abhāva-pratyayālambanā vrttir nidrā
abhāva-pratyaya-ālambanā vrttiḥ nidrā
“Sleep is supported by a thought-wave toward not-being. It also conditions the mind.”

The fourth type of  patterning (vrtti) is sleep. Many commentators  interpret sleep here to refer to non-dreaming sleep, since the sleep here is a movement of the consciousness toward not-being or nothingness (abhāva).

Patanjali says sleep is “supported by” (ālambana) a thought-wave toward not-being. This is an awkward construct in English, but it is an important idea in yoga philosophy. The consciousness is fluid, and a thought is described as a movement toward an object. Indeed, one might say that the most harmful aspect of vṛttis is if they rigidify the mind, cause a restriction to the natural flow of attention.

Practitioners of yoga āsanas will recognize the root ālamb from sālamba sarvangāsana, supported shoulderstand. As the body is supported on the arms in this pose, so the mind and attention are supported by chosen points of focus, perhaps the  press of the arms down and the reach of the legs up.

Sleep affects the state of the consciousness, as do all the vṛttis Patañjali describes.

“In sleep, the senses of perception rest in the mind, the mind in the consciousness and the consciousness in the being.” –B.K.S. Iyengar, Light on the Yoga Sūtras of Patañjali, commentary on I.10

“It may also seem odd that Patañjali regards sleep as a pattern of consciousness. After all, deep sleep, at least, feels like unconsciousness. Imagine, though, that from the perspective of omnipresent pure awareness, or puruṣa, sleep is just another set of forms appearing on the screen of consciousness–in this case, dark and silent.” -Chip Hartranft, The Yoga-Sūtra of Patañjali, p. 5

“Great masters affirm that we return temporarily to God during deep sleep. They explain that after a good night’s sleep, we feel refreshed not from physiological rest, but from the fresh energy that flows to us from God. This energy nourishes the spiritual, psychological, and physical levels of the human being.” –Bernard Bouanchaud, The Essence of Yoga, p. 15

Questions:
•  Notice the moment of falling asleep, the quality of your sleep when sleeping, the nearness of the moment of waking, your dreams. What does awareness of sleep teach you about your state of well-being? (Thanks to Bernard Bouanchaud for this question.)
• How does sleep affect your waking life–your mind, your emotions? How do the events of the day affect your sleep? What do you consider restful sleep?
• What kinds of circumstances might disturb your sleep?