Someone told me about an article from last November, where Ledger was saying that after filming his role of The Joker for the new Batman movie, he was having trouble sleeping... he said something along the lines that his body was exhausted, but his mind could not stoip working.
I'm guessing he was using sleeping pills to help him get to sleep... and lack of sleep makes you do crazy things, like maybe thinking that taking a few more will help him get to sleep faster or that the regular dose was just not working anymore...
Sad really.