While I don’t have subscriptions to all those journals, so I can’t check exactly what those studies proved and didn’t prove, all I can say is that this example: “filling gyms during in the pre-dawn hours” tells me we’re still not talking about the same thing, i.e. mental energy. I think there’s a big difference between feeling physically energetic on one hand and feeling mentally focussed and creative on the other.
Also, while I find it easy to accept that there are two kinds of people as mentioned above, I will still be looking for explicit proof that “most people” are “morning larks”, like the original quote said.
Thanks for your patience.