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One of the recommendations for sleeping warm is to empty your bladder before bed,

When your bladder is full, your body is expending energy keeping that liquid warm. If you empty it out, your body needs to expend a little less energy to stay warm.

Source

There has been some discussion on this answer as to whether or not this is true, especially since it is also recommended to sleep with a warm water bottle.

A hot water bottle in your sleeping bag can stay warm for hours and help you sleep soundly when it’s well below freezing in your tent.

Source

What would be the scientific reason for why this is so or possibly why it is not?

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    Note that this question is asking for scientific basis. Please avoid opinion-based answers. – Roflo Feb 7 '17 at 21:02
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    Ryan's and Ben's answers clearly explain why the notion is physically unsound. However Ryan and some others brought up, that there may be biologic factors that actually make a difference. I hope someone with knowledge about this factors comes around, or otherwise it would be a good idea to post this on biology.SE. – imsodin Feb 7 '17 at 23:34
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    The main reason for emptying your bladder is so that you don't have to get up in the middle of the night to pee (seriously). The heat loss from getting up and going out is enormous, especially when compared to any minuscule loss from differences in body volume, etc. – Hot Licks Feb 8 '17 at 0:06
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    Personally, I don't think I could manage to go to sleep with a full bladder, making the warmth issue moot. – jamesqf Feb 8 '17 at 0:49
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    Another point: The kidneys (or the glands supporting them) produce several hormones that control blood pressure and blood flow in the body. I can say from personal experience (due to kidney stones) that fluctuations in these hormones can make the feet, in particular, run cold or hot. I suspect some people may be quite sensitive to these effects and may have found, in their own experience, that emptying the bladder, by reducing kidney pressure, improves blood flow in the feet, and hence comfort while sleeping (or attempting to). This would be a highly individual phenomenon, though. – Hot Licks Feb 8 '17 at 13:28
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Humans have a few intuitions about how temperature, heat, and liquids work. These intuitions lead us astray for the specific case of a bladder full of urine.

"Keeping Warm" / "Cooling down"

The first intuition is that things cool down on their own: If you leave a pie from the oven on your counter, it eventually cools down to room temperature.

We believe things cool down (and thus must be "kept warm") because ambient temperature is nearly always colder than any object which we consider to be warm. In reality, things only cool down when their temperature is higher than the temperature of their surroundings.

This is thermal equilibrium: in the absence of other processes, an object and its surroundings will eventually match temperature.

Does this principle of things cool down on their own apply here? No. Urine in your bladder is already at thermal equilibrium with the rest of your body.

"Cool Liquids"

We also intuit that liquids are colder than normal matter, because wetting your hands results in a very high surface area-to-mass ratio, causing rapid evaporation. Evaporation is a cooling process, of course.

We also think liquids are colder because they tend to have good thermal conductivity. We perceive the temperature of an object based on the rate of heat exchange with it; that rate is determined by the difference in temperature and its conductivity. Thermally conductive things feel warmer or colder than their nonconductive counterparts because they "move" heat more quickly. This is why metal objects at room temperature feel colder than e.g. cotton at the same temperature -- they are the same temperature, but the higher-conductivity object exchanges heat more quickly. It's also why you can put your hand into a 450 F oven without getting burned, but can't safely grab a metal pan at the same temperature.

Do these properties of cool liquids apply here? No. Urine in your bladder cannot undergo evaporation (it's a closed system), and its high thermal conductivity is irrelevant because it's already at the same temperature as the rest of your body.

Physics

There are only a few factors we need to consider to determine the nighttime temperature of a camper in a sleeping bag:

  • Starting heat energy: How much thermal energy are we starting off with? Going to bed cold is going to be worse than going to bed warm. Starting with a hot water bottle in your sleeping bag is better than starting with a bagful of ice in your sleeping bag.
  • Insulation: What is the thermal conductivity between the sleeping bag and the surrounding environment? Well-insulated bags are better than poorly-insulated bags
  • Exterior temperature: How cold is the surrounding environment? All things equal, colder exterior temperatures will cause a higher rate of heat loss.
  • Endothermic / exothermic processes: How much heat is the camper generating (e.g. from metabolism)? Humans convert chemical energy (food calories) into heat; if we burn more calories in this process, we'll stay warmer.

This leaves us with two possible physically-possible mechanisms of action for the "full bladder makes you colder" hypothesis:

  • Endothermic reaction: It's in theory possible that urine undergoes some endothermic chemical reaction, like a one-time-use ice pack. There's no evidence for this happening; urine does not meaningfully change chemical composition over time when sitting in the bladder.
  • Exothermic inhibition: It's in theory possible that the human body inhibits its own metabolism when its bladder is full. It definitely seems like we'd notice if this were actually the case. Either way, the mechanism of action here is not that we are "keeping our urine warm".

Running the numbers: A Sanity Check

The original claim is that you don't have to "keep warm" the contents of your bladder. We've shown that this isn't really a thing, but what if it were? A healthy full adult bladder is about 300 mL of liquid, which would weigh about 300 grams (or half a pound). For a hiker weighing 80 kg / 175 lbs, that's about 0.3% of your total body mass. Is a 0.3% change in body mass really going to significantly change your overnight heating situation? That's implausible at best. Note that the change in surface area (assuming you expand with a full bladder, rather than simply compress other parts of your body) is going to be smaller than 0.3% because surface is a quadratic measure but volume is cubic.

If this effect were actually noticeable, it should be twice as strong for taking a normal #2 (weight 1-3 lbs) right before bed, because you should have to "keep warm" your poop just as much as your urine (presumably). No one seems to comment on this. Humans are pretty good at sensing their bladder fullness but not great at sensing bowel fullness when it's within normal parameters, which suggests there's a strong psychological component going on here.

What about a hot water bottle in the sleeping bag with you, a common suggestion? Does this matter?

If we heat up a 1 L bottle of water (1,000 grams) to 100 C (boiling) and it eventually cools to 24 C (74 F, a rough guess at the temperature in a warm sleeping bag), we've added 76 Calories* of heat to our system. Basal sleeping metabolism is about 80 Calories per hour; nearly all of these Calories turn into heat (only the small portion stored as chemical energy, e.g. ATP, don't). So if the water bottle cools down to ambient temperature within an hour (a decent guess), it's almost as good as having another human zipped up in the sleeping bag with you for the first hour, which is definitely something. There's a throughput trade-off here; if you insulate the water bottle a bunch to make it cool down over twice the period of time, you get half the per-time energy output.


* Note Calorie, equal to 1,000 calories

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    If you had to introduce new unit called "Calorie" why not just use the correct kcal instead? same amount of trouble – souser12345 Feb 9 '17 at 10:48
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    @progo because Ryan didn't make it up, and is using correct terminology. (It seems good that he gook the time to clarify this for people like yourself.) kirkmahoney.com/blog/2009/01/calorie-vs-calorie – TOOGAM Feb 9 '17 at 13:43
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    @TOOGAM but the big-C Calorie is only properly used in the context of food and nutrition. Don't let it bleed into contexts where it can cause even more confusion. If you really wanted to be pedantic and stay in imperial units we should forget calories and go with BTU ;-) – Ukko Feb 9 '17 at 15:37
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    the nutritonal and colloquial “Calorie” is actually a “kilocalorie” and thus properly abbreviated kcal. – mirabilos Feb 9 '17 at 17:20
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    I would just note that the one who invented two units that differ by a coefficient of 1000 and by one capital letter in their name, this person must have lost his brain before doing so. – yo' Feb 11 '17 at 20:12
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Most important reason to pee before you go to bed: you'll be uncomfortable lying there all night feeling like you have to pee.

Second most important reason: if you try to delay it, you'll just have to get up later, in the middle of the night. This means getting out of the bag and getting cold.

The REI blog sounds scientifically illiterate to me and does not appear to be backed up by a citation of any scientific source or any experimental evidence or valid theoretical argument.

When your bladder is full, your body is expending energy keeping that liquid warm. If you empty it out, your body needs to expend a little less energy to stay warm.

This sounds like the blogger didn't understand thermodynamics very well. The rate at which heat flows out of an object by conduction into its colder surroundings is proportional to its surface area and to the temperature difference. When you surround your body with a sleeping bag, the rate of heat flow by conduction is also inversely proportional to the thickness of the bag. Having a bladder full of pee at body temperature doesn't affect any of these three variables, so heat will flow at the same rate when your bladder is full. However, when your bladder is full, it will add slightly to the amount of heat energy that your body starts with, so heat loss at the same rate will produce a slower drop in temperature. In reality I would expect this to be a small effect.

We could also worry about heat transfer by convection or radiation, but I don't think it changes the conclusion.

Your body does not, of course, spend the whole night coming into thermal equilibrium with its surroundings. It probably maintains its core temperature at close to nominal body temperature, but body parts like skin and fingers will get colder. Your body is probably in a steady state of heat flow for most of the night, with its metabolic heating staying in equilibrium with heat loss. I don't think this alters the conclusions, because the rate of heat loss is still governed by factors that are pretty much the same as in the description above for something just passively cooling.

  • You do also use the term steady state but should use that term over thermal equilibrium consistently as this is an open system in steady state. – paparazzo Feb 7 '17 at 21:45
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    @Paparazzi: The body is not in a steady state at all times, or else you would feel the same during the night as during the day. This is discussed in the final paragraph of my answer. The body is not in thermal equilibrium with its environment, and the only time I mention equilibrium in my answer is to say so. – Ben Crowell Feb 7 '17 at 22:22
  • @Roflo "thermodynamics might not be all there is to this issue" Do you know of a competing model of temperature on macroscopic scales? I sure don't. There's statistical mechanics, but it reduces to thermodynamics at macroscopic scales. (There might be a few edge cases where it breaks down; I'm not sure. But thermodynamics is a very useful model.) Biological processes can't violate thermodynamics. Also, urine is not sterile. At all. – jpmc26 Feb 9 '17 at 23:35
  • @jpmc26 I agree, biological processes can't violate thermodynamics, but they're not taken into account in this answer (see this comment). Anyway, what I meant to say is that this might not be the whole picture. Human biochemistry may be involved. And it's not a passive system. – Roflo Feb 9 '17 at 23:49
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From the viewpoint of physics it's exactly the opposite.

You have taken some liquid and you have already spent some of your energy to heat up that liquid. This liquid increases your total heat capacity making your cooling a tiny bit slower. Ryan estimated that your increase in volume might be about 0,3%, your increase in mass will roughly be the same.

Yes, you increase your surface area as well, but your area increases slower (quadratically) than your volume/mass (which increases cubically).

So the upside of full bladder in this rough model is: Full bladder => 0,3% increased mass => 0,3% more heat in you.

And the downside is: Full bladder => 0,2% inscreased surface area => 0,2% faster heat loss.

Thus you will be warm for longer if you keep those 300ml of warm liquid instead of peeing out all that heat.

  • "... you have already spent some of your energy to heat up that liquid" - that depends on the temperature of the liquid you are drinking, right? – Vidar S. Ramdal Feb 8 '17 at 13:23
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    There is definitely more biology involved in body temperature than pure physics of inanimate objects. The body's metabolism - a key component of life - serves to regulate temperature. – cr0 Feb 8 '17 at 14:34
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    @VidarS.Ramdal that was a bit metaphorical. The physics is that some usable heat is already inside you in form of that liquid, it doesn't matter if the heat got there because of your body or because of your teapot. – Džuris Feb 8 '17 at 14:48
  • @cr0 Thermodynamics is well capable of modeling a system where chemical energy is being constantly released inside it. Such complications make the math a little harder, but they don't change the fundamental equations. – jpmc26 Feb 9 '17 at 23:44
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Theoretically, it depends on the length of time you sleep.

What a full bladder does is raise your body's heat capacity by providing you with more mass, so your body temperature goes down more slowly because it is storing more energy in the additional mass of the water in the bladder. However, a full bladder also increases the surface area of your body by an imperceptible amount, and increased surface area means that you lose heat faster. So with a full bladder, your thermal equilibrium with the outside air will be lower due to the increased surface area, but the amount of time it takes to reach thermal equilibrium will be higher, due to the increased mass of your body. This is all in theory though, because the amounts we are talking about are negligible and unnoticeable. In reality, a full bladder causes you to get up during the night and become much colder.

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    The mass in one's urine came from somewhere else in the body. I maintain that there is no net increase in body mass due to urine flowing into the bladder, and therefore no increase in your body's heat capacity. – Wayne Conrad Feb 9 '17 at 16:23
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There is a really good answer by Ryan Cavanaugh, as to the scientific side of emptying your bladder. In short NO your not going to be any warmer for having emptied your bladder.

HOWEVER Big picture human stuff, you might actually "feel" warmer in the end. Now to be clear, this is not at all the same as the argument that you have to work harder to warm urine. It's just a bunch of observations.

  • If you have to wake up in the middle of the night to pee, then your going to have to exit your bag, and probably the tent. Generally speaking it's warmer during the day then at night. So it's going to feel much colder to go outside the nice warm tent at 2am.
  • If we want to talk about heat loss, then you will loose much heat to opening the bag, then tent, exposing your self, and closing everything back up, getting back in the tent, and back in the bag, then just staying in the tent and bag all night.
  • Along those same lines, when you have been out all day in one temperature, your likely to get acclimated to it. Same is true at night. Your nice warm 70 degree tent is going to make that 20 degree outside feel much much "cooler" then the 30 degree outside you spent all day in. In other words, the fast temperature change from tent to outside is going to be more of a shock then the just being outside. This is amplified by the fact that your sleeping bag is probably warmer then the tent.
  • Lastly cause were all human, when we have one problem we tend to go looking for others. If you wake up at 2am and have to pee, your probably also going to start looking for other things that are wrong. It's cold, that rock is in your back, your shoulder itches, the damn crickets won't stop (keep in mind there probably aren't really crickets at this temperature), your partners breathing is too loud, someone farted and now the tent smells funny, all kinds of things. Things you were perfectly happy to sleep though, now become impediments to sleep. This becomes even worse if you have pressure to sleep. For example if you MUST get up at a time to make the next leg of the journey, and you really outta be well rested.

So while you may not burn any extra energy to keep your full bladder at the same temperature as your liver, a full bladder on it's own can really suck.

TL;DR go pee before you go to bed.

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    Yup, this is the real answer. Being cold or not has little to do with actual temperature or risk of hypothermia (your body will maintain you at a safe temperature very well indeed) and everything to do with your subjective impressions. To your list I'd add that even if you don't get up to pee, being uncomfortable and awake in your sleeping bag is going to make you spend more time feeling the cold than if you pee first and sleep better. – Jack Aidley Feb 9 '17 at 17:55
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As Ben noted in an answer, "the body doesn't spend the whole night coming into thermal equilibrium with its surroundings." Where the body needs to focus its energy is important because if your body is busy heating your core to avoid death, then your fingers and toes can't get as much warming attention and will be more likely to go numb and eventually become frostbitten. Thus, keeping a comfortable core temperature without expending extra internal energy to do so will help keep your fingers and toes a little warmer. (Consider how a proper jacket can help your overall comfort in cold weather, especially if you get blood flowing in your hands and feet. That warming blood flow lasts longer in my extremities if my core is well insulated than if my core is rapidly losing warmth.

Related to that, I always think of the need to relieve yourself before bed at camp as related to muscles and energy expenditure with them. If you need to pee and are holding it in whether consciously or not, muscles are working to do that. The blood and energy going to the muscles that are keeping you from wetting your sleeping bag, could be going to your fingers and toes.

That said, the real scientific answer is in biology, and is beyond the outdoor experiences I've had and been advised by.

protected by Rory Alsop Feb 8 '17 at 23:27

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