There are lights for the tanks we have/are getting. However, the tanks are in my office, and sometimes I hold long hours working on homework or reading articles for my research. How long is too long for light in the room? I can turn the tank light off at night, of course, but I am concerned about the light in the room.
Don't worry about the light in your room. Having the light on your tank turned off should be fine for them to rest. If you are really want to be sure, you can maybe put a cloth over the tank to give your betta more darkness.
I think of bettas like people eight hours of rest is healthy. If you have a cave or some dense planting real or fake the betta will find a nice dim corner and take a betta nap. And like Kevin said a cloth ore towel if you think it's too brright will help but if the betta has a dark spot in the tank I am sure it will manage. :)