Being that it is not throughout the day and only when you are sleeping I would suspect either your neck is out or possibly a bone spur in the neck.
When you are sleeping it is very important to make sure your head and neck remain aligned straight. If your pillow is too small or to big the neck will be out of alignment and over hours of sleep this can compress the nerves causing the symptoms you mentioned.
The first thing to do would be to lie down in your bed in your normal sleeping position and pay attention to how your head and neck is positioned to see if they are in alignment. If not you can add or remove pillows or change them so the head and neck become aligned. If that does not help then I would go to a good chiropractor and get x-rays to look for any spurs and an adjustment to make sure the vertebrae are not out.