I’ve been told that when using a down sleeping bag, sleeping in the buff will keep you warmer than sleeping in clothing. Is this true? (I’ve yet to do my own experimenting, as I like to wear long underwear to bed in order to keep my bag cleaner.)
Whoever told you that probably just wants to get you naked! I think I know where that old wives tale stems from: It is true that if you wear too much clothing to bed, you can over-compress the insulation so the bag can’t do its thing. You want a little bit of space between you and the bag–this air warms up and creates a nice little microenvironment for you.
There’s only one scenario where stripping down makes sense–in an emergency situation where one person is hypothermic and skin-to-skin contact with a warm body can jump start the warm up process. Otherwise, you’re right, your bag will absorb the oils and sweat from your skin and you’ll have to wash it more frequently. Plus, I think nylon feels just plain gross against the skin. Long johns are the way to go. If you want to sleep naked, use a removable, washable sleeping bag liner, like the ones you’ll find at designsalt.com.