Numbness (at least in my case) was caused by excessive pressure on the Ulnar nerve, which runs through the palms. I experienced severe and lasting numbness in one hand and weakness in the other hand earlier this year. I have since fixed the problem by: 1) gloves with more padding at the base of the hand but no padding over the ulnar nerve (creating a void so that less pressure is placed on the nerve), 2) Fizik bar gel, 3) thicker bar tape, 4) looser grip on handlebar, and 5) frequent shifting of hand position on bar. With these steps, I have eliminated numbness.