why do you sit up at night, but sit down during the day?

Answer 1

because during the night, the most common position is lying down (sleeping) so when you sit, you are rising up, hence sit up. during the day, the most common is standing up, so whenever you sit, you are moving downwards, hence sit down

Answer 2

"Sitting up" relates to the act of staying awake longer than you should or intended, it actually has nothing to do with sitting other than the fact most people sit down whilst trying to stay awake, normally waiting for something or someone.