>>303103The idea is that when sleeping naked (inside a sleeping bag) there is no clothes in between you legs as well as between the insides of your arms and body. Heat dissipating from these surfaces just moves to another part of your body, hence you essentially decrease your surface area. Which means you'll lose less heat.
Ideally, of course you would then put the clothes you're not wearing under your sleeping pad or on top of your bag for extra insulation.
Though I don't know whether that effect is negligible or even worse less than what you lose from insulation due to extra clothing.
I only know that at home I prefer to sleep with a t-shirt because I'm colder that way and I like that.
