Night flying doesn't hurt you, missing sleep does. Also, while you may not be soaking up UV like you would during the day, you still are getting dosed by atmospheric radiation.
Do some reading on WOCL, and the effects of "missed sleep opportunities". Like any shift work, your body can adapt and thrive, although the danger is frequent shifts in schedule and timezone.
Here is an article from ALPA.org that is a good primer:
http://cf.alpa.org/internet/projects...ackgr/wocl.htm