In Jainism, raat ka poshad (also called "ratri-bhojan tyag") refers to the practice of refraining from eating or drinking after sunset until sunrise. This observance is based on the principle of ahimsa (non-violence), as consuming food or water at night is believed to cause more harm to living beings (like microorganisms that proliferate in darkness).

Ekasana (one meal a day) and biyasana (two meals a day) are related but separate vows regarding how many times one eats during the day. These are different from the night-time restriction of raat ka poshad.

Regarding your question: If you have taken the vow of raat ka poshad, you should not drink water after sunset, regardless of whether you observe ekasana or biyasana. The two practices are independent:

• Ekasana/biyasana: Restricts the number of meals in a day.
• Raat ka poshad: Restricts eating and drinking after sunset.

So, even if you do not do ekasana or biyasana, if you are observing raat ka poshad, you should not drink water after sunset (often interpreted as before 10 pm, but ideally strictly after sunset).

References:

• Jain Agamas and conduct literature such as the "Sravakachara" (Code of Conduct for Laypersons).
• "Jainism: The World of Conquerors" by Natubhai Shah, which outlines the practices relating to food and drink in Jain lay conduct.

Summary: No, you should not drink water after sunset if you are observing raat ka poshad, regardless of your practice of ekasana or biyasana.