It sounds as if your girl has urinary incontinence if she is wetting during sleep. This is due to a weakening of the urethral muscle. There is medication that can help with this and you should see a difference fairly quickly when on the meds. In some cases the meds are needed short term and in other cases it is needed for life.
I am going to direct you to a site for further reading but I would suggest you see your vet to take care of the problem.