The sleeping posture of Lord Vishnu has a deep significance. At the end of a Kalpa, when Pralaya takes place and the creation is dissolved, Lord Vishnu reclines upon the coils of Ananta to gather energy for nourishing the next creation cycle. This is when Adya Shakti Mahamaya subjects Him to Yoganidra (Yogic slumber):
"Even so men are hurled into the whirlpool of attachment, the pit of delusion, through the power of Mahamaya, the creatrix of world. This Mahamaya subjects Vishnu, the Lord of the world, to the Yogic slumber. It is by Her the Jivas are deluded. Verily this Bhagavati, the Mahamaya forcibly drawing the minds of even the wise, throws them into delusion. She creates this entire universe, both moving and unmoving. It is She who, when appeased, bestows boons upon human beings for Moksha.” (Markandeya Purana, Devi Mahatmyah 1:53-56)
This Devi Mahamaya is none other than Mahakali, who is ten armed, ten faced and ten footed. She is Tamasi Shakti that deludes not just Jivas, but Vishnu Himself:
"She who holds sword, discus, mace, arrow, bow, skull-cup, conch-shell, trident, iron-pestle and severed head in Her ten arms; is three-eyed, bejewelled and of bluish complexion of sapphire; has ten faces and ten feet; whom Brahma eulogised for destruction of Asuras Madhu & Kaitabha when Lord Vishnu was engrossed in his Yogic slumber, I meditate upon that Mahakali."
When Vishnu gets deluded, at that time two Asuras, Madhu and Kaitabha are born of Vishnu's ear wax. They intend to kill Brahma, because of which the creator eulogises Mahamaya, the Tamasi Shakti, who deluded Vishnu:
"She is eternal, embodied as the universe. By Her all this is pervaded. Nevertheless She manifests in manifold ways; hear it from me. When She manifests to accomplish the purposes of Devas, She is said to be born in the world, though She is eternal. At the end of a Kalpa when the universe was one ocean, Lord Vishnu reclined on Shesa and fall into a Yogic slumber. Then, two terrible Asuras, renowned as Madhu and Kaitabha, were born of the ear wax of Vishnu’s ears and sought to slay Brahma. Prajapati Brahma, who was born of a lotus that sprout from Vishnu’s navel, saw those two fierce Asuras and Vishnu asleep. Hence, with a view to awakening Hari, Brahma extolled Yoganidra, dwelling in Hari’s eyes. The resplendent Brahma eulogised that incomparable Vishveshvari Jagadhatri, the creatrix, sustainer and destructrix of the universe; that Bhagavati, who subjects Vishnu to Yogic slumber, Brahma praised Her thus. (Markandeya Purana, Devi Mahatmyah 1:65-71)
This is followed by Kalika Stuti, also known as Tantrokta Ratri Suktam. Appeased by Brahma's prayer, Mahamaya withdraws Her spell, thus awakening Vishnu:
"There, that Mahamaya, extolled thus by Brahma, the creator, in order to awaken Vishnu from his Yogic slumber for the destruction of Madhu and Kaitabha, withdrew Herself out from Vishnu’s eyes, mouth, nostrils, arms, heart and chest, and appeared in the sight of Brahma of inscrutable birth. Janardana, Lord of the universe, freed by Her spell, rose up from his couch on the universal ocean, and saw those two wicked Asuras, Madhu and Kaitabha, of great valour and strength, with eyes blood-shot in anger, eager to devour Brahma. (Markandeya Purana, Devi Mahatmyah 1:89-93)
This followed by a five-thousand year long battle, at the end of which Madhu and Kaitabha were killed. We know that Mahamaya is Tamasi Shakti. But who are Brahma, Vishnu, Madhu and Kaitabha? Vishnu is Satva, Brahma is Jiva, Madhu is Rajas and Kaitabha is Tamas.
Due to Mahamaya's spell, Satva (Vishnu) lies inactive. During this time, Rajas (Madhu) and Tamas (Kaitabha) become active. These two plague the Jiva (Brahma) to an endless cycle of birth, death and reincarnation. Only when Jiva (Brahma) surrenders to Mahamaya, only then She becomes Mahavidya and bestows Moksha to Jiva (Brahma) by awakening Satva (Vishnu), who eliminates Rajas (Madhu) and Tamas (Kaitabha) and thus clears the path to Nirvana:
"She creates this entire universe, both moving and unmoving. It is She who, when appeased, bestows boons upon human beings for Moksha. She is Brahmavidya personified, the cause of Moksha, and eternal; She is the cause of the bondage of transmigration and the sovereign over all lords.” (Markandeya Purana, Devi Mahatmyah 1:57-58)
"That Sati alone when worshipped for ever confers good fortunes. She is the great goddess who bestows everything auspicious on her devotees. That Sati alone when propitiated for ever destroys the fear from worldly existence. She is the goddess who confers what we desire and who removes all disorders. That Sati alone when propitiated for ever bestows fame and wealth: She is the great Goddess who confers worldly pleasures and salvation. That Sati alone is the creator of the universe, the protectress of the universe, the cause of destruction of the universe at the end of the Kalpa. She is the primordial Sakti." (Shiva Purana, Rudra Samhita 2: :7-10)
“The supreme goddess Parvati is the one who bestows knowledge of Brahman” (Skanda Purana, Suta Samhita 4:13:34)
This is the inherent sublime meaning behind Lord Vishnu's slumber.