Yes, the Fitbit Sense has a pulse oximeter to track blood oxygen saturation (SpO2) while you sleep. In the morning, you get a report that shows your SpO2 levels. To measure blood oxygen saturation, Fitbit (and WHOOP) uses its red and infrared LEDs to shine light onto your skin and blood vessels. Based on the reflection of the light, Fitbit can differentiate between richly oxygenated and poorly oxygenated blood because the latter reflects less infrared than red light.