I use mi band 2 tracker for smart alarm. It has a bright green light to measure heart rate every two minutes. Can it disturb mitochondria in the skin or is the skin area too small for a noticeable effect?
I've seen Dave bringing his own tape to motels and cover every single LED light in the room xD.
But honestly, there is no way to tell. If it doesn't shine straight into your eyes I would not worry.
Pros probably outweigh the cons, if there's any at all.
Out of concern for this I went with a fitbit flex 2 (walmart.com had them on sale.) It comes with a band that the device comes in and out of. I took out the device, put electrical tape over the slots they have to let the lights out and then reinserted the device. I also set it not to track most of the stuff it does normally (walking is not exercise!)