The inversion of Sleep-wake cycle (sleeping in morning and not sleeping at night) could be because of many reasons.....physical, psychological or environmental.
First thing to do is to think if any thing in the environment is disturbing your sleep like light/noise/heat/cold
etc. Also do you smoke/drink coffea/tea/have caffeinated drinks/see tv a lot during the nights? These things will keep you awake at night and hence you have to go to sleep in the morning to compensate resulting in reversed sleep wake pattern. If so, correcting these small things should be enough.
If not, then there are physical problems like sleep apnoea (breathing stopping for a very short time...causing disturbed sleep), psychological problems like depression mania
disorders etc that need to be assessed for.
So, if the simple tips about environment does not help go see a physician who will rule physical things out and finally a psychiatrist
if you have no joy.