When i sleep the back of my head goes numb. does this cause brain damage? lack of blood flow?

Nerve Compression: Likely you are compressing your occipital nerve, particularly if lying on your back causes the symptom. The numbness in the back of your head may be on one side or both. Repositioning to take pressure off the nerves should lead to slow return of sensation. Compression of this nerve will not affect brain function. If you are unsure, or your symptoms seem different from this, consult your doctor.