[tokipin]

This is somewhat along the lines of my current theory, which is that night owls vs early birds is about sensitivity to the environment. The reason night owls stay up late is not because it's late, but because it becomes quiet enough that the environment they're sensitive to stops taking attention and they can direct that attention inward. According to this theory, it would make sense for introverts to be more affected by this as they would accumulate an "introversion deficit" throughout the day.