What is heart rate variability (HRV)?

Oura reports that heart rate variability (HRV) measures the variation in heartbeat timing, reflecting stress levels and recovery. Higher HRV indicates well-being, while lower HRV signals stress.
BMJ

8.6 Variability in baseline total symptom scores: association with co-morbid conditions & implications for post-concussion return-to-play decisions

Objective Variability in return-to-play (RTP) time following concussion is striking. While certain characteristics and co-morbid conditions have been associated with prolonged RTP, the correlation to ...
TCTMD

Lp(a) Can Vary Over Time in ASCVD Patients With High Baseline Levels: OCEAN(a)-DOSE

Serial measurements of lipoprotein(a) can change by as much as 10% over time for about one in five patients with atherosclerotic cardiovascular disease (ASCVD) and high Lp(a) concentration at baseline ...