https://arstechnica.com/gadgets/2018/10/garmin-vivosmart-4-review-the-attractive-new-face-of-the-vivosmart-line/

Fitness tech companies, particularly Fitbit and Garmin, want to monitor the level of oxygen in your blood. It's a metric that could help spot signs of sleep apnea and other breathing-related problems, so companies have been scrambling to perfect their SpO2 sensors and incorporate this data into their ecosystems in a meaningful way.

Fitbit still hasn't activated its SpO2 monitors in its wearables (although that feature should roll out in beta in November), but Garmin has. Its newest fitness tracker, the $129 Vivosmart 4, includes an SpO2 monitor and measures Body Battery, which lets you know when you should plan your next intense workout session and when you should take a rest day.