Sleep-timing irregularity moderates the within-participant association between training load and next-morning heart rate variability in university sport participants: a 28-day study.

Background: Heart rate variability (HRV) describes beat-to-beat variation in cardiac intervals; the root mean square of successive differences (RMSSD) is a time-domain index of short-term cardiac vagal modulation. We examined whether nightly deviation from habitual sleep midpoint moderated the association between prior-day internal training load and next-morning natural-log-transformed RMSSD (lnRMSSD), and whether analogous interactions were present for resting heart rate, wellness strain, and countermovement-jump (CMJ) height.

Methods: University students engaged in regular sport training completed 28 days of habitual monitoring. Internal load was session duration multiplied by session rating of perceived exertion. Wrist actigraphy and sleep diaries estimated sleep midpoint, and 5-min morning RR recordings yielded lnRMSSD. Mixed models separated within- and between-participant components and tested within-participant load (per 100 arbitrary units [AU]) × absolute midpoint deviation (per 30 min).

Results: The analysis included 1,232 mornings from 54 participants (57.4% female; age 21.0 ± 1.7 years). At each participant's mean midpoint deviation, greater-than-usual load was associated with lower next-morning lnRMSSD (β = -0.0089, 95% CI -0.0127 to -0.0051; p

Conclusion: The within-participant association between training load and next-morning cardiac vagal modulation was moderated by sleep-timing irregularity. These field-based associations do not establish a sleep mechanism, causal recovery effect, or decision threshold.