Aziara beat me to it! They see all spectrums of light that we can’t see. Even though the purple light is vey dim, they still see light. What happens at night when there is a light on, is they see shadows and it disrupts their sleep because they think something is trying to get them. They have a gland on top of their head nicknamed the 3rd eye that sees light and senses heat. They only need heat in heir tank at night if their tank temp drops below 65 degrees. If hat is the case then you need a ceramic heat emitter.