My hands get numb when i ride more than 20-25 miles. It's usually mostly my left hand, and the numbness mostly goes away temporarily if i shake my hand vigorously for 30 seconds or so. I've tried altering hand positions and putting less weight on my hands but that doesn't seem to do much. Any suggestions?

its mostly all about hand position
heres a good link for some explanation - https://www.nlm.nih.gov/medlineplus/e...cle/000433.htm

vary your hand position get your wrists off the top bends, ride the tops with your hands across the tops and arms tucked in, ride the hoods, spend some time in the drops.

last year I also had problems with numbness in the hands - not sure what caused it, but it took almost 2 mos of really paying attention to hand position and very frequent changes before it started subsiding.
Could it be related to your job? Cycling might just be putting the finishing touches on the main cause of the problem.

Watch your shoulders, too. Check frequently to see that they are relaxed, dropped, and symmetrical.

Thanks for the help. The numbness is in my index and middle finger, just as expected from carpel tunnel. Hopefully now that i understand the location of the nerve etc, i can try to keep pressure off it. Do you have any suggestions for positioning the drops? would rotating them higher or lower potentially help?

Bar rotation dependz - everyone has a different way of setting them up, within a range.
Being that I prefer non-anatomic (classic bend) bars I like to have a slight downward tilt of the 'drop' so that a sight line down the drop towards the rear of the bike goes a goodly amount above the rear hub, closer to wheel top. But it really depends on the bars - some bars 'finish the bend' more than others.
You just gotta 'fish' til you find a position that you like. Often the bar makes a real difference. Of all the anatomics I've tried I like the Spec Carbon bars that came on the Tarmac and Roubaix well enough to not bother changing them. I also found the old Supergo Anatomics were reasonably acceptable - for me...
I just way prefer the classic bend - Reynolds Ouzo Bars, DaKine!

relax your back/shoulders. I was having numbness yesterday at the beginning of my ride until I relaxed.

Also bend your elbows for suspension. This lets your upper-body remain stationary when hitting bumps and not getting hit with the full force of the bumps.

And rotate your hands where you grip the bars so that the weight is carried on the meaty outside heel. This relieves any force from the middle valley in the palm where the median and carpal nerves run.

The other thing not mentioned is circulation. The blood vessels go thru the same 'pass' as the nerves. If you can 'shake it out' (which I couldn't do) it could also be constriction of the BVs. Same Story though, try to let the meat of the hand heal and thumb area carry some weight. Also roll you hands back regularly and grip more with the fingers more, weight under the knuckles. But that only works for so long, cause there are BVs and nerves aplenty running under there as well.
Gotta relax from the neck down and let the arms be suspension rather than a more rigid structure.

In addition to what the others said, gloves can make a difference. I have a couple of pairs, and one is much more comfortable than others.

There's a simple test that doctors can use to diagnose whether it's carpal tunnel. It's not something that you really want to mess around with.