अधोलोकवर्णनम् (Adholoka-varṇana) — Description of the Lower Worlds and Cosmographic Measures
तस्मात्सन्ध्येति तामाहुरुषाव्युष्ट्योर्यदन्तरम् / उषा रात्रिः स्मृता विप्रैर्व्युष्टिश्चापि त्वहः स्मृतम्
tasmātsandhyeti tāmāhuruṣāvyuṣṭyoryadantaram / uṣā rātriḥ smṛtā viprairvyuṣṭiścāpi tvahaḥ smṛtam
Therefore the interval between Uṣā and Vyuṣṭi is called Sandhyā; the sages remember Uṣā as night, and Vyuṣṭi as day.