When I put on my watch it did not register the heart rate though when my wife tried it there was no problem. I have tattoos on both arms but it worked for a while on my left arm then stopped which meant no data recorded at night either.
It will not work reliably with tattoos. The sensor needs to "see" through the skin. You need to find a position where the tattoos are less dense - inner wrist, upper the wrist, or even on the ankle. Or use a HRM chest belt, but that would work only during activities. Or have a small window for the sensor removed from the tattoo.
