Background: Regular physical activity is associated with favourable cardiovascular outcomes, partly through effects on cardiac autonomic regulation. Heart rate variability (HRV) is a validated, non-invasive marker of autonomic balance. Whether graded physical activity levels relate to autonomic function in healthy young adults — before disease onset — is less well characterised. Objectives: To examine the association between self-reported physical activity levels and resting cardiac autonomic function, assessed by short-term HRV and standard bedside autonomic function tests, in young adults aged 18–30 years. Methods: In this cross-sectional analytical study, 210 apparently healthy young adults were classified into low, moderate, and high physical activity groups using the International Physical Activity Questionnaire–Short Form (IPAQ-SF). Resting 5-minute HRV was recorded under standardised conditions; time-domain (SDNN, RMSSD, pNN50) and frequency-domain (LF, HF, LF/HF ratio) indices were derived. Bedside autonomic tests (resting heart rate, E:I ratio, Valsalva ratio, 30:15 ratio, orthostatic blood pressure change, isometric handgrip) were performed. Groups were compared using one-way ANOVA or Kruskal–Wallis tests; associations were assessed with correlation and multivariable linear regression adjusted for age, sex, and BMI. Results (mock — illustrative): Higher physical activity was associated with higher parasympathetic indices (RMSSD, HF, pNN50) and lower resting heart rate and LF/HF ratio across groups (p < 0.05). Total MET-minutes/week correlated positively with RMSSD (r ≈ 0.54) and negatively with LF/HF ratio (r ≈ −0.47). Associations persisted after adjustment. Conclusion: In healthy young adults, higher physical activity levels were associated with greater parasympathetic (vagal) tone and improved sympathovagal balance, supporting physical activity as a modifiable determinant of cardiac autonomic health early in life. Causality cannot be inferred from this cross-sectional design

Physical activity; Heart rate variability; Autonomic nervous system; Parasympathetic tone; Young adults; IPAQ