The autopsy specifically noted needle marks on his arms. Whether the drugs he used/abused were street drugs or prescribed (like the propofol) clearly indicated that he had major issues. It doesn't matter whether it was prescribed or bought off the street. I personally think that he would have benefited from many, many years of psychotherapy and perhaps writing a cathartic tell all "daddy dearest" book.
It was painful to me to look at him. I thought he was an adorable kid and then he disfigured his face with all those plastic surgeries in a vain (no pun intended) to battle inner demons. I look at the before and after pictures and I much prefer the Michael before all the surgeries. I wish that he'd had spent his money on a really good therapist instead of surgery.
He wasn't able to sleep without drugs, he was dependent on them either from their addictive quality or medical issues -- perhaps adrenal fatigue or inability to produce enough melatonin.