Sutra 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.'