BACKGROUND: Healthy circadian variation in arterial blood pressure (BP) is characterized by nocturnal dipping. Elevated BP or attenuated BP dipping during sleep is associated with greater risk for cardiovascular diseases. Moreover, cardiac-autonomic activity is associated with BP dipping patterns in clinical populations, but whether this relationship exists in apparently healthy adults is unclear. Therefore, the purpose of this study was to quantify associations between cardiac-autonomic activity, indexed via resting heart rate variability (HRV), and nocturnal BP characteristics in young adults. METHODS: Twenty-nine apparently healthy young adults (n = 13 males, 23 ± 4 yrs, 23 ± 3 kg/m2, n = 16 females, 20 ± 2 yrs, 23 ± 3 kg/m2) were included in the analysis. Resting HRV was obtained in the laboratory following an overnight fast. Five-minute electrocardiographic recordings were obtained in the supine position following a five-minute stabilization period. Short-term HRV parameters of interest included the mean RR interval, standard deviation of normal RR intervals (SDNN), and root-mean square of successive differences (RMSSD). Participants left the laboratory wearing an ambulatory BP monitor on their upper arm programmed to perform recordings every 20 minutes during awake hours and every 30 minutes during sleeping hours. Self-reported bedtime and wake time were used to identify awake and asleep periods. Absolute BP dipping was quantified as awake BP minus asleep BP. RESULTS: In males, asleep diastolic BP (54.7 ± 4.1 mmHg) was associated with SDNN (58.7 ± 17.9 ms, r = -0.57, P <0.05) and RMSSD (60.5 ± 25.9 ms, r = -0.56, P <0.05). Additionally, systolic BP (12.1 ± 6.6 mmHg, r = 0.64) and diastolic BP dipping (22.8 ± 7.4 mmHg, r = 0.59) were associated with SDNN (Ps <0.05). No associations between any HRV and BP values were observed for females (Ps >0.05). CONCLUSIONS: Our findings reveal sex differences in the association between resting short-term cardiac-autonomic activity and nocturnal BP characteristics in healthy young adults. HRV is an accessible and modifiable (e.g., via aerobic exercise) biomarker that may be a useful target for young adult males to improve circadian variation in BP and reduce cardiovascular disease risk. FUNDING: Supported by Georgia Southern University Faculty Research Committee Research Seed Funding Award.

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