My children rarely watch television (and never cable - just dvd's). They don't often ask to watch anything but when they do I just tell them that we don't watch tv because it sucks the energy right out of you.
Last night Audrey asked if they could watch a short dvd before bed. I said maybe. She replied, "Well it's the perfect thing to do before bed. It'll suck the energy right out of us and we'll be all ready to go to sleep." Hmmm - can't argue with that!