The Rātrisūkta of r̥gvēda (X.127) is popularly recited during the pāṭha of Srī Durgā Saptaśatī. Two verses therein clearly refer to Bhagavatī Durgā, who is identified as Parāśakti by the celebrated commentator Sāyaṇācārya. Rātrī, referred to as the sister of Uṣas, is described as the substratum of creation, and also allegorical to Goddess Kālī. The sūkta was revealed by Kuśika, who contemplated on the all-pervading and powerful darkness as the personification of Goddess Rātrī. In the Athrvaṇa vēda (XIX:4,5,3), Rātrī is explicitly identified as Durgā and śivā. While the association of Rātrī with encompassing darkness is straightforward, she is also identified as Sarasvatī in the Br̥haddēvatā (2:74-77), the deliverer from darkness through the liberating self-knowledge. It may also be noted that while for ordinary mortals the nighttime is frightening and is spent in slumber, for a skilled Tāntric aspirant, it is the best suited time to accomplish dēvatāsānnidhya, and hence a period of utmost wakeful awareness.
Rātrisūkta sings the glory of Rātri-dēvatā, who is described to be of two forms in the scriptures: Jīvarātri and īśvararātri. Jīvaratri is the normal night when the laukika vyavahāra for that day ceases or attains laya. īśvararātri refers to the state where during the mahāpraḷaya where even the īśvara vyavahāra ceases i.e. the dissolution of the prapañca with its īśvara into Parāmbā occurs. What remains is the cidvastu which is Brahma-Māyātmaka or śabala and is the attribute-less primal cause. This is also described in Dēvībhāgavata Mahāpurāṇa:
brahmamāyātmikā rātriḥ paramēśalayātmikā |
tadadhiṣṭhātr̥dēvī tu bhuvanēśī prakīrtitā ||
The Supreme Deity Bhuvanēśvarī, in who even Paramēśvara attains laya, is the adhiṣṭhātrī of Brahmamāyātmikā rātri and the sūkta sings the praises of this Rātrī.