I like afternoon naps, too. Plus solar heating of one’s tent is an issue here at midnight north of the Arctic Circle. 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. 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.