What Causes Arm And Hand Numbness?

Hello, I am 40 yrs old, female. I wanted to know regarding numbness in my left arm and hand. At first I thought that my arm/hand had just fallen asleep because I was laying on it, but the feeling did not go away for about 3 or 4 hours. I don't know if it matters, but I had taken Melatonin because I suffer from Insomnia. Since I always hear about your left side and heart attacks...I feel concerned.