Even being LAX based, that's not a bad flight for me-- the ground transport at both ends is a bigger issue than the wake/sleep cycle. I would be awake at that time anyway, so the issues I would have are getting to LAX through DTLA for a 6 pm flight (takes about 45 minutes longer than for a 6 am flight) and then whether there are any rental cars left when I get to IAD (I've had them run out when I arrived much earlier).