Originally Posted by San Rensho
The cause is too much weight on the hands, but most so far have suggested a "gear" fix, a new stem, new bike, better gloves.
Leave everything on your bike exactly as it is and try to alter your upper body position. When you ride, you should have very little weight on your hands.
I agree with this. This is the cause of most numb hands issues. Fit may come into play here if the bike setup is forcing you to put too much pressure on your hands.
My advice is to make sure you keep a bend in your elbow, hold the bars lightly, keep your shoulders down and concentrate on keeping your whole upper body relaxed during your ride. If your face, shoulders and arms are relaxed, it should help your numb hands.