The impact of acoustic stimulation and daytime napping on heart rate variability and sleep architecture

Abstract

Receiving adequate overnight sleep is imperative for proper autonomic nervous system function. Recent studies utilizing acoustic stimulation (AS) have shown an improvement in sleep quality when applied during nighttime sleep, although the implications of AS on napping are not known. The purpose of the current study was to assess the impact of AS during a daytime nap on sleep and autonomic function. We hypothesized that AS (0.8 Hertz monaural beats at 30 decibels) would improve heart rate variability (HRV) during non-rapid eye movement (NREM) sleep and increase time spent in slow wave sleep in young adults. 25 young healthy adult subjects (12 men, 13 women, age: 23 + or - 1 yrs, BMI: 24 + or - 1 kg/m2) took part in the study. Participants were enrolled into a randomized, placebo-controlled crossover design where they completed two 90-minute afternoon nap opportunities with or without AS, at least one day apart. During each testing session, participants were fitted with gold-standard polysomnographic (PSG) equipment and five-lead electrocardiogram (ECG). Subjective questionnaires assessing sleepiness (Epworth Sleepiness Scale) and anxiety (Spielberger State Anxiety Inventory) were given prior to and following each nap opportunity. ECG was recorded continuously starting 5 minutes prior to lights out until awakening, allowing assessment of wake and sleep HRV. Contrary to our hypothesis, there were no significant changes in total sleep time (Control: 73 + or - 4 vs. AS: 72 + or - 4 min, P=0.850), slow wave sleep (Control: 30 + or - 4 vs. AS: 31 + or - 4 min, P=0.855), or other objective sleep measurements between conditions (All P>0.05). AS did not impact sleep HRV measures (All P>0.05). However, there was a significant main effect for time where napping resulted in a significant increase in the R-R interval (F(1,19)=25.683, P<0.001) and reduction of self-report anxiety (F(1,24)=8.229, P=0.008), but these responses were not different across conditions (time x condition, P>0.05). Napping elicited a paradoxical increase of diastolic (F(1,24)=14.483, P=0.001) and mean (F(1,24)=10.857, P=0.003) arterial pressure, but again these responses were not different across conditions (time x condition, P>0.05). In summary, the current study supports beneficial impacts of daytime napping on anxiety levels, but minimal effects of AS on sleep architecture and autonomic function.