I have discovered that on those rare nights I can't get to sleep because my mind just keeps going that silently chanting the heart sutra puts me to sleep, often before I can finish it. I think it's probably the rhythm of it, so it might work with anything chanted, take your pick, but by making it the heart sutra it becomes part of my practice, so I figuratively sleep on it so that it literally seeps into my subconscious... or whatever. Anyway, for all you Buddhist insomniacs, feel free to try.