Why do i act different towards my bf at night/day?

for some reason, i feel...more blunt and kind of bitchy towards him and i dont care as much about his feelings when we talk during the day. i still love him i just am in a different mood, and dont treat him as well. but at night, i can talk on the phone with him for up to 8 hours all night long and i feel like being so sweet to him. has anyone else had this problem? does anyone have any ideas as to why my mood changes so drastically during the night and day? im not trying to be a *****! and i feel bad for him because im not consistant.