Fitbit quietly added a new feature to some of its products that the Apple Watch doesn't have (GOOG, GOOGL, FIT, AAPL)

· Fitbit has begun rolling out a new feature to some of its devices that monitors the variation of oxygen levels in the blood, which can be used detect changes in breathing during sleep.
· The feature could help Fitbit further compete with Apple, the worldwide market leader in wearables, whose smartwatch does not support this...