There sure is some speculation.
AP- Ledger said in an interview in November that "Dark Knight" and last year's "I'm Not There," took a heavy toll. He said he "stressed out a little too much" during the Dylan film, and had trouble sleeping while portraying the Joker, whom he called a "psychopathic, mass-murdering, schizophrenic clown with zero empathy."
"Last week I probably slept an average of two hours a night," Ledger told The New York Times. "I couldn't stop thinking. My body was exhausted, and my mind was still going." He said he took two Ambien pills, which only worked for an hour.