Waking up with a stiff neck is the result of a strain on the muscles of the neck. This is nearly always caused by sleeping in an improper position that stretches the muscles in the back of the neck for a prolonged period. A stiff neck can be so severe that you may not even be able to get out of bed or as mild as a throbbing pain through the day. In either case, a visit to the doctor would definitely be useful as he can prescribe the appropriate pain medication but what is most important is that you correct the position that you are sleeping in.
Usually when we sleep, we elevate our heads above the rest of our body by cushioning the weight of the head on a pillow. However, one of the most neglected muscles in this position is the muscles of the neck. Most of the time, the head would be supported and so would the shoulders on a foamy medium but the neck is not and with the head bent forward, the neck muscles are pulled throughout an eight hour sleeping stretch. An optimal sleeping position is one in which you should be able to only see the ceiling of your bedroom and nothing else. The body should be flat and your pillow should be adequately contoured so that your head fits in the depression of the pillow, while your neck fits on the raised border of the pillow. There are special pillows that are available to fit these very contours exactly and these may take some time for you to get used to but the investment would be worth it.
Whenever you do wake up with a stiff neck, the treatment is usually the same as with any other muscle strain, like a hamstring injury for example that most sports person's experience in their career. The prescription is always rest, ice packs to reduce the inflammation and compressing the area to reduce the swelling on the neck. It is also important to keep your head always at the same level as your heart to keep blood from accumulating in the muscle tears – if any. This effectively means that you should spend the entire day in bed until your neck muscles recover. To alleviate the pain, a menthol-based balm can give some relief and there is no substitute for analgesics during this time as the pain can sometimes be unbearable.