There are few moments during sleeping when there is obstruction or partial obstruction in blood supply of the limbs, which cause these transient numbness
sensation in the arm.
Most of the times they are normal in nature if left for few minutes or gentle massage of the arm.
If you find it more frequently then there might be some neurological problem, or nerve weakness, so you are advised to take some Vitamin and mineral supplements. You can also contact your nearby neurologist
to get your EMG and other tests done, to rule out peripheral nerve
problems. Also get your blood sugar level checked too, as most common cause of peripheral neuropathy
is Diabetes Mellitus
hope my info helped you
Dr Hari Om Chandrakar