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Is it Warmer to Sleep Naked in a Sleeping Bag?

Sleeping Naked in a Sleeping Bag
Sleeping Naked in a Sleeping Bag

No. It’s a myth that sleeping naked in a sleeping bag is warmer than wearing in long underwear. I’m not sure how this Internet meme started but it’s dead wrong.

How Sleeping Bags Work

Sleeping bags are designed to trap the heat your body produces and prevent it from escaping. When you wear long underwear in a sleeping bag, you increase the amount of insulation between your skin and the cold air outside. This will make you warmer than if you sleep naked.

If that doesn’t make sense consider the following analogy:

A sleeping bag is like a house. A sleeping bag has insulation like a house that prevents heat from escaping in cold weather. Like your house, a sleeping bag has a furnace inside it that heats it up. In this case, its your metabolism which produces body heat. If you wear long underwear inside your house in winter, you will feel warmer than if you walk around the house naked. That is why wearing long underwear in a sleeping bag will keep you warmer than sleeping naked. It’s another layer of insulation that traps hot air and keeps it close to your body.

Exceptions to the Rule

There are times when wearing clothing in your sleeping bag will not keep you warmer. These are extreme exceptions, but I will list them here for completeness.

  • You wear so much extra clothing or fill your sleeping bag with so much extra stuff that you compress the insulation in the sleeping bag’s baffles and reduce the amount of warm air it can trap.
  • You wear such tight-fitting long underwear or socks that it reduces the blood circulation to your extremities and makes them feel colder.
  • You wear wet clothing which compromises the insulation in your sleeping bag as the heat of your body dries it. The moisture in your clothes doesn’t just disappear: it gets trapped by the sleeping bag’s insulation which degrades its effectiveness.
  • You wear too many clothes in your sleeping bag and sweat. As your sweat dries it degrades the insulation in your sleeping bag, just like wearing wet clothing. You’d have to sweat a lot for this to happen, so just de-layer or unzip your sleeping bag if you feel a sweat coming on.

Best Practice

The best practice is to wear a dry base layer (top, bottom, socks, and hat) in your sleeping bag at night to keep it clean and to keep you warmer in cooler weather. These should be loose-fitting to prevent your hands or feet from getting cold due to loss of circulation and to help trap warmer air near the surface of your skin.

While you can augment the insulation in your sleeping bag with an insulated coat or pants, you want to make sure that you can still move inside your bag and that the shell of the sleeping bag isn’t pushing hot air out of its own baffles. If your base layer is wet or damp, it’s best to dry it out before you get into your sleeping bag.

If you start to sweat at night, unzip your sleeping bag to cool off and re-zip it when you start to get cold. The amount of heat your body produces during the night changes, to a large extent based on how recently you ate food. So if you wake up cold at night, eat something sweet and fatty like a candy bar (without caffeine) or some nuts.

You have to understand that sleeping warm at night isn’t something that just happens. Your experience is very much influenced by the steps you take to sleep comfortably like wearing loose dry clothes to sleep, venting your bag when you’re too warm, or revving up your metabolism by eating something when you get chilled. Sleeping warm is a skill.

See Also:

REI: How to Choose a Sleeping Bag

The Art of Sleeping Warm at Night

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47 comments

  1. Wow, this canard isn’t dead yet? I remember being told this when I first started backpacking, along with the explanation that “if you’re wearing clothes, your body heat can’t get out and heat up the bag.”
    Well, if it can’t get out out into the bag, then it’s still in your body!

  2. That’s all logical. That said, I’m not significantly warmer sleeping when wearing additional clothing. I’ve hypothesized that this is because it reduces trapped air: That is, the sleeping bag has an essentially fixed volume. What keeps you warm is trapped air. If, in that fixed volume, you now replace some of the air with solid materials, you potentially reduce the amount of trapped air.
    I’ve experimented a bit with wool layers although nor for long enough times to claim anything. I find that beyond 2 layers or so, the additional insulation effect is limited. Even with 4 or so layers, you won’t get the level of insulation a down jacket provides. I reckon this is due to the extra mass of material that needs to be warmed. Then again, maybe it’s all an issue of non-linear perception, i.e. maybe two feel that something insulates twice as well, maybe one needs 4x more insulation or something similar. All very unscientific, but I can’t say that I experience twice the warmth by simply doubling layers….

  3. It is actually warmer for the wife and I to sleep naked than to add a bunch of clothing that just gets in the way of things…but, I am sure you meant retaining heat, not generating it.

    I tend towards a light, medium or heavy set of long-johns and long wool socks. This works to add about 2, 4, 6 degrees F to the warmth of the bag. Adding a down jacket only lets me add another 2F. So, an older 1#11 bag will take me to 32F comfortably. Adding more layers only squashes the insulation of the bag. It doesn’t add more to the warmth of the bag.

    Like Chris said above, warmth is NOT linear in a bag. Following heat loss through radiation, I believe this is inversly proportional to your bag and layers down to about 20F or so. After that, the other things become as important to heat loss: transpiration & insensible perspiration, base covering (tents) and ground covering.

    Transpiration is heat lost through breathing, and evaporation of moisture through your nose, mouth and lungs. Insensible perspiration is what keeps your skin flexible enough to hold your “bag of bones” together. Both mean you loose about two pints per night in colder weather. This is a LOT of water. The water in the insulation can make you cold, so a vapour barrier is needed at cold temps. Sleeping in a small tent can help by as much as ~10F. At the least, it provides moisture to the air, reducing your need to dampen the air through other means. Evaporation can loose a LOT of heat to keep your lungs wet enough to transfer oxygen to your body. A tent will help reduce this. Sleeping naked will also put insensible perspiration into the system. Again, the dryer the air, the more that is required. Adding a vapor barrier can add as much as 5F warmth to the bag (ignoring ice buildup in the insulaton) by reducing this loss to near zero in colder temps. The phase change between liquid and gas(evaporation) carries a LOT of heat. The ammount of water that can saturate air at 0F is a LOT less than 40F. One is well below the freezing point of water(vapour pressure of a solid.) The other is well above it(vapour pressure of a liquid.)

    Conduction through the ground can be a huge loss if you try to sleep on ice or rock. Sleep on a good pad (at least R5-R6) on snow or forest duff. Snow is an excelent insulator to 32F. Your pad(s) will keep you warm above that.

    Sleeping naked? Nope. It doesn’t work. Unless you are wet, it is always colder.

    • I must disagree. Perhaps words like “always” and “never” should not be used in proclaiming what will and will not work for everyone. For myself, I have been camping in sub-freezing temperatures for over thirty years, and have found that I’m the toastiest when I sleep in nothing (provided of course that my gear is up to the job). This is especially true for my feet and legs. I am well aware of the function of my down bag and the pad I’m on, but I think what is being ignored here is the process in which the body regulates its own heat output, and how it may differ for everyone. I think the end result of how warm one stays is more than a product of heat retention, but also of heat production. For me, my body seems to crank it up when I’m “buffo” and shut it down when I’m dressed.

      I also often sleep in an unheated room (upstate NY) in winter and test it there too, with the same results year after year.

      These posts remind me of aeronautical engineers explaining how a bumblebee can’t possibly fly, when all of a sudden………..

  4. Excellent advice, Philip.

    If you get changed to go to bed your clothes are cold and take time to warm up. So my tip is to put on the clothes that you are going to wear at night well before you go to bed. Then you remain warm when you get into your sleeping bag.

  5. That naked lady looks so happy. I need a pre-bag warmer like her.

  6. Any comments regarding a silk liner? JD

  7. Every one is different. Some sleep warmer than others so there is no real answer to the question. At Sea level with a group I have seen experienced hikers of all shapes and sizes preparing for bed in different ways, Some are naked, others have on a full set of long johns, some a T-Shirt top no bottoms some just underpants and maybe one with a full thermal set with a Balacava in prepping for an expected 30 degree night. And I almost forgot sleeping bag liners which add some more insulation. I have not see many long distance hikers use Cotton based liners since they tend to absorb and hold mositure most including myself use silk.

  8. “Internet meme”??? That was meme-ing long before Mr. Gore (didn’t) invent the internet. I subscribe to the idea that the idea was started by (trying to be) clever members of 50% of the population. Not the 50% represented by the model in your photo:-) I’ll leave the motivation to your imagination.

    But returning to the backpacking realm … your model is gonna be cold clothed or not if trying (and failing) to sleep on a cot without some insulation underneath!

    • Agree. Heard it from my Dad in the early 70s. And had a boss who said his Sargent in Alaska used to check and make sure they weren’t sleeping in clothes when they were out on maneuvers in the early 60s. But maybe there was something else going on.

  9. So all those long cold sleepless nights out in the field while i was in the marines was due to sleeping naked like i was told to do.

  10. Why do people forget about physics in the woods? I mean I’ve had so much people arguing that OF COURSE sleeping naked will keep me warmer that I’d be tempted to believe this if experience hadn’t taught me otherwise. It’s happened more than once to reach desperately for the extra clothes on my backpack to combat a sudden drop of the temperatures

  11. Thanks you so much for posting this! I’ve had multiple people try and tell me that wearing cloths in their sleeping bag made them cooler and it’s was probably due to the reason you stated. I’ve been thinking for awhile my long underwear is what made my legs cold. I thought they were supposed to have a taught fit, but maybe not for sleeping.

  12. I got this idea from reading Julie of the Wolves as a kid, but I think it was actually stated in the book that she slept naked to keep her sleeping furs dry. I just made the naked is warmer association on my own.

  13. hahaha…pretty sure someone of the male gender started the sleep naked nonsense when he took someone of the opposite gender camping on a cold night. >:-0

  14. And to think of the $$$ I could have saved by buying looser fitting long underwear rather than investing in a new bag. Why didn’t this article come out a few weeks ago? j/k :)

  15. Plus, wearing sleep clothes keeps your sleeping bag much cleaner.

  16. I think you’re exactly right, Philip, though I suspect I do know why this myth lives on.

    When I hop in my bag, the skin under my merino long underwear has already been partially shielded from the cold external air. So there’s a bit less of a perceptible difference in warmth when I first get in the bag. Any exposed skin, though, gets a much bigger jump when going from direct exposure to cold external air to the radiant warmth of that inside layer of the sleeping bag. My guess is that folks who have relatively little experience sleeping in sleeping bags probably base their assumptions on that first 30 seconds of perception and think “wow, my bag feels so much warmer when I jump in naked” and that’s as far as they think.

  17. If I’m getting into an ice cold sleeping bag and planning on sleeping in my long johns, I’ll crawl into it fully clothed (except for the boots, of course!) and strip off my outer layers while in the bag. All the contortions involved in doing so will warm up the bag quite nicely by the time I get to the long handles. I’ll usually leave the outer layers in the bag so they won’t be a shock to get into in the morning.

  18. Forget the debate…I’m more interested in meeting the woman in the bag! SectionHiker rules!

  19. I have eared that one in France too, 10 years ago. Always found it hard to believe except while bringing a girlfriend camping.
    Also a closed silk liner is a great heat booster.

  20. This is no Internet meme – this idea has been around since at least the early ’70’s when I was first exposed to it.

  21. I think this comes from a confusion about old advice for hypothermia victims. From what I have read, if you are trying to warm up someone with hypothermia, one way to way to warm them is to get in the sleeping bag with them with them naked. This kind of makes sense, because if they are wearing clothes, they will inhibit the transfer of heat from you to them.

    • That is old advice. The new advice is not to get in there with them because they’ll freak out when they wake up and you’ll have two victims instead of one.

      • Agreed, it’s not a good idea anymore (if it ever was). I just think that’s how this general line of thought probably came about.

    • Yes, I agree with Grumpy Old man regarding the hypothermia issue. Not advisable to have a man jump in the bag with a women, especailly a stranger………could be deadly.

  22. I’m one of those that sleep better au natural. I find that my sleep system, hammock tq uq set up, seems to warm quicker when I start that way too.

    If I’m out in cold or cooler weather I keep a set of sleeping socks, and I put a base layer in my hammock when I get in and put it on if needed. I always use the socks. I also have my socks and shirt for the next day in with me.

    The house analogy is off tho because of the space to heat ratio imho. Back in my youth I was told that the old long johns lost their efficiency the longer you wore them because of dirt and oils from your skin.

  23. Philip, making a logical argument is not the same as proving a fact. Just because adding an additional layer of insulation around your body will keep you warmer sounds logical, it is not necessarily a fact. Facts are something that can be repeatedly tested and verified by multiple people. Your article provides no such evidence that any independent test were either performed or verified. In other words you’re merely trying to replace one possible myth with another.

    • Ron – grumpy as usual, I see. :-)
      If you want the evidence for my reasoning, look into the EN13537 sleeping bag temperature rating standard test procedures conducted by Liz McCullough at the University of Kansas and replicated worldwide. Test dummies (test mummies) clothed in long underwear and hats sleep warmer than naked ones. Is this a fact? Only until someone disproves it, I guess.

  24. Eddie’s reply is the most on-target for me – different folks prefer different sleep outfits. This is as true at home in private as when camping with or without a group. The key word to add to the conversation is comfort. Warmth might be measurable in degrees of Fahrenhiet preserved, but comfort includes how those degrees are distibuted. At night camping I slip into a liner in a bag on a pad in a hammock wearing very little, then take all off. I’ve done this with 6 inches of snow around and been toasty. The liner affords modesty, avoids the shock of cold nylon against the skin, contains body-generated heat and keeps the sleeping bag clean. Wearing little or nothing allows heat generated by the body’s core to distribute via trapped humid air to cold extremities. I’d be very uncomfortable sleeing in long johns and my feet would be uncomfortable – cold without socks, confined and sweaty with them.

  25. It would be an easy hypothesis to test. Until then however, and in the total absence of any verifiable data, a hypothesis is all it is, just like the other one. No matter how many unsupported “facts” are listed..

  26. I believe the sleeping bag warms up faster if you sleep naked (actually, I wear only shorts) due to the immediate transfer of heat to the air surrounding you. If you already have clothes on, they keep some of that heat near the body, thus increasing the amount of time it takes to warm the bag. I also know this, if I am sleeping naked and I get cold, I put on some clothes. This would seem to me to be at least some sort of evidence against the sleeping naked concept. I also know that I can go to a lower temperature comfortably in my bag if I wear clothes than If I don’t. Perhaps another bit of anecdotal evidence.

  27. I find it funny how many non-scientific replies this post has generated:-) When it gets right down to it I’ll trust the well established scientific facts of heat retention and leave the goofy theories to those that want to practice them.

  28. I don’t profess to know whether sleeping naked helps you stay warmer or not (though I’ve always taken issue with the “if you put on too much clothes, your body heat can’t heat up the bag” line of thought because, as someone else mentioned, if it’s not reaching the bag, then it’s staying nearer to you, so who cares?), but your analogy isn’t the best. I hope you don’t put on a fresh, dry baselayer over your furnace in your home. ;)

  29. Good provocative reaction of Ron Moak and Philip’s answer seems adequate to me.

  30. The study you reference may indeed be a valid one and probably should have been noted in the original article. At the very least it would have provided a bit more credence to the piece.

    However there are still several issues that aren’t and can’t be addressed by article or the EN study.

    First, there is cold and then there is the perception of cold. The two are vastly different. At may times we feel cold when our bodies are quite content and fully capable of maintaining a constant temperature. Then there’s the cold that if not properly addressed will slowly draw us into a state of hypothermia. For the most part we react to the perception of cold long before we’re in any actual danger. Plus what we wear or don’t wear to bed can significantly alter our perceptions.

    Second, while the study may be a good start it’s hardly conclusive. Simply because a dummy is not a person. As such it doesn’t react like one. In addition to giving off heat, people also give off a significant amount of water vapor. This is not something that is taken into consideration with a dummy. The affect of this water vapor can vary widely depending upon the person and the environment within the sleeping bag. At one level, it may simply enhance our perception of cold. However, if it condenses within our clothes, it can more rapidly draw heat from our bodies.

    In reality there are few, if any, really hard and fast rules. We all respond differently to our environment depending upon our on unique chemistry, conditioning and perceptions. There are general guidelines that can be discussed with the knowledge that they will vary wildly among individuals. It is therefore up to the individual to determine what methods work best for themselves.

    Ron

    • It’s not a study Ron, it is an outdoor industry standard originally developed in Europe that was adopted in the US about 3 years ago. It’s so widespread that manufacturers can’t sell their sleeping bags at REI unless their temperature ratings have been certified using it by an independent testing laboratory. While all the variables you cite do in fact affect the perceived and actually temperature experienced by people in sleeping bags, we can thank the standards organizations who came up with this certification process (EN) for factoring all of those things into the testing process so that sleeping bag ratings can be objectively determined and compared by consumers. And yes, the dummies wear long johns and a hat because they “sleep” warmer that way. There are still individual differences, but R-Values are additive, so you will sleep warmer in a sleeping bag if you wear long underwear.

      Unfortunately it costs about $260 to buy the text of the standard in PDF so I can’t share it with you, but if you search on EN13537 or EN13537:2012 you’ll find a fair amount of information about it.

      Here are some links free links that are helpful to read.

      http://www.mammut.ch/images/Mammut_Sleep_well_pt1_E.pdf
      https://www.k-state.edu/ier/testing/TRS.html

  31. Under very cold circumstances its difficult to getting warm..i use a trick from the Dutch army. ..I undress myself in the sleeping bag… @Gerjan72 (Twitter)

  32. No no no!
    The whole point is that sleeping naked in your sleeping bag Does Keep you warmer as you have naked buddy in your sleeping bag with you!
    That was how it is suppose to work!
    Weren’t any of you in Cub Scouts??

  33. Here on web research, late to the game, but…

    I’m with Ron on the perception of cold and hypothermic cold being two different sets of factors for which the laws of heat loss physics – as presented – only explain the latter. Still dealing with laws of physics, of course, but when talking about perceptions of cold, you’re talking about a dynamic energy source in the human body, and specific dynamics with the body’s physiological reaction to the perception of cold (or perhaps the perception of warmth?) that affect the variables that make up the physics equation.

    In other words, to say that insulation is blindly “additive” without qualification of variable factors seems quite asinine to me, at least when talking about camp type sleep situation.

    I agree, its about what works for individuals more than a technical script to be followed. It seems to be a little too complex/varied to nail down something that works for most in almost every situation. Sometimes adding clothing does insulate further. Clearly, that doesn’t work for everyone, at least not within the system of insulation that they are using.

    If only the really smart technical people (I don’t qualify) would get off their high horse about what is “myth” vs “the laws of physics”, maybe we might get a lot closer to what could be a really good explanation of how all the factors get put together, and a lot better insight into staying warm and confidence in our gear choices.

  34. I always feel much warmer when I sleep naked. Having LOOSE long underwear is ok, but tight long underwear that sits right against your skin doesn’t allow for a pocket or warm air to be trapped. It’s like going outside in fitted clothes compared to loose ones. You’ll feel the cold air hot you much harder in fitted clothing. If it’s just you in a big insulated sleeping bag there’s a large pocket of warm air that’s trapped.

  35. It’s been a LONG time since I last camped out in a sleeping bag, but I remember the same arguments about this at the summer camp I went to as a teen. I think that the outside air temperature would have a good deal to do with the end results as well, during the summer in upstate NY on a cloudless night the outside air temperature could drop into the upper 40’s, hardly a hypothermia risk even if you are outside buck naked.

    I remember trying it both ways, and I was more comfortable sleeping naked in the bag, than with my clothes on (I didn’t wake up sweaty), but I kept the next days change in the bag so that dressing in the cool morning was less of a shock.

    OTOH, if one were to camp out in the dead of winter, things would be quite different! So maybe the important question, isn’t if you are warmer, but if you are more comfortable and sleep better.

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