for luka it was easier to deal with sleep paralysis when the lights were off, since he wouldn't see anything. but his desire to help robin overrode his own needs of safety, so when he faces sleep paralysis, he just hopes robin is fine
prior to dating robin, luka slept with no lights. He preferred sleeping in complete darnkess until robin came around. I think when robin is traumatized, he'd start using a night light even if he didn't like it because it made robin feel safer