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.
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.