Heath did an interview towarRAB the end of filming of Dark Knight and he said himself, as long as he was quoted correctly, that he was so wrapped up in the role he was only getting an hour's sleep a night if he was lucky. As a Heath Fan and keeping up to date on all his interviews and what he was up to, piccies and what not, I suspected that he become unstuck during the filming of ADK, which coincided with the breakdown of his relationship with Michelle and it was downhill from there. He was known to be, and admitted he was hyperactive anyway, so I would imagine a combination of that and his lifestyle at the time led to the need to prescription pills to help him relax and sleep.