The Collaborative International Dictionary of English v.0.48:

Hypnagogic \Hyp`na*gog"ic\, a. [Gr. ? sleep + ? a carrying
   away.]
   Leading to sleep; -- applied to the illusions of one who is
   half asleep.
   [1913 Webster]

WordNet (r) 3.0 (2006):

hypnagogic
    adj 1: sleep inducing [syn: soporific, soporiferous,
           somniferous, somnific, hypnogogic, hypnagogic]