Some do, some don't. Most people go back to their natural patterns during the holidays, though. The classification is based on forms asking questions in a context of freedom from external constraints.

I'm not very familiar with sleep research, but I'd guess that both types will underperform if their natural penchant is disturbed (by work hours, for example).

Edit: Here is the actual screening form (in French) used for the studies: http://www2.ulg.ac.be/crc/in/CIRCADIEN_ONLYweb.html ... Adapted from: Horne, J. A. and O. Ostberg (1976). "A self-assessment questionnaire to determine morningness-eveningness in human circadian rhythms." Int J Chronobiol 4(2): 97-110.

Extreme early and late subject are further "diagnosed" with more exhaustive tests.

Interesting, thanks.

I do want to make it clear I wasn't trying to attack the study so much as explain why people get upset and respond to something like this with anecdotal comments.

It was clear, to me at least :-).