I find it painful to lie on my right arm for a few years now. it starts to throb and hurt after a few minutes. however i wake up on this arm every morning without pain? why does it hurt to sleep on?

Several causes: One causes of this type of pain would be Rotator cuff disease in the shoulder with pain radiating into the arm due to position of the shoulder when sleeping.Another cause would be nerve pain(neuralgia) due to compression of a nerve that supplies the arm while sleeping.This neuralgia could be due to a Thoracic Outlet Syndrome or possibly a cervical radiculopathy due to neck position>check Wikipedia

Many causes: As you have had pain for some time it is unlikely that the problem is very serious however the best way to find out would be to see a doctor who can examine you. There are a lot of causes for this sort of pain, the most common is nerve related where pressure on the nerve due to the position that you are in causes a shooting or tingling type pain.

Pins and needles?: Hi beckyb, its likely you are cutting off the circulation in certain positions. This can cause pain, sensory loss, and a "pins and needles" sensation. It usually resolves once circulation is restored. We do move while we sleep, and it may be less uncomfortable as your body is more relaxed. Otherwise, make sure your bed and pillow are in good nick for good ergonomic support. Hope that helps

Sore arm: I guess more information is required. But some times the shoulder pain may not be just because of sleeping on the arm. It can be due to other conditions like frozen shoulder/bursitis. If there is an on going issue you may need to get an USS of your shoulder. I am assuming that the pain is in the shoulder. See your GP for review as an X-ray of arm/shoulder may be requireD.

See doctor: Your pain can be explained if there is injury of muscles around shoulder joint called rotator cuff but that would also affect your shoulder during day time when moving shoulder. Other explanation could be if circulation is cut off in certain positions . Please consult your doctor fro examination to confirm the diagnosis