2. Why evening twilight is called Asura Sandhya or the time of demons?
When it comes to the name Asura Sandhya or the twilight of demons, a story follows from the holy epics. Long long time ago, as we know, a demon king Hiranyakasyapa did vigorous penance and pleased Lord Brahma.
Lord Brahma appeared and asked Hiranyakasyapa, what his wants are. Hiranyakasyapa got a boon from Lord Brahma that he wants to be invincible and not to be killed by humans or animals at day or night and inside or outside home.
After this we know the story that Lord Vishnu took the incarnation of Narasimha in the form of half human and half lion and killed him sitting on the doorstep during evening twilight which is neither day nor night.
So, our ancestors decided not to perform any worldly activities like eating and sleeping during this time and dedicated it to the ritual of Sri Maha Vishnu.
It is said that demons forces are active during evening twilight. The active demonic energies are negative and there is a chance that they affect food and energy levels if we sleep. That is why it is said not to eat and sleep during Asura Sandhya Vela.