Let's put this red led / green led thing to bed. The green led came 1st. It operates at about 540 nano-meter wavelength. The LEDs shine light onto the surface of the skin, and down to a given depth into tissue, to 'read' variables like O2 saturation of blood, heart rate (blood pulsing), etc. For Fitbit, they took a bit of flak for the accuracy of their heart rate monitoring. The 'simple' green light is easily absorbed by the human body and is prone to produce artifacts. The green light originally used by Fitbit does not penetrate tissue to an optimum depth for reading physical data, it gets absorbed by the skin / tissue. If any of you have been to a doctor's office or hospital you no doubt saw that the O2 sensor / heart rate monitors used are red leds. This is because the body is a poor absorber of red light and thus the red led is able to penetrate the tissue deeper and get a more refined reading of blood O2 levels and heart rate. Apple watch, Garmin and a few others use. the green led as the it was the 'gold standard' for many years in the fitness industry. The red led uses a longer wave-length and is better at penetrating body tissue and providing feedback readings on vitals such as oxygen saturation and blood pulse. The red led also reduces 'noise' from sunlight as the light spectrum from the sun contains more green than red. FitBit support is notorious for giving vague and nonsensical responses to questions. The response that the 'red and green pulsing lights does not interfere with the function of the monitor' is silly because the red and green leds are a part of the monitor function. The fitbit was designed that way. The red light led is more active at night so as to not disturb the person wearing the Fitbit and because the red light is more sensitive for evaluating oxygen saturation, motions, heart rate and thus - the overall sleep pattern. The green is used during the day but is accompanied by the red for more thorough and detailed information gathering. It isn't that there 'is nothing wrong' with the monitor, or that it 'will not cause a malfunction' of the monitor. In simple terms, the red and green LEDs have been incorporated into the new FitBit wrist monitors to improve overall accuracy of the varied functions read. Yep, I know that this is some 3 years after the OP but it may still be relevant. These leds should be flickering, activated by motion or inactivity, but they should NOT be constantly lit. If either one is always lit / on, there is something wrong with that FitBit fitness tracker.
