As Robin and Spoke said, PHX used to not be so hot, and it would cool off at night. The population didn't take off in the valley until A/C became commonplace in the 70's. All that asphalt, concrete, stucco, and landscaping rock soaks up all that heat, then releases it at night. When I lived in Tucson, my A/C died, my dog and I went to stay with a friend at his parents' house. They lived out toward the base of Mt. Lemon and I loved going out there at night because it was so cool in the less densely populated area, it was actually pleasant to be outside.
It sucks being there when the A/C goes out. Everything gets hot, candles get mushy, you wonder about medications that say "store between 65 and 85 degrees F" . . .
The best part about going up hills is riding back down!