Law2Doc, I actually went ahead and googled a little, and I found something to support both our points of view. There seems to be a considerable personality component and some genetic dispositions.
"Individual differences in tolerance to shift work--a systematic review" from Sleep Medicine Review, published last year.
http://www.ncbi.nlm.nih.gov/pubmed/20851006
"Personality factors related to shift work tolerance in two- and three-shift workers"
Some of the same researchers here in Applied Ergonomics, also published 2011.
http://www.sciencedirect.com/science/article/pii/S0003687010001936
Quoting the first paper, "Overall, the studies indicate that young age, male gender, low scores on morningness, high scores on flexibility and low scores on languidity, low scores on neuroticism, high scores on extraversion and internal locus of control and some genetic dispositions are related to higher shift work tolerance." Some of this made me laugh out loud, to be honest! But seriously, thanks, Law2Doc, for pushing me to think about this from a different perspective.
Importantly, it appears there's at least one variable in there that probably contributes to your being able to 'will yourself', as you put it, and one that I would think a person can work on, which is hardiness. (The Appl Erg paper defines this as "resilience to stressful life events.") I did another quick search for this too -- I love research, can anyone tell?

-- and it brings up a number of interesting articles.
Mindfulness Meditation Will Make You Tougher Too
http://www.psychologytoday.com/blog...dfulness-meditation-will-make-you-tougher-too
Prayer, meditation, exercise, and special diets: behaviors of the hardy person with HIV/AIDS. http://www.ncbi.nlm.nih.gov/pubmed/8400157
(I wonder about causality here, though.)
So in conclusion, shift work tolerance has been researched, it is a complex issue, and tolerance is largely down to personality, with some genetic dispositions.