My dad keeps telling me off for having the lights on when I am using my computer or playing on my PS3. But I do this for a reason:
I get eye-strain when I look at a screen when there isn't ample background lighting. Although my dad tells me that cinemas have all the lights turned off when they project a movie, the difference between a movie projector and a lit monitor/TV screen is too great as a projector doesn't provide enough light to effectively display what's on-screen if there are other lights on in the same area. TV Screens and monitors don't have this problem as they are lit brightly enough to view, even when the lights are on. I don't know about dad, but I would greatly prefer retaining decent vision even if it meant higher electricity bills, rather than skimping on electricity costs if it meant deficiency in my eyes, repeated eye tests and shelling out for new glasses.