When camping in cold weather, some people say it is actually warmer to sleep naked than to sleep with clothes on.
Is that true, or is it better to leave warm clothes on while in the bag?
When talking about fresh, dry clothes then it's not true. More layers definitely equals warmer!
As pointed out in the comments if you really go to extremes then more layers doesn't necessarily equal warmer, but to get to that point you have to really cram yourself in the bag so there's no insulating air between the layers. You could also make yourself so warm you sweat which would make your clothes damp and cool you down again, but if you just wear warm enough clothes to be comfortable, not sweltering (which is surely what you want anyway!) then you shouldn't have an issue.
Where this saying may come into effect (I've heard it branded around before as well) is when you wear clothes to bed that you've already been wearing so they may be damp, or clothes that are next to damp clothes in your rucksack (somewhat surprisingly regular for the more novice hikers!) and so on with similar situations. In those cases, it's probably best to sleep without than put on damp clothes.
Yes, in certain contrived / specialised / odd cases this may be true. But on all practical levels my experiences, and the experiences of those I know suggest that it's a myth with dry clothes and only comes into play with damp clothes, which most seasoned campers (and many other people too) know tend to easily cool you down anyway!
There's no paradox.
Possibly. the reason this is a consideration is best way to stay warm is with loose layers (multiple depending on the temp) that trap air pockets close to the body that are heated BY the body. if you are in your birthday suit you will trap a decent larger pocket of air around you.
BUT a single sleeping bag will NOT keep you warm this way. if you go this route here are a few things to keep in mind
My experience tell me this: sleep naked always if there's no sign of a possible avalanche.
I've been in many high altitude expeditions in three Continents and have explored many vertical and horizontal caves and underground systems. Sleeping bags are best when they're good. Don't try to get a cheap offer and trade it for your safety or comfort.
In mountaineering, especially when camping at very high altitudes (5,000 metres or higher) it is recommended to sleep naked.
First, we use only down feathers sleeping bags OR linear single fiber ones -Hollofill is one of the materials made that way. The specs can go from good for 0ºC to -40ºC. Kelty, Big Agnes, Valandre, Marmot... there are many options with high quality standards. Differences are huge in price between a good sleeping bag and a bag for laying down.
One of the main reasons why sleeping naked is more comfortable and safe is that moisture in our clothes stays there for a much longer period of time than on the skin.
Second, the air cushion of warm air that our body produces by means of its own heat, is maintained between the skin and the inner layer of down -or fibers- and these let the moisture pass through their loose structures, then it's evaporated or frozen on the outside shell of the sleeping bag, keeping your body dry.
One should always make sure to undress when already inside the bag. The boots and all layers of socks must be kept inside the sleeping bag as well. This helps to dry them and keep them warm for the next morning. You can leave your parka and windbreakers outside and use them as pillows.
Going to the loo in such situation will take its time. But, if weather is just around -10ºC, you can grab your thermic undies and go outside without much fuss. Just make it fast!
Camping at warm places asks for another layer, if possible: use a light cotton sheet inside the sleeping bag. It will pass the sweat to the outside as vapour and you'll feel that smooth and fresh sensation when in contact with cotton.
Never cover a proper sleeping bag with anything. This will shut the shell's properties of heat exchange and you'll feel colder or smothered.
Try sleeping naked at home. Always take a shower before bedtime and you'll experience why sleeping naked is much better. Keep your pj's under the covers and use them if you have to get out of bed. You'll find yourself rested and fresh in the morning.
Only in contrived or extreme examples does wearing less clothing about your body in fact make you warmer when camping.
The areas where I might consider it warmer to not wear clothing inside a sleeping bag are:
There is more than one reason, which makes you feel warmer sleeping with less clothes (even if it's perfectly dry):
There is some balance, actually: you should wear something, but not too much. I personally prefer sleeping completely naked, but high up or in winter I use Polartec-100 linen, socks and a cap with a scarf (though in these conditions I'm usually sharing the bag).
And all this doesn't mean you have to put your clothes in your backpack. You can use it to insulate your most exposed parts: the one which are closest to the tent and to the earth (actually snow). Just don't put it on but keep it loose between yourself and the sleeping bag, effectively improving bag's thickness.
I generally sleep naked in my sleeping bag. Ive slept nights where I went to sleep in my clothes, and then woke up because my feet were freezing in the middle of the night, so I took off my clothes and when back to bed, and then woke up at dawn toasty warm. And nights where I didn't do that in the same exact conditions, and suffered the night.
And these are dry clean clothes kept away from any moisture.
My theory is that my core produces a lot of heat, but my extremities are always cold. Sleeping naked lets the heat from my core make the whole sleeping bag warmer so my extremities can warm up. Layering up only reflects heat from your body back at that same part of my body, so I end up with an especially warm core, but freezing feet.
Both clothes and sleeping bags are insulators, the problem with wearing too many clothes in your sleeping bag is that you will actually insulate yourself independent of the bag, as a result, your bag won't get warm or it will take a long time to warm up. You will eventually have issues with moisture control, as your bag will slowly warm up after you fall asleep as heat escapes your layers and eventually replaces the cold air in the loft of the bag, causing you to sweat. There's nothing worse than waking up with that clammy sleeping bag feel.
Layer your bag! – Fleece, cotton or silk bag liners–although some are not technically designed to add warth to your bag–can actually add close to an extra 10°C to your bag, in addition, adding a vapour barrier keeps warm air next to your skin, and reduces moisture build-up in your bag, keeping your bag dryer and warmer by a few more degrees. Not only will you stay warmer, but you will also be way more comfortable. Overbags add an additional 5-10°C by adding a layer of insulation and by reducing built-up condensation from frost and the sleeper's own perspiration. The result is your sleeping bag's insulation stays drier and more effective. In really cold weather you can even consider layering with a second sleeping bag.
There are multiple benefits to layering your bag:
The best routine: strip down naked and put on a DRY base layer before getting into your bag. I usually sleep with nothing more than a polyester base layer and a pair of wool socks in cold weather (except for a touque and neck-warmer to keep the drafts at bay). In the morning, I change out of my sleeping layer back into my hiking clothes.
This is of course assuming that you already have a good ground sheet and sleeping pad, which are essential components of a warmer sleep system.
From my experience - no way. If the temperature outside is below the comfort range of your sleeping bag, you'll have to put some extra things on you, otherwise you risk hypothermia (in best case - sleepless night). The best things are the fleece ones.
However, when it goes to trousers and jackets (normal or rain-proof, without warming), it's usually a better idea to put them under your sleeping bag or foam-pad, to increase isolation from ground (the most head you loose through ground). So actually it may be a source of that hypothesis - better to sleep without trousers, and sleep on them instead.
However, many people claim that it's warmer to sleep naked. Well, the ones I know have very warm (and expensive) sleeping bags, so the temperatures were above their comfort rage. I have an impression that such rumors are produced by sleeping bag producers, which want people to buy warmer sleeping bags for cold weather, instead of trying to keep them warm in their (cheaper) summer-time sleeping bags with extra layers of clothing.
Note that sleeping bags are relatively new. As a soldier in 1st World War, you had only a blanket to keep you warm and you had to sleep in everything you have on you during winter.
TL;DR: Honest answer: depends on too many things, different situations will be different. Best answer: do what you feel like, whatever makes you feel comfortable.
If you cannot sleep naked in your bag comfortably, you should consider getting a better bag.
If you cannot sleep fully clothed comfortably, you should consider getting better clothes. Note you can get clothes cheaper than sleeping bags.
The more insulation you use the more heat you keep.
If insulations are used the same way all that matters is how insulating it is. For practical purposes, nothing else matters, not even what you call it (bag or clothes).
Worn clothes insulate body parts from other body parts.
This stops them from sharing heat efficiently. For this reason a sleeping bag insulates you more efficiently. This is where the "naked sleeping is warmer" idea comes from.
If your bag barely keeps you warm so the sleeping method matters, then your problem is you need a better bag.
In the end, there's a catch-22 here:
If your bag can't keep you warm, you need to wear clothes.
If it can keep you warm, you don't need clothes so do whatever because it doesn't matter.
If your clothes can keep you warm enough by themselves, then you don't even need a bag.
Where is the line? It depends on too many variables, including your own metabolism and recent activity level and comfort preferences. Only you can know this, and only by trying it out. There will be configurations where one is warmer and some where the other is warmer. That's not the answer you want to hear, but it's the truth.
Which is warmer? Option 3!: take your clothes off, wrap them around you like a blanket so they add insulation but not between body parts. This is both warmer and more annoying to do than either option we discussed.
If you are like most people and camp above freezing temperatures and without a cold weather bag then you probably want to wear your clothes. If you wear enough clothes to keep yourself warm anyway or have a good enough bag, both of which you should have, then it doesn't really matter.
I've done both under tarps and in hammocks down to about -10F/-25C. My findings are that you do whatever you feel like doing as long as you have good clothes and a good bag. Again: it really doesn't matter.
You get in your sleeping bag to sleep, but other than that the same rules apply as during the day: wear whatever makes you feel warm enough, your body will help you decide.
But why get naked and then redressed in the morning when you don't have to? It's such a hassle. I prefer to sleep in what I will wear the next day, even the jacket. In the morning I just step out and start my day.
I also camped at about 40F/5C with no bag. I gave my bag to someone else who joined last-minute and I wore everything I brought plus their jacket. It was very windy (there was even a high wind weather warning), yet I was still quite comfortable sleeping under the stars, on top of a tarp, in slightly-above-freezing weather with no bag. As I said: with enough insulation, just do whatever you feel like.
I've never been to the arctic but been considering it for a couple years. I hope to get some -40 degree nights at least, if not colder. In this situation I would want to use my insulation as efficient as possible, so I plan to use arctic bag inside a good bivy bag, and possibly a very warm blanket inside the arctic bag.
With all that insulation, I plan to strip down to bottom 1 or 2 clothing layers but not naked, and the rest of the clothing layers I would wrap around me to keep them warm for morning. That way my body parts can share some heat hopefully keeping my legs and feet warmer, and there's plenty of insulation around to keep the body as a whole warm.
Keep clothing in your bag with you and try it out and see how you feel. Listen to your body and adjust as you feel you need it.
I did several (short) winter hikes below zero degrees Celcius (freezing point) while having only a plastic cover and sleeping bag.
I always took of my clothes, especially jacket (since it was slightly wet due to snow), and put ALL clothes inside the sleeping bag but not wearing it.
Because of your body heat the sleeping bag will get warmer and it will dry and keep the clothes warm (relative) so in the morning you don't have cold clothes.
What is important, to keep as much air inside the sleeping bag. I don't mind to sleep with just an inch of gap opening of the sleeping bag, but some feel suffocating. Of course you need a so called 'mummy' type sleeping bag.
The only thing I keep on is (preferably clean) socks; somehow my feet don't get warm otherwise when they are cold.
Also it is important to put a layer under you (like an isolation mat).