Yesterday's 400k started at 4am. I slept 3 hours Thursday night and 3 hours Friday night leaving my house at 2:30 am to get to the start yesterday.
A 400k in rain and lots of hills is going to test you way more than getting up early, so, I say it doesn't much matter when it starts but a 7 am start sounds nice to me. You are going to get night time riding in during a 400k unless the route is flat and you are fast, so, there isn't really any advantage that I can see starting at night because you will also be riding thru the day, too.