Night shift seems to have a very strong causal effect on my sleep cycles. Up until about ten years ago I was a night owl, rarely falling asleep before midnight and rarely waking up before 8. Then I started getting serious about light hygiene and using night shift and now I'm a serious day person, rarely staying awake after 11 and rarely waking up after 7. But the real clincher is that when I travel I don't change the time zone on my computer (because it screws up my calendar). But my sleep cycle continues to track my home time zone for a very long time. I life in California, but at the moment I'm in Hawaii. I've been here three weeks so far. At home I'd fall asleep around 11 and wake up around 7, but here I'm getting sleepy at 9 and waking up at 5.

My wife, on the other hand, is a hard-core night owl even with night shift. So apparently there is a lot of individual variation.

This article has inspired me to do a control experiment by switching night shift off. Check back here in a week or so for the results.