Suunto's watches have done that for a while now (5-6 years?), though they use a capacitive chest strap, rather than the optical method used by smart watches. E

Suunto's watches have done that for a while now (5-6 years?), though they use a capacitive chest strap, rather than the optical method used by smart watches. Edit: And as far as I know, Suunto was really the first company to pursue measurement of HR variability. They have some interesting whitepapers, if you use the Google.