Locomotor-Autonomic Coupling is Phase-Lagged and Behaviorally Driven: Evidence from long-term simultaneous heartrate and activity monitoring

The temporal coordination between behavioral and autonomic rhythms is a critical feature of circadian physiology, yet the precise alignment and causal structure of this coupling remain poorly characterized in free-living humans. Using long-term wearable data spanning 30 days from 52 individuals, we analyzed accelerometry (ACC) and heart rate (BPM, via IBI) to quantify circadian phase alignment, inter-day stability, and temporal directionality between locomotor and autonomic systems. Across individuals, behavioral activity rhythms consistently peaked earlier than autonomic rhythms (mean lag: –1.8h, p < 0.001), with the lag largely attributable to greater variability in locomotor phase. Despite this temporal dissociation, both signals exhibited coherent 24-hour patterns and relatively stable inter-day acrophases. Lag magnitude was negatively correlated with nighttime BPM (r = –0.55, p < 0.001), suggesting a link between autonomic hyperactivation and desynchrony. Crucially, behavioral acrophase more strongly predicted daily lag fluctuations than BPM acrophase, and causal analyses revealed asymmetric dependencies: same-day activity levels were significantly predictive of nighttime heart rate, whereas the reverse relationship was weaker and less consistent. Granger causality confirmed a predominant flow from ACC to BPM across subjects ( p = 0.0045). These findings establish that autonomic rhythms lag behind and are shaped by preceding behavioral activation, supporting a behavior-first model of internal circadian organization.