I get that way too.  When I'm really in writing mode, I can spend literally a whole day sitting at the computer, researching and writing (and getting distracted and wasting time on here and on Twitter LOL).  I don't watch TV, I forget to eat, I don't leave the apartment if I can help it.  I'm in the zone.  I had that kind of day yesterday, and I didn't actually write a thing, but I had a new story idea I couldn't let go of and needed to get out of my system, so I spent the day making a four-page outline.
Then there are days where I can't ever get the writing going and will spend the day reading or watching TV instead.  I almost always read before I go to bed, though.  I love sleeping, but I'm not a big fan of going to bed, and it takes me awhile to fall asleep, so I always delay it a bit by reading in bed.  I don't read a lot during the day, though, unless it's a book I just can't put down.