Your heart rate is more uneven than you think. This is what it says about your health - BBC

Tiny beat-to-beat changes in the timing of heartbeats—detectable by consumer wearables—are emerging as a useful signal of mental health, stress levels, fitness and biological ageing. The report that ageing speed shows two abrupt peaks reinforces the idea that continuous physiological signals could reveal sudden accelerations in ageing, not just gradual decline, and could be used to detect or monitor those critical windows. Researchers suggest these micro-fluctuations provide a noninvasive window into how the body responds to strain and recovers.