I thought I heard he was taking Ambien. If that's true, I'm fairly sure that, pharmaceutically, you can't die from an overdose of Ambien. Unless, of course, you combine it with other meds or alcohol.
But then again, Ambien realy messes with your head. I've had clients do all sorts of crazy things while taking it. He could have taken a couple Ambien, and done something "normal", but the combination was lethal. I'm withholding judgment until the final coroner's report.
Yahoo mentions Ambien use, Heath's trouble sleeping.