Sleeping Naked is Warmer?

No, it is not basically true. However, it may be true in one way. Your sleeping bag is warm because it has loft. That means inches of insulation. The thicker (loft) the insulation the better or warmer it can keep you. If a person has a bag that is perhaps a little too snug for them or, too small, then wearing extra clothes works to compact the loft making it less effective and making you think that wearing clothes in your bag is not a good idea.

A sleeping bag that is not too snug, combined with 1-2 layers of clothing is the best way to get the most insulation value. Your sleeping bag's girth (usually found in the specs) should ideally be approx. 18 inches or so, wider than your chest measurement. Most bags are made a certain size and so don't be concerned with the 18" exact. If it's 16, if it's 22, don't worry. Just make sure you not putting your 200 pound frame and 46 inch chest in a sleeping bag with a 54 inch girth made for teenage boys. If you do, you might be better off naked.

Cheers!