Some of it is genetic but mostly it's conditioning, like in the Military.
EDIT: To clarify, veterans often (but not always) wake up before dawn consistently for decades after their service. I don't really recommend it as an option, but it's proof of concept.
Yeah, I got the conditioning during college. Work 32 hours a week while taking 15 credits, you won't have time for sleep. Work 7:30am-4pm MWF, go right to campus for some homework and dinner before class 6pm-9pm. Then go home and do homework/sleep. Tu/Tr have class from 10am-2pm and work 3pm-7pm and then work on homework, housework, gym. You won't have the energy to stay up late.