[paublyrne]

> Also, if your waking hours are say 0800-2300 then you can easily work late and move your leisure time to the morning without changing your sleep schedule.

Not necessarily. Early birds like me find it difficult to work in the evenings, despite being technically awake. I find it way easier to get up early than to stay up late, which can actually feel like torture sometimes.

As for the split, I've read it's more like 10% for each extreme with the majority in the middle, though I can't find a reference, and I suspect it's very hard to actually know.