I like afternoon naps, too. Â Plus solar heating of one’s tent is an issue here at midnight north of the Arctic Circle. Â Not only is the light annoying, but the solar heating makes it uncomfortable inside the tent (where you are to avoid the bugs).
I suspect, more than the R-value of the sleeping bag, you’re blocking the solar input for that area of the tent. Â So while the sleeping bag works great, you already have it along, and it dries it out; a tarp could also be used or used in addition to a sleeping bag to provide more shade for the tent.
In Baja, we’d spread our ground cloth over a spindly bush to create a spot of solid shade for a siesta in the hottest part of the day.