I've found with time that the easiest way to fall asleep is to try to meditate. Sit there or lay there in a quiet room and just watch your breath going in and out, not chastising yourself or getting frustrated when your mind inevitably wanders. Simply acknowledge that you've gone off track and come back to your breath. Normally in meditation, the goal isn't to fall asleep, but it is definitely something that you can let happen if you want to.