Something that is hypnagogic makes you sleepy. If you often fall asleep in movie theaters, you can say that movies put you in a hypnagogic, or drowsy, state.
The Greek word hypnos means "sleep," and when it's added to agogos, "leading," it makes hypnagogic, "leading to sleep." You can use the adjective hypnagogic to describe something that makes you yawn or feel sleepy. Hypnagogic is also a psychological term for the moments just before you fall asleep; it's a period that's commonly full of dream imagery.