From about 60F to 90F I stay comfortable by adjusting my level of clothing and my activity rate. Remember that 60 in the midday sun feels 20 degrees warmer than 60 before the sun rises.
It is easier to adapt to cold than to heat. Another layer of insulation can always be worn but when it is hot, eventually you run out of things to take off. Indoors, fans are your friends and clothing is your enemy.
I don't hike steep hills or big distances above 85F. I don't do any hiking above 90F. Shade and a breeze or recreational water can kick this up a few degrees but not much. In hot weather, if I can't get any shade, then I have to wear something light-colored and loose.
If the day's high is 100F, it probably dropped to 70 at night. Maybe even 60. Depends on atmospheric moisture and how late in the season. So at least in the desert, the nights are lovely in the summer. You can do your work in the evening and morning and then estivate in midday. Siesta time!
Aestivation - Wikipedia
Cooler temperatures are great altho I seriously start to think about clothing somewhere in the 60s, depending on my activity level. One time I climbed a thousand-foot hill to get back to my car at night. When I got there, the car's thermometer said it was 48 degrees. I wasn't a bit cold until I stopped hiking.