I did an archaeological project in the Grand Canyon some years ago, down in the Inner gorge, not far from the suspension bridge (the small pueblo ruin we worked on is immediately adjacent to the trail as you proceed to Phantom Ranch).

Our daily schedule was to be on site at 4 AM (as soon as it was light) - worked till noon. We then retreated to PR, got in the shade or in our rooms until after dinner, when we returned to the dig for a couple of hours. That is dark granite country, which insures that you will be warm all night long, as the rocks re-radiate heat. When I joined the project, I hiked down from the South Rim, departing at 2:30 AM.

"Only mad dogs and Englishmen are out in the noonday sun."