Scientifically, around 15% of the population in Western nations are rated as "owls", and another 15%, roughly, to be "larks" - says sleep researchers. It is a probabaly genetic disposition for going early or going late to bed, and we talk about very huge time shifts here, 5-8 hours. Such people do not sleep more or less than everyone, it also is not a habit, or symptom of a deeper disease. People affected that way never get used to needing to violate their inner clock, it ticks the same way like other's people's, they do not sleep more or less than others, but their phase is totally desynchronized to that of the majority of people.
I once read that they also seem to be more sensitive to moon influence, but that was still under examination back then.
I know what all that means, since I am heavily affected from that "problem" myself, since my earliest childhood days (when I drove my parents crazy). School's first two hours were hell, only teachers knowing me for a longer while left me alone in the first two hours, knwoing that that guy is only physically present before 10 am. Today I raise at 10:30 or 11:00 am, means: i wake up, if I can manage to delay all dates of the day to the afternoon, and go to bed at around 3:30 or 4:00 am. At midnight I gain another mental climax, I do kitchen work 1 am in the morning, and dinner is at 9 pm. Deep in the night I am as awake as most of you are at 11 am. Raising with you for brakefast at 7:30 would feel for me like you being thrown out of the bed at 2:00 am. Interestingly, certain health problems I had in the past, disappeared when I finally gave in to my shifted biorythm and allowed it to have it's way. In the years I gave daily meditations classes from 0900 to 1030, were hell for me, concerning the time - for me it felt like monks in a monastery starting their day at 0400 in the morning. My grandfather, his brother, and their father also were known to dance around until the sun went up (antoher hint for the genetic disposition). My grandfather was teacher. He used to come home from school, ate something, went to sleep, raised in the early evening, and then did the remaining work, did what we do - but late in the night, and then went to work again.
No problem if you have an envrionment where you can have that way. If you are depending on a timetable saying 8 a.m. "job", it is very bad, and messes up your life. Me - one can hardly be any more night-active than I am. Should have been born as a vampyre, maybe.
It is assumed to be a genetical disposition indeed. Usual explanations like ill metabolism, probolems with the tyroid gland, cardiovascular probolems, do not explain those people being identified as owls and larks.
If it is a disease, so far noone has found the cause, and remedy. I tend to think it is just natural fluctuation of a population's characteristic, the two extremes at the upper and lower 15%. Even when staying in the african and ME countries, these things posed problems to me. Glad I am in a position i can avoid needing to violate this differently ticking clock, at least most days. And amongst those being affected, not all are affected as massive as extreme examples like me.