What Causes Lifelessness In The Arm?

From time to time I wake up at night and my arm is like dead weight and does not feel like it is attached to my body. I literally have to lift it with my other hand to move it to another position and start massaging it until it starts to wake up. Is it simply falling asleep, or is something more serious happening. The pins and needles feeling comes after I start moving it around again. When it feels like dead weight I think it might be my husbands arm and I lift it to move it away from me and it falls right back down. Really creepy feeling....