"Flow state" happens when I'm into something and I look up at the clock, see that ten hours have passed, my stomach is rumbly and my bladder is so full it's pendulous. Natural urges to eat and pee take a back seat when in that Zone. Goes with work, same with writing and reading. Can't count the number of times I've put a book down or stopped typing, thinking it was around midnight and it was 3am. I hate and love it at the same time. You're tired the next day... but it's worth it!!