Sandhya means `Twilight` In the Vishnu Purana it is mentioned that Sandhya is a form of Lord Brahma. It is also the name of the period preceding a Yuga.

In the mythology it is mentioned that the night is called Usha, and the day is denominated Vyushta, and the interval between them is called Sandhya. On the occasion of the dreadful Sandhya, the terrific demons, attempt to devour the sun. Brahma then denounced this curse upon them, that, without the power to perish, they should die every day and revive by night. For this reason a fierce contest occurs daily between them and the sun.

In this season pious Brahmins scatter water, purifies by the mystical Omkara, and consecrates by chanting Gayatri mantra. By this water, as by a thunderbolt, the foul demons are consumed. When the first oblation is offered with solemn invocations in the morning rite, the thousand-rayed deity shines forth with unclouded splendor.

Omkara is Lord Vishnu the mighty, the substance of the three Vedas, the lord of speech and by the enunciation of it the Rakshasas are destroyed. The sun is a principal part of Vishnu, and light is his immutable essence, the active manifestation of which is excited by the mystic syllable, Om. Light effused by the utterance of Omkara becomes radiant, and burns up entirely the Rakshasas and demons. The performance of the Sandhya sacrifice must never therefore be delayed, for he who neglects it is guilty for the murder of the sun. Protected thus by the Brahmans and the pigmy sages called Balakhilyas, the sun goes on his course to give light to the world.