"pertaining to the state of consciousness when awaking from sleep," 1897, coined by English man of letters Frederic W. H. Myers (1843-1901) from hypno- "sleep" + second element from Greek pompe "sending away," from pempein "to send" (see pomp).
Hypnagogic -- Illusions hypnagogiques (Maury) are the vivid illusions of sight or sound--"faces in the dark," etc.--which sometimes accompany the oncoming of sleep. To similar illusions accompanying the departure of sleep, as when a dream-figure persists for a few moments into waking life, I have given the name hypnopompic. [F.W.H. Myers, "Glossary of Terms used in Psychical Research," Proceedings of the Society for Psychical Research, vol. xii, 1896-97, supplement]
By hypnagogic paramnesia I mean a false memory occurring in the antechamber of sleep, but not necessarily before sleep. Mr. Myers' invention of the word "hypnopompic" seems to me unnecessary except for pedantic reasons. I take the condition of consciousness to be almost the same whether the sleep is coming on or passing away. In the dream I have recorded it is even impossible to say whether the phenomenon is "hypnagogic" or "hypnopompic"; in such a case the twilight consciousness is as much conditioned by the sleep that is passing away as by the sleep that is coming on. [H. Ellis, "A Note on Hypnagogic Paramnesia," in "Mind," vol. vi, 1897]