Here's the thing: the way HRV is measured reliably when used as a metric is with 'paced breathing'. You synchronize your inhalations and exhalations to a displa

Here's the thing: the way HRV is measured reliably when used as a metric is with 'paced breathing'. You synchronize your inhalations and exhalations to a displayed pace (in for X seconds, out for X seconds, or whatever). The primary source of variability under those conditions seems to be that HR increases when inhaling and decreases when exhaling. Seen in that light, it makes more sense. Essentially, what HRV may measure is ability to actively and acutely lower HR (which is most naturally performed during exhalation). This is not so unlike how some exercise equipment will provide a fitness measure based on heart-rate recovery: how long does it take you to return to some percentage of your resting heart rate after having exercised? At this point, I'll make an analogy, which is meant only to be that. If you have a high income, you can afford to spend a lot of money on a day-to-day-basis. Depending on how you set things up, you might see a wild flux in bank balance, and that's just fine -- because you can afford it. Likewise, performance athletes experience highly-elevated heart rates regularly. Indeed, it's that stress which leads to fitness adaptations. Just as you need to spend money to make money, you need to stimulate the heart in order to strengthen it. High HRV (as far as I can tell), may largely correspond with the naturally-adapted ability to rapidly return to the calm, low-HR state characteristic of the well-trained athlete. When you live check-to-check, you have to budget very carefully and avoid variability in spending. You can't afford to venture, and you don't gain. When you know huge cash infusions are around every corner, you don't worry about spending. Likewise, the fit heart doesn't need to damp and suppress stimulus. Elevated heart rate is good (think about the positive moments in life which can cause this), as long as it doesn't lead to chronic stress but instead evaporates in the space of a breath or two.