Some of the causes of arm numbness are not serious and fairly commonplace, while other causes are reason for concern and should be treated by a doctor as soon as possible. Simply sleeping the wrong way on one's arm can cause numbness by constricting the blood vessels that deliver blood to the arms. Relieving that pressure often relieves the arm numbness. Some environmental effects such as extreme cold can lead to numbness in the arms as well, as can raising the arms above the level of the heart, which limits blood delivery to the length of the arm.
More serious conditions that may lead to arm numbness include multiple sclerosis, a pinched or damaged nerve, and disc injuries or other spinal problems. Multiple sclerosis (MS) is a disease that affects the brain and spinal cord, and it can affect how the brain communicates with the rest of the body. MS is a complex disease that must be addressed and diagnosed by a doctor. It is not one of the more common causes of arm numbness, but it is certainly one of the most serious.
Nerves run through the arms and around the spine to help the body function properly. When one of these nerves becomes pinched or damaged, the area of the body serviced by that nerve can be affected adversely; one may lose motor control in more serious instances, or may experience tingling, pain, aches, or arm numbness. A pinched nerve can be caused by many conditions, including tight muscles or tendons, joint problems, or even a herniated disc in the spine that puts pressure on that nerve. If nerve pain or numbness occurs, one should seek medical attention immediately to address the underlying cause of the nerve pain.
Herniated discs in the spine occur when a spinal disc that sits between two vertebrae becomes compressed, leading to a bulge or rupture. Such a condition can cause arm numbness by pressing on the nerves that service that arm, and by causing other issues with the proper functioning of the spine, shoulder, and arms. Most herniated discs heal on their own given enough time and rest, though some will require the attention of a doctor. The most serious instances of a herniated disc in the spine may require a surgery that will repair the damaged disc to prevent further pain or neurological issues.