Wearable-derived sleep quality and heart rate variability recovery after critical illness: implications for cardiovascular risk monitoring

Background
Survivors of critical illness have elevated long-term cardiovascular risk. Autonomic dysfunction and disrupted sleep, both adverse cardiovascular prognostic markers, frequently persist after discharge. How nightly sleep quality relates to autonomic recovery after ICU discharge is not well defined.

Purpose
To determine whether wearable-derived sleep metrics reflect day-to-day and week-to-week autonomic recovery patterns relevant to cardiovascular risk stratification in adults recently discharged from intensive care.

Methods
 14 adults recovering from critical illness were monitored for six weeks over a three-month period using a wrist-worn photoplethysmography-based wearable. Sleep efficiency and percentage non-rapid-eye-movement sleep were derived nightly. Autonomic indicators were heart rate variability (root mean square of successive differences) and resting heart rate. Analyses included pooled correlations, within-participant Fisher-z correlations, cluster-bootstrap correlations, mixed-effects models with participant-level random intercepts, and individual-pattern inspection.

Results
Cluster-bootstrap correlations demonstrated a positive same-day association between sleep efficiency and heart rate variability (r = +0.25, p < 0.001) and an inverse association with resting heart rate (r = –0.18, p = 0.004). Week-to-week analyses showed that higher mean sleep efficiency predicted higher mean heart rate variability (r = +0.45, p = 0.001) and lower resting heart rate (r = –0.37, p = 0.016).

Mixed-effects modelling indicated that higher nightly sleep efficiency predicted higher next-day heart rate variability (β = +1.66, p = 0.002). Higher mean non-rapid-eye-movement sleep in week 1 showed a borderline association with higher week-2 heart rate variability (β = +0.69, p = 0.063). Longitudinal modelling demonstrated a significant week-on-week improvement in heart rate variability (β = +0.038/week, p < 0.001), with sleep efficiency trending upward but not significantly (β = +0.065/week, p = 0.52).

Participant-level inspection revealed heterogeneous physiological patterns, including tightly coupled sleep–autonomic relationships, delayed effects, paradoxical responses, and resilient profiles with minimal variability.

Conclusions

Wearable monitoring is feasible for tracking sleep - autonomic dynamics after critical illness. Higher sleep efficiency is associated with higher heart rate variability and lower resting heart rate which are markers linked to cardiovascular protection. Sustained sleep quality shows week-to-week associations with autonomic improvement. Given the heightened cardiovascular vulnerability of post-ICU survivors, sleep - autonomic trajectories may serve as accessible indicators for early cardiovascular risk identification and remote preventive follow-up. Larger studies are needed to define cardiovascular implications and integration into preventive cardiology pathways.SE vs next day HRVFor image description, please refer to the figure legend and surrounding text. NREM vs next week HRVFor image description, please refer to the figure legend and surrounding text.