śeṣetmanā nija-sukhānubhavo nirīhaḥ
turye sthito na tu tamo na guṇāṁś ca yuṅkṣe
As explained very clearly in the Brahma-saṁhitā (5.47):
yaḥ kāraṇārṇava-jale bhajati sma yoga-
ādhāra-śaktim avalambya parāṁ sva-mūrtiṁ
govindam ādi-puruṣaṁ tam ahaṁ bhajāmi
“I worship the primeval Lord Govinda, who lies down in the Causal Ocean in His plenary portion as Mahā-Viṣṇu, with all the universes generating from the pores of hair on His transcendental body, and who accepts the mystic slumber of eternity.” The ādi-puruṣa, the original Supreme Personality of Godhead — Kṛṣṇa, Govinda — expands Himself as Mahā-Viṣṇu. After the annihilation of this cosmic manifestation, He keeps Himself in transcendental bliss. The word yoga-nidrām is used in reference to the Supreme Personality of Godhead. One should understand that this nidrā, or sleep, is not like our nidrā in the mode of ignorance. The Lord is always situated in transcendence. He is sac-cid-ānanda — eternally in bliss — and thus He is not disturbed by sleep like ordinary human beings. It should be understood that the Supreme Personality of Godhead is in transcendental bliss in all stages. Śrīla Madhvācārya concisely states that the Lord is turya-sthitaḥ, always situated in transcendence. In transcendence there is no such thing as jāgaraṇa-nidrā-suṣupti — wakefulness, sleep and deep sleep.
The practice of yoga is similar to the yoga-nidrā of Mahā-Viṣṇu. Yogīs are advised to keep their eyes half closed, but this state is not at all one of sleep, although imitation yogīs, especially in the modern age, manifest their so-called yoga by sleeping. In the śāstra, yoga is described as dhyānāvasthita, a state of full meditation, but this is meditation upon the Supreme Personality of Godhead. Dhyānāvasthita-tad-gatena manasā: the mind should always be situated at the lotus feet of the Lord. Yoga practice does not mean sleeping. The mind should always be actively fixed at the lotus feet of the Lord. Then one’s practice of yoga will be successful.