Circadian rhythm refers to the body's biochemical sleep cycle. Here's a wikipedia entry that is not bad if you are interested.
All medical students and residents learn the hard way that staying up all night on call and being tied to a beeper is not good for one's general health or mental health. I still can't understand how you can sleep well with a beeper on... I never succeeded in doing anything more than catnap on call, but some people have the gift of quick and restful sleep. I am not one of these people.
The worst thing about Emergency Medicine has to be the nights. Whereas many physicians can kiss the overnight shift goodbye after residency we can not. To be sure, surgeons and some other medical subspecialists still have nights, but they can sleep, if the gods deign, between calls, and they never switch to a night schedule.
Coming up to a string of night shifts still provokes the fight or flight response in me. In the wee hours with ambulances rolling in and kids crying and drunks yelling and phones ringing one is hard-pressed to believe that it will ever end.
Add to this the quick switch back to a day schedule and our 'airline pilot' schedule (working about 15 shifts a month) looks less attractive. For me, it takes me two days to recover from two overnights, and more days to recover from three or four in a row.
It's like this. Start your workweek in Atlanta, for your third day of work catch a flight to London and work regular morning hours there for a couple of days, then fly back to Atlanta for some days off. Repeat.
Admittedly, some ER groups try hard to create some regularity in the docs schedule by staggering the shifts and moving them forward a few hours every few days, but there's just no good way to do this absent having a full time night doc or two.
To the goddess cirdadia... you are a douchebag.