Sleep had been, until recently, the dark one-third of our lives and, like the far side of the moon, a persistent mystery for all humanity. As with all areas of mystery, it has become entangled with folklore and ritual. Known only indirectly through dimly remembered dreams and the drowsiness associated with falling asleep and awakening, sleep has always been linked with poetry, myth and time-honored superstition. None of us can study himself or herself while asleep nor converse with another who is asleep. But just as the space age has given us instruments to view the far side of the moon, nurturing a more sophisticated knowledge of our planetary environment, so too has it given us the means to investigate, in a far less speculative manner, aspects of brain function leading to a more sophisticated knowledge of our inner environment: Questions regarding the kinds of sleep we are subject to, the variation in the intensity of this sleep, and even the very purpose of sleep can now be entertained. And it is precisely because of everyone's "indirect" experience of it that sleep lends itself so well to an exposition of the power, validity, and fascination of a scientific approach to the understanding of a human activity.
But it has not always been so that we have even appreciated the fact that the seat of sleep resides in the brain. That pillar of Greek enlightenment, Aristotle, attributed to the heart, and anatomically located there, the function of sense perception and sleep. It was not until the 13th century, during the late Renaissance