Sandhyavandhanam is a ritual that all Brahmanas (in fact all Dvijas) are required to perform three times a day. This chapter of the Vishnu Purana describes one reason why Sandhyavandhanam is done, namely to destroy demons called Mandehas who try to eat the Sun:
The night is called Ushá, and the day is denominated Vyusht́a, and the interval between them is called Sandhya. On the occurrence of the awful Sandhya, the terrific fiends termed Mandehas attempt to devour the sun; for Brahmá denounced this curse upon them, that, without the power to perish, they should die every day (and revive by night), and therefore a fierce contest occurs daily between them and the sun. At this season pious Brahmans scatter water, purified by the mystical Omkára, and consecrated by the Gáyatri; and by this water, as by a thunderbolt, the foul fiends are consumed. When the first oblation is offered with solemn invocations in the morning rite, the thousand-rayed deity shines forth with unclouded splendour. Omkára is Vishńu the mighty, the substance of the three Vedas, the lord of speech; and by its enunciation those Rákshasas are destroyed. The sun is a principal part of Vishńu, and light is his immutable essence, the active manifestation of which is excited by the mystic syllable Om. Light effused by the utterance of Omkára becomes radiant, and burns up entirely the Rákshasas called Mandehas. The performance of the Sandhya (the morning) sacrifice must never therefore be delayed, for he who neglects it is guilty of the murder of the sun.
My question is, why did Brahma curse the Mandehas to die and come back to life every day and night? What did they do to incur this curse?
By the way, another chapter of the Vishnu Purana uses Mandeha as a term for certain low-caste inhabitants of Kushadvipa, but I think that's unrelated.