Riddle Me This

Why am I often in my worst moods on Friday evening?

(And I was in a shitty mood before I realized I'd still be working after midnight, when I ought to be letting it all hang out.)

Let me know if you have any ideas, but the Friday evening blues seems to have become a bit of a trend with me. (And it's not that I don't have shitty moods at other times, because I am a very moody person, but there does seem to be a Friday night spike with me.)