What causes sternum pain while sleeping/lying down?

I have been having a lot of problems with my sternum hurting. It is always while I am lying down. It wakes me up at night and hurts really bad. I haven't injured myself that I am aware of. It is also tender when you press against it. It isn't anything like heartburn or reflux. I know what that feels like. This is like it is bruised or cracked. What causes this type of pain in your sternum?