Note: This is a sequel to this list, which also happens to be full of rare, delightful words to incorporate into your repertoire.
adjective : of, relating to, or occurring in the period of drowsiness immediately preceding sleep
While those of us who are distracted by words during the period of drowsiness immediately preceding sleep do not necessarily wish the same fate on those who are spared such things, we see no reason to go on as though there is no term to describe the hypnagogic realm, and if you recall this word just as you're dropping off, well, so be it.
Hypnagogic (also spelled hypnogogic) is of 19th century vintage. It's from French, which got it from Greek: hypn- means "sleep," and -agōgos means "inducing, leading."