What wearable metric should a man over 40 actually pay attention to for cardiac health?

In order of clinical value for cardiac health specifically: first, resting heart rate trend over eight weeks or more (not daily variation, the directional trend is what matters, and elevation above 80 bpm or a sustained upward shift of 10+ bpm is a finding worth clinical attention); second, any cardiac rhythm alerts from ECG-capable devices (Apple Watch, Kardia), these have documented positive predictive value for AFib and warrant physician follow-up when they occur, particularly if they repeat; third, sleep duration trend, specifically whether you consistently get less than six hours, which is an independent cardiovascular risk factor with solid epidemiological evidence; and fourth, HRV as a contextual indicator of training load, illness, and alcohol effects on recovery, with the explicit understanding that population-level cardiovascular risk stratification is not its clinical role in otherwise healthy adults.

What to give far less attention to than most wearable users do: the specific daily score, the "readiness" or "strain" number, the HRV rank compared to others, and the detailed sleep stage breakdown. These are products of algorithms that vary across devices, have not been validated against cardiovascular outcomes, and generate anxiety disproportionate to their clinical information content. The clear signals are rate, rhythm, and duration. The noisy signals are everything else. (Thayer et al., International Journal of Cardiology, 2010)

Cardiologist's calibrated position, Solid (1) for resting heart rate trend and rhythm alerts as the clinically prioritized wearable metrics for cardiac health.

What to do: If you could only look at two wearable metrics for cardiac purposes, make it resting heart rate (weekly trend) and any rhythm alerts. The rest is useful context, not clinical signal.

For the full picture, read What Your Apple Watch Is Trying to Tell You.