Problem is, and I'm not saying this to be a smart-ass, you aren't nocturnal. Humans have never been nor will ever be nocturnal (maybe thousands of years down the road as our 24-hour lifestyles evolve us into damn vampires--not sparkling ones, though!!). You can screw up your circadian rhythm though. However, while your mental state might adapt, your physiology won't, and eventually it will all affect each other. Also, insomnia is a lot of times due to stress (anxiety/depression). Oh, and take it from me, you can be very stressed and "not know it."

http://www.nursezone.com/nursing-news-events/more-features/Night-Shift-Work-Deemed-a-Carcinogen_21068.aspx

Since early childhood, I've never been very successful at living a day-adapted schedule. Going to bed at dawn and waking up at noon is the natural rhythm I've always fallen into, given the chance. I sleep just fine during the day, and have never needed blackout curtains. And I do get sun exposure for Vitamin D; I just get mine in the afternoon.

I finally chose self-employment so I could dictate my own hours, and have been contentedly nocturnal for the last few years. I've always seen a noticeable improvement in my mental health whenever I've been on a night schedule, and in the last three years I've seen a big difference--I've finally been able to make certain changes in my life for the better.

That said, day-people (who are the majority) really do suffer when they have to work night shifts for very long. Just as I've always struggled when keeping a day schedule (and all the while performing at a sub-par level and being extremely stressed out), day people have the same difficulty on a night schedule. Either way, it can be extremely stressful. But if you're a night person, and that comes easily to you, and your ability to think and work and function is better on a night schedule--there's no real need to change.

Is it working for you? That's the most important question to ask yourself. Because you'll get plenty of people telling you that no, it can't possibly work, and that if you think your idiosyncratic way of doing things is really working you're deluded, or broken, or just plain wrong. Screw that noise. Is it working for you?

I am concerned! I urge you to fix it - sleeping at the correct time, and for long enough, is quite possibly the next most important health factor after sensible eating. You should be looking for the same fixes an insomniac seeks (q.v. here).

Do remember (without wanting to sound parsimonious) that some discipline is needed during a period of adjusting to new habits, but enjoy it - after all, we also embrace shiny new things!

I think I'd start by skipping your daily sleep (so get tired), do a heavy resistance work out around 5pm, eat soon after, avoid TV/PC that evening, then read for roughly an hour before bedtime - you probably won't make it past 10pm. Usual prescription applies - make sure it's completely dark in the bedroom.

You have hormonal systems that synch with daylight. When you're "nocturnal" you interrupt some of those cycles so your body may not be processing things like cortisol or leptin correctly or appropriately. It's not ideal.

It is absolutely possible to be born with a disregulated circadian rhythm (I was, and my son was) so that being more awake at night feels natural. We are about 5 generations into this experiment with artificial light in the evening and that has only intensified with LED TVs and computers, combined with reducing our exposure to sunlight, and removal of animal fats that contain vitamins A & D from the diet, so I expect this phenomenon to only increase over the next few generations.

I am a resolute night owl, and naturally feel more alert in the evening, but my health slips when I let my schedule drift too late. I did full nocturnal for years and my health really paid a price for it. Sleeping 3am-11:30am seems to work just fine, but 7am-3 or 4pm kind of destroyed me, my vitamin D tanked, and my thyroid went haywire. I tried doing sun exposure by doing my shopping run in the morning before sleep but that made falling asleep really difficult and messed up my stomach.

I don't know what your situation is, but if you get a chance to spend some time away from electricity and artificial lights camping or staying out in cabin somewhere that can do wonders if you feel like you want to give a circadian reset a try. I have found it almost impossible to change in my normal surroundings without a good dose of Ashwagandha every night, the siren song of late night TV or the computer is just too much, but travel to somewhere quiet usually helps when I need to reset and rejoin that land of the living.