There have been studies that confirm sleeping outside results in sounder and healthier sleep.

I think about it in terms of germs. Yes, on paper it's more safe to sanitize everything, but in doing so we prohibit our ability to build defenses, and actually become less healthy.

In the same vein, maybe by sleeping on mattresses indoors all the time, we fail to build a tolerance to adverse sleeping conditions, and maybe get more sensitive to them too.