Deep sleep is that state of mind which is based on an absence [of any content].

Bryant Commentary:

The commentators acknowledge here that there is some difference of opinion regarding whether or not sleep is an actual vṛtti. Based on the Chāndogya Upaniṣad (VI.8.1), the Vedāntins do not consider any vṛtti to be present in the citta during deep sleep, but that the ātman, or puruṣa, undisturbed by any citta vṛtti in the state of deep sleep, experiences Brahman (Vedānta Sūtra I.4.18; II.3.31). Vyāsa and the Yoga commentators, in contrast, view deep sleep as a type of vṛtti on the grounds that when one awakes, one remembers that one has either slept well or restlessly or in a stupor. One would not be able to do so, in their view, if these impressions did not relate back to a state of mind that existed during deep sleep. In Yoga psychology, memory is the product of saṁskāra, and saṁskāra is caused by experience. Therefore, the memory of having slept well must relate to a state of mind experienced during deep sleep, which is recorded in the citta as memory (the topic of the next sūtra) and remembered upon awakening. This state of mind according to this line of reasoning must therefore pertain to a category of vṛtti distinct from others.

It might be useful to note along with Vācaspati Miśra that the fourth vṛtti being discussed by Patañjali in this sūtra does not refer to the state of mind represented in the dream state—dream sleep corresponds to the vṛttis of memory (since it involves the activation of saṁskāras). Nidrā, sleep, then, refers to deep dreamless sleep. It takes place when the tāmasic element of the mind densely covers the sāttvic nature of buddhi, the intellect; in dream, more rajas is active, and this churns up past saṁskāras, which produce dream experiences. In deep sleep, rajas is inactive, and so the mind is not stimulated to assume the form of the objects of knowledge, as it does during the waking and dream states; thus puruṣa is conscious of darkness alone. Another way to put this is that due to the preponderance of tamas, there is a suppression of the other vṛttis. However, needless to say, while deep sleep can be considered a type of citta-vṛtti-nirodha, cessation of thought (at least of the four vṛttis other than sleep), since the sattva or knowledge aspect of the citta is smothered by tamas, as is any rājasic stirring of citta, this is not the citta-vṛtti-nirodha defined by Patañjali as the goal of yoga in I.2. In the samādhi state, sattva is at its maximum, and the yogī’s citta-vṛtti-nirodha occurs in full vibrant wakefulness and in complete lucidity as to the nature of reality; in deep sleep, awareness is simply aware of the dense motionless darkness of tamas in which it is enveloped. Additionally (and importantly), this verse informs us that consciousness is eternal; it is never “switched off,” not even in deep sleep. In deep sleep, it remains fully aware, since it is eternally and inherently fully aware, but its object of awareness is (almost) pure tamas; hence there is nothing to recollect when one awakens (other than whether one has slept well or not).

If the tāmasic element that covers the intellect during deep sleep is accompanied by a measure of sattva, the commentators inform us, a person feels refreshed and lucid upon awakening; if accompanied by rajas, one feels that one has slept restlessly and one is confused and distracted; if tamas has almost completely dominated sattva and rajas during sleep, one feels sluggish and tired upon awakening. Pointing back to I.3 where Patañjali states that yoga is the cessation of all vṛttis, which therefore includes deep sleep as well, Vācaspati Miśra notes that sleep, too, can be controlled in samādhi, meditative absorption. And certainly the hagiographies of saints the world over are replete with claims that many indulged in a very minimal amount of sleep.

There are two important technical terms introduced in this sūtra: pratyaya and ālambanā. Pratyaya has a number of meanings, two of which are relevant to the sūtras. It can mean cause, as it is used in Buddhist sources, which is how Vācaspati Miśra takes it here (as also in I.19).77 Elsewhere (II.20; III.2, 12, 17, 19, 35)—and it can also be read here in this sense—it refers to the image of an object imprinted on the mind, that is, a cognition, which is how it is understood in the Vedānta tradition. In Yoga cognition, the powers behind the five senses, jñānendriyas, flow out through the senses with the mind (as the antaḥkaraṇa), to grasp their objects (pratyaya, from prati + i, to go forth), and then imprint images of these objects on the mind (which then presents them to puruṣa as outlined previously via its faculty of buddhi). These imprints or cognitions are pratyayas. Although sometimes used synonymously with vṛtti by the commentators, it differs in my understanding insofar as it represents a singular momentary imprint, while a vṛtti is more a flow of thoughts or images and may contain a series of pratyayas. The second term, ālambana, is the support for the mind and refers to any object upon which the yogī has chosen to focus or concentrate the mind. A list of possible ālambanas for meditation is presented by Patañjali in I.23–39.

Dreamless Sleep Is The Mental Modification Produced By Condition Of Inertia As the State Of Vacuity or Negation (Of Waking And Dreaming).

Deep sleep is a pattern grounded in the perception that nothing exists.

Sleep is a vacillation of understanding dependent upon the absence of mindfulness.

That modification of the mind which is based on the absence of any content in it is sleep.

Taimini Commentary:

This is an important Sutra and should be studied carefully. Of course, the literal meaning of the Sutra is obvious. Even that modification of the mind in which there is no content in the mind is classed as Vrtti which is called Nidra. And for a very good reason. During the time a person is in this state his mind is, as it were, a blank or a void. There is no Pratyaya in the field of consciousness. This state outwardly appears to be the same as that of Citta-Vrtti-Nirodha in which also there is complete suppression of mental modifications. How does this state then differ from the condition of Nirbija-Samadhi for the two are poles apart? The difference lies in the fact that in the state of Nidra or deep sleep the mental activity does not stop at all, only the brain is disconnected from the mind and so does not record the activities which are going on in the mind. When the person wakes up and the contact is established again, the brain again becomes the seat of mental activity as before. When a car is put out of gear the engine does not stop, only the effect of the running of the engine on the car disappears and so there is no motion of the car. In the same way, in deep sleep although there is no Pratyaya in the brain the mental activity is transferred to a subtler vehicle and goes on as before. Only the brain has been put out of gear. Experiments in hypnotism and mesmerism partly corroborate this view.

Now, in Yoga it is the activity of the mind or Citta which is suppressed and for this it is necessary to stop the vibrations of the lower mental body while in the waking state. In the waking state the brain is connected with the lower mind and by controlling the activity of the mind in the brain we can control its own activity. When the engine of a car is in gear, by regulating or stopping the motion of the car we can regulate or stop the movement of the engine itself. It will be seen therefore that the state of deep sleep and the state of Citta-Vrtti-Nirodha though superficially they may appear similar are quite different.

Dreamless sleep is an inert state of consciousness in which the sense of existence is not felt.

Iyengar Commentary:

Sleep is a state in which all activities of thought and feeling come to an end. In sleep, the senses of perception rest in the mind, the mind in the consciousness and the consciousness in the being. Sleep is of three types. If one feels heavy and dull after sleep, that sleep has been tamasic. Disturbed sleep is rajasic. Sleep that brings lightness, brightness and freshness is sattvic.

In the states of correct knowledge, perverse knowledge, fanciful knowledge, and knowledge born of memory, one is awake. Mind and consciousness are drawn by the senses into contact with external objects: thus, one gains knowledge. In deep sleep, these four types of knowledge are absent: the senses of perception cease to function because their king, the mind, is at rest. This is abhava, a state of void, a feeling of emptiness.

The sadhaka, having experienced this negative state of void in sleep, tries to transform it into a positive state of mind while awake. Then he experiences that pure state in which the self is free from the knowledge of things seen, heard, acquired or felt through the senses and the mind. When he has learned to silence all the modulations of mind and consciousness, then he has reached kaivalya. He has sublimated the vrttis and become a master: his citta is submerged in the soul.
Sleep gives one a glimpse of the seer, but only indistinctly because the light of discrimination, viveka, is clouded. Simulation of this state of sleep when one is awake and aware is samadhi, wherein the seer witnesses his own form.

That mental modification supported by cognition of nothingness is sleep.

Satchidananda Commentary:

This is the fourth type. Normally, we say we do not have any thought in the mind during sleep. But actually we have the thought of having no thought. That is why when we wake up we say, “I slept very well; I knew nothing.” You knew nothing, but you know that you knew nothing. Don’t think there is no thought in sleep. If there were no thought and you were completely unconscious, you would not even feel that you had slept. All other thoughts are temporarily suspended except this one thought of emptiness in the mind, which leaves its impression upon waking.

Sleep is a Vrtti which embraces the feeling of voidness.

SV Commentary:

The next class of Vrttis is called sleep and dream. When we awake we know that we have been sleeping; we can only have memory of perception. That which we do not perceive we never can have any memory of. Every reaction is a wave in the lake. Now, if, during sleep, the mind has no waves, it would have no perceptions, positive or negative, and, therefore, we would not remember them. The very reason of our remembering sleep is that during sleep there was a certain class of waves in the mind. Memory is another class of Vrttis, Patanjali Yoga Sutras 18 which is called Smrti.

“Sleep is the mental modification which has for its objective substratum the cause of non-existence.” And this is a particular kind of notion, because it is called back on awakening. How? ‘I have slept well. My mind is clear ; it renders my intellect bright,’ ‘I  have slept badly; my mind is listless; it wanders and is unsteady.’ ‘I have slept with great stupidity ; my limbs are heavy; my mind is. tired ; it stands as it were lazy and absent.’ This calling back would certainly not exist on awakening, if there were no recognition of the cause : and there would not be memories dependent thereupon and having that for their object. Therefore sleep is a particular kind of notion, and further it is to be checked in trance like any other modification. 

~ Rāma Prasāda translation.

Yogasūtrabhāṣyavivaraṇa
or the Pātañjalayogaśāstravivaraṇa
by Śaṅkara

Patañjali Sūtra I.10

The mental process which rests on the notion of nonexistence is sleep

This is a special notion arising from the recollection on waking in the form of ‘I slept well; my mind is calm and has cleared up my understanding’, or else ‘I slept badly, my mind is dull and wanders aimlessly’ or again ‘I slept sunk in stupor; my limbs seem heavy and my mind is limp and faint, as if some force had seized control of it.’ There would be no recollection on waking unless caused by an experience; without an experience there would be no memories based on it and corresponding to it. Therefore sleep is a particular notion, and like all the others it is to be inhibited in samādhi.

Now the sūtra describes sleep: The mental process which rests on the notion of nonexistence is sleep (nidrā). Right knowledge, illusion, and logical construction, which have just been explained, are in the waking condition, and sleep begins when they cease; so sleep is now described, immediately after them.

Non-existence means absence of the waking state, not absolute nonexistence, because there could be no notion of that. The notion of nonexistence means that there is a notion about non-existence. The mental process of which this is the support is one which rests on the notion of nonexistence, and that is sleep (nidrā), a dreamless state (suṣupta-avasthā).

(Opponent) The dreaming state (svapna-avasthā) also must be included in sleep.

(Answer) It does not come under sleep as defined here, because the sūtra (I.38) will distinguish them: Or by meditation on the knowledge of dream (svapna) and sleep (nidrā). There sleep (nidrā) refers only to dreamless sleep. It is only dreamless sleep that rests on the notion of non-existence. Dreaming does not rest on that notion, but on memory, and memory is of something experienced. The commentator illustrates this point about memory when he says that in dream, things remembered become actualized (comm. to sūtra I.11).

It is evident that dream is a mental process, and there is no doubt about that. But the question does arise about dreamless sleep, and so the commentator says, Dreamless sleep is a special notion arising from the recollection on waking. Unless there had been a notion, there could hardly be a recollection. And when one wakes, one does recall, ‘I have slept well’ and so on. The recollection itself is a reflection of the notion that I have experienced something; unless there had been some experience, that reflection would not be there, nor could there reasonably be any memories about it. If dreamless sleep were not a notion, there would not be any effects or notional experiences such as my mind has cleared up my understanding (of a problem), where mind is active.

Again, there is the experience I slept badly, my mind is dull, static, ineffective; or again My limbs seem heavy (guruguru) – following the sūtra 8.1.12 of the grammarian, the repetition of the word indicates some resemblance, so that the meaning is that they feel heavyish; my mind is limp, as if some force had seized control of it out of my possession, as it were. There are these various memories, effects and recollections, and the commentator has presented three of them by which it is established that sleep is a particular notion.

Again, a man who has been asleep in an inner room, without any hint from outside however slight, has recollected immediately on waking ‘I have slept a long time’, and this would otherwise be inexplicable.

(Opponent) These are not memories, because a memory must be of some particular thing which one has experienced, whereas on waking from dreamless sleep one does not remember any definite thing.

(Answer) In the infant as soon as he is born, we see the desire for union with the breast, which he has never experienced in this life. It is not something automatic; purposeful activity is always consequent on some memory, as we see in adults. So here, the recollection whose object is dreamless sleep must follow from some notion just because it is a recollection, like recollections of what has been perceived in the waking state. The heaviness of limb and so on of a man waking from dreamless sleep is an effect, preceded by some experience of discomfort or the like; the heaviness is an effect in its field, the limb. Just as it is certain that this effect has been preceded by some experience of discomfort, so it is certain that sleep is a special notion.