This would still be consistent with there being genetic (or otherwise hard to change in adulthood) factors that make a person function better with a shifted sle

This would still be consistent with there being genetic (or otherwise hard to change in adulthood) factors that make a person function better with a shifted sleep cycle. It's not at all clear that it's all environmental (like the post I replied seemed to imply) and I find it irritating when people make such sweeping claims without evidence.