I have a "gut" feeling putting all the pieces together that Heath Ledger was either suffering from a clinical depression or perhaps part of the condition "bipolar" since he wasn't able to sleep. But I am merely speculating.
The reason I believe this might be the case is 1) the prescription medication "Xanax" (sp?) was part of the medicines he overdosed on. 2) There was a statement on the news -- not yet put into context in which Jack Nicholson said he had "warned him".
My Hunch....Heath Ledger was playing the "Joker", a psychotic killer and so had Jack Nicholson. My acting training taught me how the lines between a role and one's own life may become blurred. I am afraid that what happened may have overpowered him. Heath Ledger was very possibly dealing with an area of his "mind" that was not clearly understood by him. It is my opinion, but a strong one, that he was unleashing some dark forces, due to the fact that in his acting role, he was uncovering some things.... "his demons" "his problems" "his neuroses" and stretching them out so wide that he didn't realize the dangerous pandora box he was unleashing.
Actors/actresses usually emerse themselves in the characters they play by delving deeply into their own history -- observations. They use their own lives to draw upon, imagination, of course, and absorb environments and people that may inspire them. I think Heath Ledger was warned by Nicholson to be careful, because Mr. Nicholson had prepped himself for roles of psychotic killers even back to The Shining...and he knew the dangers.
I studied with William Hickely at HB Studios who once said that if you never killed you don't go out and practice killing....you may draw upon yourself killing an annoying insect. I never played a psychotic killer or killed anyone. However, I have seen more than two people in manias who have been threatening to me and I have killed insects.
So... perhaps Mr. Nicholson was warning him about taking too much medication. Whatever the warning...it is still a sad fact that at only 28 years of age -- a young talented man and father of a 2-year old girl, is no longer alive. Possibly because he didn't get the help he needed. Possibly because of a lack of knowledge.