Every individual has a unique sleep requirement. Generally an average adult requires 8 hours of night sleep. However, for you even 12 hours could be normal. I would say consult your doctor about this.

Are you stressed for some reason?

Maybe it's a thyroid problem (under or over active.) I'm not sure though, because I'm no doctor, so I say go see a doc too.

Or, as Concernedn said, it could just be the way your body works. I feel for you - I have sleep issues myself. It's no fun at all.

I agree with musicspirit. You should have your thyroid checked. Both under and overactive thyroid can cause this. Some people will say that you should have more energy if you have an overactive thyroid, but for many people, it makes for very restless sleep. Your body never really stops and fully rests, so no sleep is ever enough.

It could also be a sleep disorder that is preventing you from having a restful night's sleep. If your blood tests come out normal, you should go to a sleep specialist or clinic to make sure that you are having normal sleep patterns.

Are you drinking a lot of caffeine during the day because you are tired? Sometimes this can be a vicious circle. It can make getting restful sleep more difficult.