TV attacked me

On Sunday I found myself with nothing to do at the last minute. After a prolonged lie in I got up and moped about. After around 6 hours of TV marathons I looked at my watch and thought I'd just woken from a coma. I felt partially brain dead....maybe that's why I still feel so tired.

Why does long periods of TV tire you out? Intense concentration? Atrophy in your muscles due to the lack of movement?