The operation of the mind associated with the cognition of that which is non-existent is called sleep.
Yoga Sutra 1.10 Recitation
Audio Recitation by D.C. Rao, PhD
abhāva = a + bhāva
bhāva existence; feeling; emotion; being
vṛttiḥ modification of mind; operation of mind; thought construct; that which revolves
Sleep like a sage
During sleep, a veil of darkness normally overcomes us. But the quality of sleep can be improved by understanding how to sleep with awareness.
According to the yogis, if we learn the proper technique for sleeping we can provide a more profound rest to both body and mind in less time.
According to yoga, the mind remains active even in sleep. This is why you may not feel completely rested, even after sleeping for hours. The quality of rest depends on how concentrated and one-pointed the mind has been in sleep, when the activities of the brain, the nervous system, and the senses are almost shut down. Unlike the waking state, in sleep the mind no longer has these tools for going into the external world, gathering the experiences of worldly objects, and bringing them back to the ego and the intellect. Yet because of its roaming tendency, the mind runs to the world of memory even while sleeping and begins to reclaim the contents of the past deposited in the unconscious mind in their subtle form. Often the mind mixes those unconscious contents with false perception and imagination. This is what causes you to dream.
Even when the mind is not caught in the dreamworld, it remains aware that it is not perceiving anything from the external world. It is busy joblessly, but fools itself that it is not busy at all. This state is called dreamless sleep. Yogis call it "mental stupor." In dreamless sleep, the mind has put a thick veil of darkness (tamas) over itself. Consequently it is not aware of its own thought constructs.
But the mind as such does not cease to exist during sleep. That is why when you wake up, you remember that you have been asleep and you remember that you either slept well or that you had a disturbed sleep, depending on what you were experiencing while you slept. But the activities of the mind (or mental modifications) during dreamless sleep move at a slower pace than they do in the waking state. Thus the nervous system gets some degree of rest. That is why, after six to eight hours of sleep, we get a sense of restfulness in body and mind.
According to the yogis, if we learn the proper technique for sleeping we can provide a more profound rest to both body and mind in less time. That technique is known as yogic sleep (yoga nidrā). During yogic sleep, you are fully awake and aware that you are sleeping; the mind is not roaming. During normal sleep you are neither awake nor aware that you are sleeping. During yogic sleep, you sleep like a master; in normal sleep you sleep helplessly, like a slave.
According to the yogis, normal sleep is a waste of time except that it gives some rest to your body and mind. In yogic sleep, you get complete rest. The mental stupor that descends during normal sleep partially erases the clarity and crispness of what you had learned before going to sleep, and that is why, in order to memorize something, you have to keep refreshing that knowledge day after day. In yogic sleep the mind remains so fresh, alert, and clear that not only does it retain what you had learned before, it becomes even clearer. This is because during yogic sleep your body—nervous system, senses, brain, and mind—are fully relaxed and free from all distractions and disturbances. Even time comes to a standstill. Whatever enters the mindfield during yogic sleep becomes firm forever.