Atandrita is another name for Śiva-Mahādeva (Siva-Mahadeva), one of the holy trinity of Hinduism. The Sanskrit word tandrā (tandra) means ‘sleep’ or ‘slumber’. Therefore the word atandrita literally means ‘one who does not sleep’ or ‘one not addicted to sleep’. Śiva, as manifesting as the Lord of the Universe, is always busy in the process of Creation – creating and maintaining the universe. He has his keen and responsible eyes on each and every organic and inorganic element of the universe and nothing in the universe is unknown to him. That is why he is called Atandrita, the one never to sleep.
Then again, tandrā or sleep stands as the metaphor for darkness and ignorance, and therefore has been traditionally identified as tamomayī vṛtti (tamomayi vritti; the faculty of darkness). Mahādeva himself being the manifestation of pure knowledge cannot be touched or maimed by the darkness of ignorance. In this sense also he has been hailed as Atandrita —
atandritaḥ tandrā tamomayī vṛttiḥ