Thanks for using the forum. I am happy to address your questions, and my answer will be based on the information you provided here. Please make sure you recognize that this forum is for educational purposes only, and it does not substitute for a formal office visit with your doctor.
Without the ability to examine you and obtain a history, I can not tell you what the cause of your symptoms are, however, I will try to provide you with some useful information
Most of the scalp, up to the middle of the head, is supplied by a nerve that exits the spinal cord in the upper cervical area (this is called the C2 nerve root region). It is conceivable that certain positions that you sleep in might in one way or another cause pressure on that nerve, leading to the numbness sensation you have been experiencing. The nerves exit the spinal column at each vertebral level (under or above the vertebral bones). They exit through small holes, and they may be pressed on by various processes such as arthritis, herniated discs, etc. Positioning of your neck in certain areas may be increasing compression on a nerve. Since your symptoms have only occurred when lying down in certain positions, other causes of transient numbness such as transient ischemic attack (TIA, or what is often referred to as a "mini-stroke") is unlikely.
I suggest you share your symptoms with your primary doctor/family physician. After examining you, he/she may or may not choose to do further testing.
Thank you for using the forum I hope you find this information useful good luck.
Thank you so much! This was very helpful. I have had neck problems and have seen a chiropractor on a regular basis until 3 months ago when we moved. My Sister is a PT and she said the same thing. If it continues I will see a regular doctor. At present, it has not happened since.
Happy Thanksgiving.