The green and red lights on the back of your Garmin Lily Sport watch are connected to the heart rate and oxygen satellite measurement (SpO2).

- The green light is typically used for heart rate measurement.
- The red light is used to measure blood oxygen saturation (SpO2), which is often done during the night when the watch automatically performs these measurements.

These lights are part of the watch's health and wellness tracking functions and should not normally cause concern unless you experience unusual or unexpected results, which you should then discuss with a doctor.