Temporal and sex-specific dynamics of autonomic–somatic coordination in young adults: nocturnal parasympathetic activity and fatigability associations

Background Autonomic-somatic coordination, which reflects the dynamic interplay between autonomic regulation and physical activity, may serve as a physiological marker of fatigue and recovery. However, its temporal characteristics and sex-specific patterns remain poorly understood. This study utilized a correlation-based framework to quantify this macroscopic coupling. Methods This study examined the correlation between high-frequency normalized units (HFnu) of heart rate variability and physical acceleration (PA) during one-hour (1-h) periods before sleep onset and after waking in 125 healthy adults (69 men and 56 women). The participants wore a wearable device for 24-h to record their triaxial acceleration and R–R intervals. Nocturnal parasympathetic nervous system activity (PSNA) was indexed using the HFnu during sleep. Before the monitoring period, subjective fatigability was assessed using the Japanese version of the Cornell Medical Index. Results Women showed more negative correlations between HFnu and PA after awakening than men ( p < 0.01). Among women, less negative HFnu–PA correlations before sleep onset were associated with higher subjective fatigability ( p < 0.01) and higher negative correlations after waking were associated with higher nocturnal HFnu ( p < 0.05). In men, more negative HFnu–PA correlations before sleep onset were associated with higher nocturnal HFnu ( p < 0.05), while higher fatigue was linked to stronger negative coupling, demonstrating a contrasting pattern to that observed in women. Conclusions These findings revealed sex-dependent temporal patterns in autonomic-somatic coordination quantified via correlation-based indices. Before sleep onset, higher subjective fatigue is linked to reduced coordination in women, whereas men exhibit a distinct and contrasting negative correlation pattern. Wearable-based monitoring may help to capture sex-specific autonomic dynamics related to fatigue and nocturnal parasympathetic activity.