The lark and owl distinction has been driving me crazy since I thought about my sleeping habits. I spent about 2 years with completely self-determined sleep (I even collected sleep data) and do not even approximately fit into those schemes. I alternate between a lot of sleep and no sleep over the course of 10 days continually shifting my waking hours into the night until I reset. Some people claim that this would be unhealthy, but (measured by introspection) I felt good and haven't been sick during that time. And I have spoken to some people that feel similar.

While all this is not based on anything that can withstand analysis, it adds a strange flavor to advice when it starts with broad categories which seem not entirely correct.