Starting anything at 6am means sleep deprivation.
Entirely dependent on the person;
A lark should have no problem with a 6am start, it could easily be well into their normal day; but may suffer later on depending on how far their sleep cycle allows them to push it past their normal sleep time.
An owl on the other hand may struggle with the early start, but since the world is set up for early starts we're possibly more used to it and handling having to get up when we should be asleep; the up shot of that is riding into the night is easy as you're not trying to go to sleep when your body's awake.
I've so far had no problems with lack of sleep towards the end of a 24hr ride. (which the one 400km I did last year turned out to be); but I can reasonably comfortably allow for staying up past 3am even if all I'm doing is reading or mashing a keyboard. Longer than 30hrs though...
You've got to learn your own sleep needs and work that into your game plan;
I remember a few years ago Jason Miles got a bit of a shock as Keith Forsyth absolutely hammered it off the front at the Puffer and as he had no idea if Keith was going to sleep or not, all he could do was batter on and try and keep up. Keith slept, Jason didn't and Jason won by quite a decent margin.
Some riders would sleep a few minutes, some a few hours; it gets surprisingly quiet out in the forest in the wee small hours for those that just keep plodding on (me).