in your arm can occur for a number of reasons. This can be caused by minor causes that will improve on there own, but can also be caused by life threatening issues. I would recommend discussing this with your doctor
if it persists or if you have any other symptom discussed below.
To answer your question, numbness can be a sign of a heart attack. Normally, this will present with other symptoms as well, so numbness alone is rarely a heart attack. Often chest pain, shortness of breath, lightheadedness, palpitations, chest pressure and fatigue are the major signs of a heart attack. Pain that radiates to the arm or the jaw is also classic. If you are at risk for a heart attack (over the age of 40, have diabetes, obesity, high blood pressure, high cholesterol) that this could be an issue. If this seems to fit you -- go to the emergency room.
Other causes of numbness can be also dangerous. The same risk factors for heart attacks are risk factors for strokes -- which can also cause this symptoms. If you have any other weakness, headache
or visual changes -- go to the emergency room.
Sleeping on your heart --compressing the nerve-- can also cause it. This will improve over hours. If not -- see your doctor. Good luck!