I had this in the latter stages of both my previous pregnancies and I assumed the weight of the baby was pressing on something, but I'm only 11 weeks now and I keep waking up with numb hands. It's worse when I sleep on my back, and when I sleep on my side then one hand is worse than the other. It's also not the whole hand - usually my little finger and ring finger and the side of the hand below that. It feels like pins and needles but less painful.
I'm not really concerned about it but it's bugging me that I don't know why it happens!