its your ulnar nerve in your hand,goggle it and you will see its position in your hand,and the fact it comes under pressure when you grip your bars,
i get it from time to time and it comes on faster on rough rides
what can you do?
change your grip from time to time on a ride,don't grip to hard
wear a pair of good padded cycling mitts,
another tip i have been given by a physio i ride with,is when you can tuck your chin into your chest,it releases the pressure feels a bit unnatural but works!
when you feel the hand numbing,one at a time hand off the bar place behind back open and close hand,should be job done
it does get better and may even go away for good
good luck and happy riding
