Nobody believes the official spokesman, but everybody trusts an unidentified source.
Question #91966 posted on 02/07/2019 5:04 p.m.

Dear 100 Hour Board,

A little while ago, I was out camping with an experienced outdoorsman. He explained that folks who want to warm up their sleeping bag will sometimes put hot water in a Nalgene bottle and tuck it into their sleeping bag with them. He believes that, while this is great at the start of the night, by the time the water cools down to below body temperature, it will actually make you colder.

He reasons that as the water cools off, some of your body's heat output is used keeping the water warm instead of keeping you, or the inside of the bag, warm.

I'm not sure if he's right. Adding a water bottle to the inside of the sleeping bag definitely increases the heat capacity of the interior of the bag. At the same time, the body heat used to keep the water warm is still inside the bag, and it wouldn't seem to make a difference unless the water is somehow better at radiating heat through the bag.

Is my friend the outdoorsman right? For bonus internet points, I'd love to see some supporting math describing how heat moves through the sleeping bag - human - bottle system.



Dear you,

I unfortunately don't have the mathematical skills to describe the heat transfer, but I can give some good ol' basic thermodynamic analysis!

The second law of thermodynamics basically says heat naturally flows from low temperatures to high temperatures. What does that mean in this system? Basically, we have 4 different parts:

a) person
b) water bottle
c) air inside sleeping bag
d) surrounding air

All 4 entities have a stored level of heat. Only the person produces heat, which happens due to your body metabolizing food and producing body heat. Bodies are pretty good at maintaining their heat, and it's unlikely your internal body temperature will change much, but the temperature of the air in the sleeping bag can change quite a bit. How warm the air inside the sleeping bag is what will determine how comfortable you are. Okay, now that we have that out of the way let's get to the thermo!

Phase 1

When the system starts off the water bottle is the highest temperature, followed by you, the air in the sleeping bag, and the surroundings--in that order. This means the water bottle is heating both your body and the air in the sleeping bag. Your body also produces heat which warms the air in the sleeping bag. Some heat leaves the sleeping bag, but if it's a good sleeping bag that number is pretty low. You are pretty warm.

Phase 2

After some time, the water bottle has lost enough heat that it is the same temperature as your body. At this point no heat transfer occurs between you and the water bottle. However, the water bottle is still likely hotter than the air in the sleeping bag and both the water bottle and your body heat up the air in the sleeping bag. Again, some heat is lost to the surroundings.

Phase 3

The water bottle is now colder than your body. At this point some of your body heat goes to keeping the water bottle warm instead of keeping the air warm. However, the water bottle is still warmer than the air and also transfers heat back into the surroundings. Note: how much goes in and out of the water bottle depends on temperature differential and method of transfer. Basically, If you're touching the water bottle it'll take your heat quicker than it puts it back into the bag, so you might feel colder.

Phase 4

The water bottle is now the same temperature as the air. Heat is now going from your body to the air in the sleeping bag and the water bottle. Very little heat transfer occurs between the bottle and the bag and both basically remain at the same temperature. At this point the specific heat capacity of the bag and the bottle combined than that of the bag alone. That doesn't necessarily mean that you will be colder, but that it will require a greater amount of heat loss/addition to change the temperature. If the sleeping bag isn't losing much air to the surroundings, you'll probably stay the same temperature as it will be harder for the surroundings to cool the bag off. If you're losing a lot of heat through the bag it will be a bit harder for you to keep the bag warmer as you will have to warm both the bag and the surroundings.


After the analysis, I conclude that the water bottle would help keep the bag warm early and would not be likely to make you colder later. However, this conclusion comes with two important caveats:

  1. If you're touching the water bottle, even if it's the same temperature as the surrounding air it will feel colder than air at the same temperature. So if you do use a bottle, I'd recommend placing it at the very foot of the sleeping bag.
  2. If you use a water bottle and it leaks you will become wet and miserable and cold. This isn't an engineering conclusion. I tried it once and it was not fun. So keep that lid on tight, yo'.

Hope this helps! Personally I'm excited to see what conclusions the other writers draw. However, I think that the best way to see if you like it would be to try it for yourself!




Dear Geraldine,

There are various partial and ordinary differential equations to model heat flow. However, they all come down to one basic model, which we conveniently call the heat equation (though it is also referred to as the diffusion equation):

ut= cΔu

Here, Δ is the Laplace operator, c is a diffusion constant, and u is the heat. Essentially what this equation is saying is that heat flows through a medium as a function of space and time. So, we have that heat is moving through the water, sleeping bag, or what have you. 

I could come up with equations to more precisely describe what would happen with your exact scenario, but honestly all I'd be doing would be translating the answer below mine into math. You wouldn't get any extra information (unless you actually worked out the equations to determine the exact moment at which the water bottle started being a heat suck instead of source, and other such details). So, I'm not going to furnish more precise equations.



Dear G,

I asked my friend, another experienced outdoorsman, about this question, and here's what he had to say.

Excellent question! I LOVE when people think critically about how outdoor systems work and we get to discuss them, so I’ll try to contain myself [retrospectively: I failed miserably on that. But! it could have been worse! I didn’t get started sharing my enthusiasm for the best and lightest backpacking stove, a DIY costing under $1.60!]. Bit ‘o background: I’ve been backpacking since I was 4, am a NOLS grad, and have been a professional outdoor educator since 2012. Helping people to enjoy their time outside is one of my great life passions.

I’m with your friend; in fact, when Alta sent me the question it was such déjà vu I wondered if I was the friend. Anathema is working on the math (and hopefully science), and I’ll tackle the practical stuff. Ok I’ll hit a little on the science, because that’s practical too and a helpful foundation.

The Hot Nalgene Question

Water is a fascinating set of molecules, and behaves rather abnormally and in doing so, allows life as we know it. It gets bigger and less dense when it gets colder! Do you realize reversing that alone would likely kill much of our planet’s life?! Think about marine life if ice sank. But I digress. One of water’s incredible properties is its ability to absorb energy (think heat) before it changes forms, and it’s all thanks to HYDROGEN BONDING (so yes, there was a reason you learned about those. They’re also why ice floats). Relative to other substances, those hydrogen bonds can absorb a TON of energy, and so it takes a ton of energy to get water to heat up and boil, which is great during the first half of the night because the water can then dispense all that stored heat back to us. Of course there’s an angle of repose however, when the source keeping the water warm in our bottle switches over from the stove to our body. Basically, as your water bottle cools, its hydrogen bonds become a black hole for heat—and that heat suck is snuggled up with you. The same thing is going on within your bladder if you’ll believe it, which is why sooner or later, if you hang around enough outdoor geeks, you’ll hear the advice to go pee before going to bed, and to go pee in the night if you so need to (don’t go far though—there’s really no need—and thus don’t get lost). Furthermore, we change during the night—we produce less and less heat (explained below), so a heat suck in the middle of the night is really not welcome. (Btw, this is also why your roommate who just pulled an all-nighter is so much colder than you who just woke up and ate breakfast)

The Science of How We Make and Preserve Heat (and also lose it)

So, what can you do to stay warm while sleeping in the great outdoors? Or indoors, for that matter? You can memorize my below list, or you can derive it (and then some!) yourself by thinking about how we get warm and how we get cold.

As should be obvious but few take the time to think about it—sleeping bags do not provide heat, they only trap some of the heat your body produces. As warm-blooded mammals, we produce our own body heat through metabolism, meaning we eat food and without even exercising we produce heat, and if we’re exercising, a lot of heat. That heat just dissipates into the atmosphere though unless there is something to trap it near ourselves, which is what fur does for animals and clothes do for us. We also get heat externally from the sun’s radiation, fire, and a million modern inventions. In our tent however, we don’t have most of those, so we’ve got to make do with our own metabolism. Thus, the first rule of staying warm at night? Eat a lot of calories and do enough exercise before bed to be nice and hot, but not sweating (we’ll get to that later).

So how do we lose heat? We radiate heat out and it dissipates. We breathe out warm air and take in cold air. Conduction from that cold snow we’re standing on. Evaporation from sweating (those hydrogen bonds again!). Wind rips heat away from us via convection. Water sucks it out of us. Moving water sucks heat out of us and then flows off with it.

Thinking about how we lose heat should help you with your clothing choices next time you’re headed into the cold: are you cold because it’s cold out? Puffy insulating layers will be your friends. Are you cold because it’s super windy out? A protective layer, like a rain jacket (or heck, even a garbage bag) will be your friend, as that puffy just won’t trap the heat. Both cold and windy? Better put that rain coat over your puffy. Standing on snow? Thicker boots, and stand on a piece of foam pad, or even a log. Maybe you’re cold because you’ve been wearing too many puffy layers and exercised and got sweaty? You’ve probably heard of wicking layers that you wear next to your skin, and this is the time they shine as they pull that sweat and evaporative cooling away from your body! (Unfortunately, no church garments do this well, and ALL cottons are not only NOT helpful, they are harmful in this department. You’ll have to make your own judgement calls but know this: a lifelong ranger in the Teton’s looked back over all the hypothermia incidents, and in every instance the victim was wearing cotton. This means you, jeans and hoodies!). In summary, you’ll need one or all components of the VIP layering system: Ventilating (wicking) next to your skin, Insulating (puffy, lots of dead air space) sandwiched in the middle, and Protective (from wind and water) armor on the outside.

Sleeping Warm: The List

Here’s a number of things you can do to stay warm at night, camping and otherwise:

  • Go to bed warm!!!
  • Use a sleeping pad to insulate you from the cold ground.
  • Stay on your sleeping pad – tuck a shoe under each side or pad each side with protective layers to give your pad a more cradled shape.
  • Go to bed dry, in dry clothes
  • Be fed, also hydrated
  • Go pee before bed, and go if you wake up in the middle of the night (don’t go far, don’t get lost)
  • Wear lots of layers, and if you’re too hot in them all, sleep with your sleeping bag unzipped (then during the night once your metabolism has slowed and you’ve cooled off, all you have to do is zip up yer bag, not crawl out of it and try to put on some cold layers!)
  • Think mittens for your feet—mittens are warmer than gloves because your fingers get to stay together, it’s same for your feet—put ‘em together in a hat or balaclava or something.
  • If you bring extra blankets to supplement your sleeping bag, think loft—it’s the dead air space that’s insulating, so if those blankets are over your sleeping bag and crushing it, they’re not helping much. If they’re inside the bag making it more puffy, they’re doing great!
  • Wear a hat, and don’t breathe into the sleeping bag—that’ll just make your bag wet.
  • Take a hot water bottle to bed and then push it out of your bag in the middle of the night.
  • Sacred socks. They’re the best. A pair of cozy socks that literally never leave your sleeping bag the whole trip, so you always have warm dry socks to sleep in. You’ll swear by them.
  • Wake up cold? Do some sit ups in your bag.
  • Hot water in a thermos/hydroflask: make herbal tea or hot chocolate to sip on if you wake up during a cold night, or have ready in the morning before you’ve even lit the stove! Alternatively, you could pour your hot thermos water into an empty Nalgene bottle in the middle of the night for round two of the hot water bottle method.
  • Place your tent well – in a less windy spot and angled to shed the predominant wind, on a local high spot where you won’t get flooded, and above and away from lakes and rivers. Trees reflect heat back to you, but make sure there are no leaning or dead ones around to fall on you!
  • Your shelter—snow is a better insulator than a sheet of siliconized nylon, and thus igloos are warmer than tents: if you’re insulated from the snow with your sleeping pad that is.
  • Pretrip, try out your sleeping set up in your backyard in similar conditions as you’re anticipating. Adjust as necessary.

Precautions: it’s true you can dry out a damp pair of socks in your bag, but don’t try to dry out more than that!


Yes, a hot Nalgene will provide warmth for the first few hours, but then it will cool off and your own body heat will start to be absorbed by the cold water and it will make you colder, and it will do so during the coldest part of the night and when your own body is producing the least amount of heat.

Happy adventuring,

Corsica S.