Insomnia severity modifies diary-based sleep-continuity and sleep-initiation associations with next-day daytime HRV in a real-world wearable dataset.

Objectives: This study examined whether baseline insomnia severity modifies within-person associations of diary-based sleep-continuity and sleep-initiation metrics with next-day daytime heart rate variability (HRV) in a wearable-based dataset. Methods: We conducted a secondary analysis of a publicly available dataset combining daily sleep diaries with wearable-derived HRV over 4 weeks. The analytic dataset was constructed from the publicly released raw files. Daytime heart rate variability (HRV) was aggregated from eligible 5-minute windows recorded between 09:00 and 23:00, and the primary outcome was daytime RMSSD on the natural log scale. Sleep efficiency was operationalized using a Consensus Sleep Diary (CSD)-style definition that explicitly incorporated wake after sleep onset (WASO). Within-person associations were estimated using participant fixed-effects linear models with standard errors clustered at the participant level. Effect modification was tested using sleep metric × baseline Insomnia Severity Index (ISI), mean-centered, interaction terms. All analyses were restricted to observations meeting prespecified coverage thresholds and day-level complete-case criteria. Results: The analytic dataset comprised 1,372 diary-days from 49 participants. The primary sleep-efficiency model included 451 person-days from 39 participants. Baseline insomnia symptoms were largely non-severe, and no participant met criteria for severe baseline insomnia. The CSD-style sleep efficiency × baseline ISI interaction was positive (β=0.0060; 95% CI, 0.0023 to 0.0097; P=0.002), and the sleep latency × baseline ISI interaction was negative (β=-0.0034; 95% CI, -0.0051 to -0.0018; P<0.001). Interactions involving WASO and CSD-style sleep duration were not statistically significant. Conclusions: Baseline insomnia symptom burden modified the associations of diary-based sleep efficiency and sleep latency with daytime RMSSD. The findings support symptom-dependent sleep-HRV coupling in a predominantly healthy cohort, but higher-ISI estimates were based on few participants in the moderate-ISI range and on person-days meeting daytime HRV coverage criteria. These findings should not be extrapolated to populations with severe or clinically diagnosed insomnia.