I'm prone to having trouble knocking out- even alcohol keeps me awake.
For some, like myself, even the purest most flooring MDMA will keep me up, rubbing my feet together like a happy bumble bee. I pretty much have simply grown to accept that it's going to be an all nighter for me, I usually plan a relaxing day of laying around, watching mindless movies, DJing, massaging my friends, and playing with my pets. The following night I sleep hibernation style.
Bearlove nailed it if you want a drug induced sleep- benzos. I also find ketamine helps me relax and pass out sometimes (but not always) after any stimulant.