Breast milk supply is typically highest during the night and first thing in the morning- the hormone prolactin is highest then. Caffeine doesn't usually affect milk supply and as long as you drink to thirst you don't have to worry about water intake (you have to be very dehydrated for lack of water to cause a problem and increasing water above what is needed will not increase milk).
Do you feel your milk is low because baby is fussy? Most babies have a fussy time of day- often it is around dinner time but my youngest would turn into a monster between 11 pm and midnight. Is baby otherwise seeming unsatisfied? Or is it just feeling less full? More information about why you feel your milk supply is low might help you to get more helpful advice. In the meantime check out www.kellymom.com for great breastfeeding advice.