presented at the American Physiology Summit 2025 and is only available in HTML format. There is no downloadable file or PDF version. The Physiology editorial board was not involved in the peer review process.