Circadian rhythm is genetically determined and all long term studies of delayed sleep phase show it's essentially impossible to change. However it's one of those things that's a disorder only because it goes against what our societies expect.
I used to think I had insomnia for years, because I would try to sleep at "normal" times and struggled. When a sleep doctor suggested I change my sleep patterns to match my genetics, my perceived insomnia immediately disappeared. Now I just have to deal having with a sleep pattern that everyone else identifies as belonging to disaffected teenagers.
FWIW, a recent study found a physical cause for this - something about a genetic mutation that causes our sleep-signaling hormones to be created without the tails they usually have to navigate to the correct spots in the brain. So once again, the people claiming "It's just willpower" are proven wrong. Funny how the more we learn about the brain, the more we realize that willpower as a concept fully within our control doesn't necessarily exist.