I thought I was a night owl. I got fired once for showing up to start work at closing time after being warned several times.
10 years later, I'm one of those people that wakes up at 5AM so they can go to the gym before work.
What changed? I hit my forties, I cut out caffeine, I stopped using the computer after 9PM, I got married & had kids, and I go out dancing 6 times a year rather than 6 times a week.
Want to know if you're really a night owl without radical experiments like cutting out caffeine? Ask your Mom if you slept in on Saturdays when you were 7 years old.