I get this when the grips are hard.
Also I reduced it by moving the brake levers more horizontal so your resting position on the hand when braking etc twists under rather than in or over the bars.
Also heightened when I ride the smallest of my bikes.
Off to look up more about Carpals Tunnels
I get it. I can ride off road for hours and I am fine, but I ride bout 20 miles[round trip] getting to off road and I get it, I tend to move my hands about a bit and it is a bit better; it seems to be where the heel of the palm is pressed against the grip.
You mention hard grips, I have the sort of ribbed soft rubber grips on one bike and it's worse then the other bike which had hard grips. But the more comfortable of the two has a longer top tube, maybe the position
is slightly better.
And I have set the better bikes levers so they are inline with my arms[the other bike is all how I bought it]. I also twisted the bars very slightly, to prevent the bars denting the frame.
I was looking for grips, I have a bike without grips, I was wondering if those odd shaped[probably ergonomic] things are any good?