Yoga Sutra - Samādhi Pada 1.10
भावप्रत्ययालम्बना तमोवृत्तिर्निद्र ॥१०॥
abhāva-pratyaya-ālambanā tamaḥ vṛttiḥ nidrā ||10||
The sleep fluctuation is based on the intention of nonbecoming.
Deep sleep is that state of mind which is based on an absence [of any content].
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.