Age-related losses in cardiac autonomic activity during a daytime nap

In healthy, young individuals, a reduction in cardiovascular output and a shift from sympathetic to parasympathetic (vagal) dominance is observed from wake into stages of nocturnal and daytime sleep. This cardiac autonomic profile, measured by heart rate variability (HRV), has been associated with significant benefits for cardiovascular health. Aging is associated with decreased nighttime sleep quality and lower parasympathetic activity during both sleep and resting. However, it is not known whether age-related dampening of HRV extends to daytime sleep, diminishing the cardiovascular benefits of naps in the elderly. Here, we investigated this question by comparing the autonomic activity profile between young and older healthy adults during a daytime nap and a similar period of wakefulness (quiet wake; QW). For each condition, from the electrocardiogram (ECG), we obtained beat-to-beat HRV intervals (RR), root mean square of successive differences between adjacent heart-beat-intervals (RMSSD), high frequency (HF), low frequency (LF) power and total power (TP), HF normalized units (HFnu), and the LF/HF ratio. As previously reported, young subjects showed a parasympathetic dominance during NREM, compared with REM, pre-nap rest, and WASO. On the other hand, older, compared to younger, adults showed significantly lower vagally-mediated HRV (measured by RMSSD, HF, HFnu) during NREM. Interestingly, however, no age-related differences were detected during pre-nap rest or QW. Altogether, our findings suggest a sleep-specific reduction in parasympathetic modulation that is unique to NREM sleep in older adults. Impact Statement Sleep is naturally modulated by the autonomic nervous system (ANS), with greater dominance of parasympathetic over sympathetic activity during non-rapid-eye-movement (NREM) sleep. As such, sleep has been termed a “cardiovascular holiday” and has been associated with positive health outcomes. Aging, however, is linked to decreases in cardiac autonomic activity and sympathovagal imbalance. While the impact of aging on ANS activity during nocturnal sleep has received some attention, the cardiac profiles during a daytime nap, to our knowledge, have not yet been studied under the context of aging. Herein, young adults demonstrated increased parasympathetic activity during deep sleep. Older adults, however, showed less parasympathetic modulation during NREM sleep, suggesting loss of the cardiovascular holiday. Importantly, no age-related declines in parasympathetic activity were detected during wake, suggesting a sleep-specific reduction in parasympathetic modulation that is unique to NREM sleep in older adults.