[TetOn]

Also seems to hinge on whether the participants "consider themselves" to be a night-owl. If the effect is there, you should be able to stratify by, say, regularly going to bed after 1am (or whatever). This doesn't seem to be that.

[brlewis]

Exactly. I think people categorize themselves relative to those around them.

At the beginning of the study period, participants were asked whether they considered themselves to be morning people or evening people, or whether they felt they fell somewhere in between those two groups.