Source : Webster's Revised Unabridged Dictionary (1913)

Hypnagogic \Hyp`na*gog"ic\, a. [Gr. ? sleep + ? a carrying
   away.]
   Leading to sleep; -- applied to the illusions of one who is
   half asleep.

Source : WordNet®

hypnagogic
     adj : sleep inducing [syn: {soporific}, {soporiferous}, {somniferous},
            {somnific}, {hypnogogic}]