We've been following this closely here, as I've always had a crush on Heath and my DF loves comics. Apparently in preparation for his role as the Joker in the upcoming "The Dark Knight" he locked himself in a hotel room for a month and wrote a journal from the point of view of the Joker while reading the comics that this movie is based on. (These comics are heart breakingly sad, and disturbing)
Since that time, he's had trouble sleeping (not helping by hopping across the globe on a regular basis) and it's been getting worse lately, to the point where he's been complaining about it in interviews. As someone else mentioned, he took an overdoes of sleeping medication. He may just have been desperate to sleep.
I'm so sad about this.