Sleep Patterns...Choice or genetic?

I have heard many perspectives on sleep patterns and I was wondering...... In regards to night owls and eary birds.... have they proven it to be a biological/chemical sleeping pattern in the body to prefer the night life or early morning? Or, is it just a lifestyle choice that can change if you choose to?

My two sons are night owls, and one of them has to be in training this week from 9 am to 5 pm. He prefers night shifts and stays up into early morning, prefering to sleep until midday. Could it be a sleep cycle pattern related to his age also? He is 22 soon, and the other son will be 19. I am 47, and prefer to bed early and up early in the morning. Does it change with age?

I think sleep patterns are a combination of genetic and learned/social:

http://healthysleep.med.harvard.edu/...terns-rem-nrem

Lots of young adults get their sleep messed up as teens when they do the night owl thing; that can end up as a sleep phase disorder that can be hard to "fix":

http://www.stanford.edu/~dement/delayed.html

Thank you, Perna. I appreciate the articles.

My 22 year old also sleepwalks. This is someething he began only recently, but his father used to sleepwalk also.