There are a lot of other sleep disorders actually that could explain it. Now if you go to a psychiatrist, they will probably lump it all into insomnia which you may have but research the drugs that they can give because you don't want to disrupt a healthy sleep architecture. Now if you have a healthy sleep architecture but are just not sleeping at the "correct" time, that's kind of a big risk to take just move your sleep time back.

But if you go to a neurologist, it could be found to also be delayed sleep phase (circadian disorder), narcolepsy, restless leg syndrome (which could be due to low iron), parasomonias (sleep walking/eating), idiopathic hypersomnia, disorders in the specific stages of sleep and much more. Most people are not aware of the vast number of sleep disorders.

One thing is that some of these are hereditary. I notice that my sleep patterns are very close to one of my parent's. And I've gone to multiple doctors to confirm my original diagnosis. It's also good to rule out other health problems like allergies - if allergies were the cause of poor sleep, your sleep study would show several microarrousals.

Good Luck!