I’ve heard this one more than once over the years. Despite the expertise of the various adherents, it’s never made sense to me. A sleeping bag, like clothing, is simply a form of insulation.
The belief seems to be that a person will sweat (and therefore end up colder) if the camper does not sleep naked. While it does make sense that sweating in your bag is a bad thing, it also seems obvious that if one is sweating in a bag, one simply has too much insulation.
Finding oneself overly-insulated is a tragic outdoors fashion faux pas. It can arise from two situations:
(1) sleeping in too many clothes/the wrong kind of clothing
This is similar to wearing a parka on a warm day. The correct amount and type of clothing varies by conditions. The warmer the conditions are, the less clothing is needed and the more breathable the garments should be. It does not follow that zero clothes is always the answer.
(2) using an overly-warm bag
If sweating occurs with any amount of clothing, then it is distinctly possible that your bag is rated for much lower temperatures than you are experiencing when using it. In other words, the bag is too warm for the conditions. In that case, you need to vent the bag to a comfortable level or get a more appropriately-rated bag.
Speaking of venting, some people seem to believe that, when using a mummy-type bag, they must be sealed into the bag. There is, however, no rule against letting cool air into your sleeping bag if you feel too warm. This is admittedly more difficult to do for your feet, but many good quality bags have small venting zippers near the footbox. If one cannot vent enough to make a bag comfortable, then it is the wrong rating for the conditions.
A word about sleeping bag ratings is important here. This is an industry measure with some quasi-standardized protocols (laboratories test sleeping bag performance inside tents, for instance) which attempts to help consumers pick out a sleeping bag for anticipated conditions. However, this system cannot account for the wide diversity of human physiology: some people naturally sleep colder or warmer than others. These kinds of factors affect how each person experiences a sleeping bag. Only personal experience can tell someone that they may need a 20 degree bag while their spouse does fine with a bag rated to 30 degrees – on the same night in the same tent.
Pay attention to the bag’s rating, know your own physiological tendencies, and understand that warm conditions require less insulation and cool conditions require more. Following this paradigm will allow you to use your bag in a surprising breadth of conditions. As an example, I have a bag rated to 30 degrees. When I camp in 100 degree temps in the summer, I don’t even take it with me. Nothing would stop me from sweating at those temps, naked or not. I sleep on a camp pad under a sheet (or at least I try to sleep when the overnight low is 90 degrees). When I camp at 50 degrees, I leave it partially unzipped and vented. I’ve used this bag at zero degrees in the depth of winter - a full 30 degrees below its rating - by wearing a ton of warm clothes.
In sum, then, sweating in a sleeping bag occurs because the resident of our hypothetical bag is too warm. We’ve talked about why this might be the case and how to remedy the situation. So get out there and experiment a little with your bag while enjoying a peaceful, restful night in the woods.