Your foot going to sleep means that the blood flow is cut of or reduced due to either the position of the foot or the tightness of the shoe.

One mistake people sometimes make is that they strap in their foot really tightly. Instead, try to keep the velcro and strap a little loose. Additionally, think about the position of your foot. You may want to change the position of the cleat a little as this might avoid the position in which your foot gets decreased bloodflow.

Because you say that it happens when standing on a flat surface, I personally suspect that the shoes are too tight near the top of your foot. When you stand on these with the cleats, the nose of the shoe points upward and there is more pressure on the top of your foot *( where the strap is).

Where the numbness starts is not necessarily where it originates from. Typically, it starts in the extremities even if that is not where the blood flow is cut off (you feel it because it is actually the farthest point from it).