Heart Rate Variability (HRV) is one indicator of both cardiovascular health and general health, a topic covered pretty well in the Wiki. Many of the wearables like Fitbit and the Oura ring track HRV as well as heart rate and other fitness parameters. I have an OUra ring, which tracks HRV only during sleep. This has been one of the most helpful uses of the Oura ring, as it shows a much lower overnight HRV if I am not well, have overworked and not rested enough, or even if I had one glass of wine.
There is a good 3-article series online at http://www.hrvcourse.com. Although one is not supposed to compare her own HRV with others, I can't help but wonder if my readings are anywhere in the range of reasonably good fitness and health. The article series provides a chart of typical healthy HRV for various age groups and also for gender. HRV can be calculated a number of ways based on which part of the heartbeat is used for measures of intervals between the beats. The Oura ring uses rMSSD.
Sleep and Activity trackers that use HRV at rest appear to be reasonably accurate according to the following review which compared wearables to electrocardiograms:
Folia Med (Plovdiv). 2018 Mar 1;60(1):7-20. doi: 10.2478/folmed-2018-0012.
Can Wearable Devices Accurately Measure Heart Rate Variability? A Systematic Review.
Georgiou K1, Larentzakis AV2, Khamis NN3, Alsuhaibani GI3, Alaska YA3, Giallafos EJ4
Eighteen studies were included. Sixteen of them integrated ECG - HRV technology and two of them PPG - PRV technology. All of them examined wearable devices accuracy in RV detection during rest, while only eight of them during exercise. The correlation between classic ECG derived HRV and the wearable RV ranged from very good to excellent during rest, yet it declined progressively as exercise level increased."