I also fall into the camp where I believe that there are probably a variety of different sleep cycles that people are just predisposed to. I haven't seen any studies definitively indicating that there are a common sleep cycle. Even anecdotally, I know several people that are just more alert at night.
I've always wondered if there was a way to structure society so that there could be more time variety in socially needed functions. Perhaps one bank could be open 9-5 but another bank could be open 5-12. Or at the very least, improved flexibility for jobs where constant communication is not needed and can be done asynchronously. A set of core hours where communications could happen and then allow workers to work on their own cycles, taking naps as needed so that they can operate when they are most productive.
Though I have heard that there are natural biological functions that depend on the sun such that night owls who are sleeping their natural pattern are STILL predisposed more towards certain physical/mental conditions. Though who knows?
Researchers who lived in African tribes that are _really_ following the "old ways" found that tribespeople followed all kinds of sleep schedules. Somebody was up at almost _all_ times, including the middle of the night.
This makes total sense: you want at least somebody to be awake at all times to raise the alarm if a pride of lions happens to wander close by.
By doing the "split day" you just switch to another fixed pattern.