The Yoga Sutras of Patanjali

10. Sleep is the psychic condition which rests on mind states, all material things being absent.

In waking life, we have two currents of perception; an outer current of physical things seen and heard and perceived; an inner current of mind-images and thoughts. The outer current ceases in sleep; the inner current continues, and watching the mind-images float before the field of consciousness, we "dream." Even when there are no dreams, there is still a certain consciousness in sleep, so that, on waking, one says, "I have slept well," or "I have slept badly."