What is HRV and why does it matter for men over 40?

Heart rate variability (HRV) is the millisecond variation between consecutive heartbeats, regulated by the autonomic nervous system, measured as RMSSD during sleep, with a normal range of 35–55 ms in men aged 40–49, and values below 25 ms associated with elevated cardiovascular mortality risk and impaired autonomic recovery (Thayer et al., Int J Cardiol, 2010).

Your heart does not beat like a metronome. A healthy heart speeds up slightly with each inhale and slows slightly with each exhale, driven by the vagus nerve, the primary conduit of the parasympathetic nervous system. That beat-to-beat variation is HRV. The more variation, the more responsive and adaptive your autonomic nervous system is. A low HRV indicates that the sympathetic "fight or flight" system has taken over and is suppressing the parasympathetic recovery system, a pattern that correlates with higher cardiovascular mortality, poor sleep quality, insulin resistance, and impaired stress recovery.

Honesty Scale: Solid (1). The association between HRV and cardiovascular outcomes is established in multiple prospective cohort studies. The association between consumer wearable HRV and clinical cardiac risk is Early (3), the correlation exists, but wearable precision varies enough that single readings should not drive clinical decisions.

What to do: Use your wearable's HRV to track trends, not single numbers. If your 30-day trend is declining without an obvious cause (illness, alcohol, disrupted sleep), bring that trend data to a cardiologist. A declining HRV unexplained by lifestyle factors is a signal for clinical evaluation, not a coaching algorithm adjustment.

For the full picture, read Your Whoop Is Worried. Here Is What It Actually Found.