The summerians used heroin poppies to induce sleep around 4,000 years ago (thats why the latin name means sleep-inducing poppy - papavaer somniforum(sp?))
And poppy has been the base of sleeping medication until quite recently....
And then not long ago (historically speaking), a whole large class of drugs, including many different catagories of drugs were introduced, called hypnotics...
And since I have no intention of doing all your homework, I'll provide a link and let you do the rest.
http://en.wikipedia.org/wiki/Hypnotic