There are so many things wrong about posting at night, but probably the top two are:
1. It has a tendency to cause me to stay up longer than I meant to. Posts yield follow-up posts, and I always spend more time writing them than I expect.
2. It, far too frequently, gives me too many things to think about when I'm trying to fall asleep. Like, just now. I think about the phrasing I used. I think more about the topics I posted on, which often are pretty fresh from the mind and haven't really had time to percolate before being posted on the internet for all to see. I have a hard enough time clearing my mind and falling asleep. Writing about something that I thought was interesting is the last thing I should be doing.
But, if I don't post at night, I'd probably almost never post on weekdays. Conundrum.