Wh why do my hands fall asleep at night?

Carpal Tunnel. This sounds like carpal tunnel syndrome. Try wearing a wrist brace at night when you sleep this takes the pressure off the nerve and can help with the symptoms. If this is not resolved i would recommend you see a hand surgeon for a full evaluation.
Nerve impingement. It is most likely a position you're sleeping in that pinches a nerve and makes your hands fall asleep. You can try different sleeping positions, but you may have a very common nerve condition called carpal tunnel syndrome. See your doctor for an accurate diagnosis and treatment.