I have heard this subject brought up a few times here on BF. I suffered from toe numbness and with the help of a professional I have overcome the issue. I am posting this to help others who have the same issue. My toe numbness came regardless of how tight my shoes were. It always came around the 12-18th mile.

1) Orthopedic shoe inserts for flat feet. I do not really have flat feet, but the way that the shoes fit do not conform to my arch well. I got custom inserts made, however I was told that buying generic heel insoles would work nearly as well.
2) Shoe tightening: Contradicting my original school of thought, try keeping the toe straps slightly more snug than the upper strap and the ratchet strap. No straps need to be any tighter than what is absolutely required to keep your shoe from moving around.

new shoes or looser straps or diff socks

Go looser on the straps, I found the top largest one, was the culprit for me.
Also vaseline'ing my toes/ball of foot like I do when I'm going out for 10+ mile runs, also has seemed to help. *shrug*

Are you wearing super-stiff shoes? I had a set of carbon Shimanos that were super-stiff, and I had toe numbness all the time. When they wore out, though, I replaced them with some Sidi Genius 5 Pros, with a carbon composite sole instead of a carbon weave. No issues since - apparently the little bit of flex was just what I needed! It's not enough to be at all noticeable, let alone cause meaningful power loss, but it sure is comfy.

Lower your saddle....

Raised mine a tad a couple days ago. Yesterday's ride resulted in toe numbness after about 30 minutes. Dropped the saddle about 2mm and rode on, no more numbness...

My guess is I'm slightly pinching a nerve when the saddle is too high....

My shoes are stiff. They have a stiffness index of 11.

I had toe numbness when I started riding. I lowered my seat just a bit so I more heel down rather than pointing my toes down. Also wear very thin WriteSocks.