Touring - Numbness in foot

When I ride for extended periods, the bottom of my right foot, under the cleat, gets numb. I am wearing Shimano mountain bike shoes with cleats/SPD pedals. I have been using the combo consistently for the past three years. The problem has been present from the beginning, although the bottom of my foot seems to get numb more quickly this summer. The numbness passes after a few minutes when I rest, but it is a nuisance and an irritant!

The cleats were positioned by a kinesiologist/bike racer who really seemed to know what she was doing. (In fact, her adjustments caused a 20 year knee problem to disappear.) I have little doubt that the cleats are in the "correct" position.

So, what are the possible causes of foot numbness under the cleat?

Tarsal tunnel syndrome.
But you may have entrapment of the nerve along the plantar region of your foot.
My small toe and the one beside it often go numb. After a few pairs of shoes I think I have it to a minimum.
You can get rid of the symptoms but not the cause, which unfortunately is genetics.

My feet and toes would go numb during medium to long rides. After much research, I bought a Brooks saddle, and no more numbness. It could be that you need a stiffer shoe, although i didn't try this because the saddle cleared up my problem.

You could also try more spinning, if by chance you don't already. It makes a difference as to the period you have direct pressure on the foot.

Or toe clips -

I have done some long touring days in 20 years - 125 miles on occasion.
Done quite a few touring centuries simply because I was stupid and didn't know when to stop.
I found that if I got numbness - even using toe clips - if I took my foot out of the clip and flipped the pedal - moved my foot significantly forward or back - with very easy spinning - the pain/numbness would go away.

Besides tarsal tunnel - there is also repeated motion stress/injury.
by changing foot position dramatically - beyond what toe clips and certainly clipless would allow -
the points of pressure were relieved.
I'm guessing that I might lose 25% to 35% efficiency -
And you sure can't do the above climbing a mountain pass.
But if it's late in the day and you are just toodling along -
It's a great way of giving your footsies a break.

PS - I also change out my cycling shoes into my Tevas late in the day.
Pedalling on the flip side of the toe clips.
I swear - I can hear my feet saying, "Thank you, thank you!"

You might have someone help you check the positioning of your saddle. Perhaps a skosh high or the fore to aft may be off a bit.