If you watch and read interviews practically every member of the cast has said that the Joker role being linked to Heath's death is media hype. They were the people working closest to him on the film, they know things the press don't and they have all said he would leave the Joker on the sound stage, that behind the scenes he was happy, excited about his daughter. Morgan Freeman said it is ridiculous to try and suggest a role in a film made him so depressed. Heath was an actor, that's what actors do, they play a part, then leave it when they go home. So regardless of whether he used the method approach or not, I tend to take the word of people who actually knew him over anything else. There could have been other reasons for depression or sleeplessness.