In one of the reports I read, the sleeping pills he was taking was Ambien. It said he was not suffering from depression, but was having trouble sleeping from all the stress of doing "The Dark Knight" and another film he was working on - and so was taking Ambien to help him sleep.
I take Ambien, and I know from experience that I can take several pills without realizing it. Ambien works as an amnesiac to an extent, it is one of the drawbacks of taking it. I took 6 pills one night, because I kept thinking I hadn't taken it yet. Hubby now keeps the prescription bottle with him and doles them out to me at night so that I do not take over and over again.
If he was taking other pills on top of the Ambien, and forgot that he had taken those doses as well, it would have been quite simple for him to overdose - without ever meaning to.
It is truly a tragedy. And if anyone ever takes these types of medications, I strongly urge that you neve take them while at home alone, and have your spouse be aware of when you are taking them.