3 am is usually when I start to 'really' mean it because that's when I'm saying to myself "Okay, I've only got five hours till I have to get up, now finish it or come to the realisation that finishing it will have to wait till tomorrow or I'll be really sorry when I do get up".
Yeah that wouldn't be good. : ) I'm a night owl so I'm usually up but if I was working at night I'd probably have a hard time staying up too. It's totally different when you're staying up for the fun of it.
If a book is good I'll just stay up and keep reading until I pass out. I'm never been a big sleeper though. As long as I get a couple hours in here & there I'm ok.