At the time of his death, Ledger had just finished playing the villain The Joker in The Dark Knight, the latest installment in the Batman series. The film is to open in July.
The role disturbed him, according to The Associated Press. He called The Joker 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 prescription sleeping pills didn't help, according to AP.
The AP story in question scatters this information throughout the story (perhaps precisely to avoid suggesting that the role killed him); it mentions his previous role, in the Bob Dylan biography I'm Not There, as also having been very stressful. In any event, even if there turns out to be another cause of death, this may account for the presence of the sleeping pills.