Light and deep sleep differ physiologically, deep sleep having much in common with being awake. Studies with cats now suggest that the two states of sleep are induced by different biochemical secretions.

Introduction

Early philosophers recognized that there are two distinctly different levels of sleep. An ancient Hindu tale described three states of mind in man: (1) wakefulness (vaiswanara), in which a person "is conscious only of external objects [and] is the enjoyer of the pleasures of sense"; (2) dreaming sleep (taijasa), in which one "is conscious only of his dreams [and] is the enjoyer of the subtle impressions in the mind of the deeds he has done in the past," and (3) dreamless sleep (prajna), a "blissful" state in which "the veil of unconsciousness envelops his thought and knowledge, and the subtle impressions of his mind apparently vanish."

States 2 and 3 obviously are rather difficult to investigate objectively, and until very recently the phases of sleep remained a subject of vague speculation. Within the past few years, however, studies with the aid of the electroencephalograph have begun to lift the veil. By recording brain waves, eye movements and other activities of the nervous system during the different sleep states neurophysiologists are beginning to identify the specific nervous-system structures involved, and we are now in a position to analyze some of the mechanisms responsible.

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