Why does it seem like my milk supply is less at night?

I am exclusively feeding and I often feel like I have less of a supply in the evening (630-930). I feel less let downs than I do in the middle of the day, he seems to swallow less and he takes both equally before he is done. During the day he takes the first one for 15 minutes or so, and then the second for a few more. Just wondering if anyone knows why, or how to ist. He often acts more frustrated at night during feedings as well. By the end he is satisfied, but gets worked up during. When I pump during the day after he gets his one bottle a day (gearing up for day care) I get at least 5 oz and up to 7. BTW, he is 2.5 months old. Any advice is appreciated.