Dreams and nightmares-what Ambrose Bierce called "visions of the night"-are the basis of some of the greatest weird fiction in literary history. The unruly images that torment us in sleep are usually dispelled by the coming of day-but can they be dismissed so easily? Do nightmares have some impalpable reality that can affect our daily lives, the lives of those around us, and perhaps the very fabric of the universe? Who can say that the nightmare is...