Also look at your grips/bar tape. The numbness is probably caused by pinched nerves, vibrations, or just squeezing too tightly. The thicker your grip/tape the better it will absorb vibrations and the larger surface area will spread these vibrations out more.
How tightly do you grip on the bars? Think of it as resting your hands, not holding on. White knuckles are an indication that it's time to ease up.
