Chapter-1, Sutra 10

Patanjali Yogasutra 1.10 

अभावप्रत्ययालम्बना वृत्तिर्निद्रा ॥ १.१०॥

Sleep is a modification (vritti) which is based on the absence of any content (abhava-pratyaya).

The first word of the sutra explains the context and the terms used in the Sutra. The sutra belongs to the first chapter (samadhi pada) of the Yogasutra, which deals with the nature and types of the mental modifications (vrittis) that are either conducive or obstructive to the attainment of samadhi, the state of absorption in the true self (purusha). The sutra defines sleep as one of the five kinds of vrittis, along with right Knowledge, wrong Knowledge, imagination, and memory. The term abhava-pratyaya means the cognition of non-existence or voidness, which is the opposite of the cognition of existence or reality (bhava-pratyaya). The term alambana means the support or the object of cognition, which is the basis for the formation of a vritti. The term nidra means sleep, which is a state of Consciousness characterized by the absence of any perceptual or conceptual activity.

The second word of the sutra explains the significance and the implication of the sutra. The sutra implies that sleep is not a state of complete cessation of the mind, but a state of subtle mental activity that is based on the notion of nothingness. Sleep is a vritti because it is a modification of the mind that arises from a particular cause and has a particular effect. The cause of sleep is the predominance of tamas, the quality of inertia and dullness, which obscures the other qualities of sattva, the quality of purity and clarity, and rajas, the quality of activity and passion. The effect of sleep is the experience of rest and relaxation, which is necessary for the maintenance of the physical and mental health. However, sleep is also an obstacle to the Realization of the True Self, because it veils the light of awareness and creates a false sense of identity with the body and the mind.

The third word explains the relation and the difference between sleep and samadhi. The sutra suggests that sleep and samadhi are similar in some respects, but different in others. Both sleep and samadhi are states of Consciousness that transcend the ordinary waking state, which is dominated by the fluctuations of the mind and the senses. Both sleep and samadhi are also states of Consciousness that involve the cessation of the gross mental modifications, such as perception, inference, imagination, and memory, which are based on the cognition of existence or reality. However, sleep and samadhi are different in their causes, their objects, and their effects. The cause of sleep is tamas, while the cause of samadhi is sattva. The object of sleep is abhava-pratyaya, while the object of samadhi is purusha, the True Self. The effect of sleep is ignorance, while the effect of samadhi is Knowledge.

The sutra can be compared with similar verses from Vedic texts. 

Brihadaranyaka Upanishad 4.3.21:

यत्र सुप्तो न कञ्चन कामं कामयते न कञ्चन स्वप्नं पश्यति तत् सुषुप्तम् । सुषुप्तस्थान एकीभूतः प्रज्ञानघन एवानन्दमयो ह्यानन्दभुक् चेतोमुखः प्राज्ञस्तृतीयः पादः ॥ ४.३.२१॥

When a person is asleep and does not desire anything or see any dream, that is deep sleep. In that state of deep sleep, he becomes one with the mass of Consciousness, blissful and enjoying bliss, his mind is stilled and he is the third quarter, the prajna.

The verse identifies sleep as the third quarter of the self, the prajna, which is the cosmic aspect of the Self that pervades all beings and things.

Mandukya Upanishad 5:

अन्तःप्रज्ञः सप्ताङ्ग एकोनविंशतिमुखः प्रविविक्तभुक् तैजसः स्वप्नस्थः प्रवर्तकः द्वितीयः पादः ॥ ५॥

The inward-turned Consciousness, with seven limbs and nineteen mouths, enjoying the subtle objects, is the taijasa, the dreamer, whose sphere is the dream state, the second quarter. 

The verse identifies dream as the second quarter of the Self, the taijasa, which is the individual aspect of the self that experiences the subtle world.

Yogavashishta 6.2.215.6:

यथा चित्तं तथा लोको यथा लोकस्तथा चित्तम् । चित्तलोकयोरैक्यं तदेव ब्रह्म तत्त्वम् ॥

As is the mind, so is the world; as is the world, so is the mind. The identity of the mind and the world is the Reality, that is Brahman, the Essence.

The verse identifies the Reality as the identity of the mind and the world, which is the Essence of all things.