As anyone experienced the same: I came home late today. It had been dark for a couple of hours. When I turned on the lights in the room, my betta must have been woken up. And he was swimming around the walls of the tank, and running into the wall or thermometer, as if couldn't see. He's a perfectly healthy betta, always has a good appetite. The only time I saw him swim like that was in the 60 seconds after I switched the lights on.