Most doctors would recommend better sleep hygiene and trying to reset your clock. However, if you have tried--really tried--and not succeeded then it might be depression. Or, do you do things at night that you feel others will not allow during the day? Ex: maybe you like to spend your time writing songs, but during the day people distract you or belittle you or bug you in some way. Since you say you prefer it, I suspect it is a social or emotional thing. Take care, and keep writing! This is a good place.