You can't move. You can't talk. You may feel the weight of some alien body pressing down on your chest -- or even kinesthetic sensations, such as feelings of being dragged from your bed, vibrating, flying or falling.

And then there are the hallucinations -- the true, terrifying colors of Borges' mirror fish ripped from the world of dream into our own. Because both the hypnagogic (falling asleep) and hypnopompic (waking) states are highly susceptible to hallucination. In the former, the descending rational mind tries to make sense of nonlinear dream images. In the latter, the emerging dream-mind tries to make sense of real-world sights and sounds in the surrounding environment.