hi shirix, you dont't have to drink so much water at night. You need to make sure you rest well, cos being tired and stressed can affect your milk supply as well.
It's good to keep yourself hydrated, but you dont't have to force yourself to keep drinking.
I think it's good enough to drink at least a litre in the day. In the beginning I would always make sure I drank at least a litre. Now that my supply is established, I just make sure that I dont't go thirsty.
The best way to make sure you have a constant supply of milk is to latch baby on demand, and frequently.