Yeah...
He actually stayed in an apartment by himself for a month studying the role of the Joker. Getting the posture just right and practicing the laugh and just other little minor things like that, he wanted to live up to everyone's expectations.
So while doing that and during the filming of the movie, Christian Bale actually said in an interview that he would come on the set looking like shit because he would only get 2-3 hours of sleep a week!
So what happened?
My THOUGHTS are that he started taking the pills to get some sleep, which he is quoted to saying that they didn't work, and he became dependent on them but still couldn't get enough sleep....so he took more and more....and eventually accidentally took TOO MUCH. Thus...accidentally overdosing.
I was a fan...and I was really excited about The Dark Knight and still am...he basically put his life into it...you could actually say that The Dark Knight killed the Joker...although he was done filming it and was actually filming another movie, he started losing sleep while filming The Dark Knight and the pills killed him in the long run.
His role as the Joker will most likely earn him an Oscar...he put so much work into the role...it cost him his life...
Rest In Peace, Heath...we'll miss you.