Dr. Free-Ride: Peeing At Night

January 11, 2009

Have you ever had a seemingly good scientific-ish explanation to a phenomenon that, after years of satisfying you, kind of fall apart?

I think mine fell apart last night.

My better half and I were talking about how freaking cold it’s been. Our thermostatic maximum in the house is 67 oF, and during the hours we’re supposed to be asleep it drops to like 60 oF. So, despite bundling up and wearing hats to bed and such, one or the other of us almost always wakes up in the middle of the night cold.

As the sandman approached, I suggested that peeing before retiring might be an effective preventative measure. Every time I’ve been camping in the winter, my experience has been that the instant solution to waking in the middle of the night on the verge of hypothermia is to trudge off and take a leak.

The story I told myself about this is that you’re offloading a bunch of water that your body no longer has to heat to body temperature. Thus, it can devote its heat to actually heating you.

Last night, my better half pointed out the flaws in this theory: Isn’t the pee pretty much already at body temperature when it gets to your bladder? Before which, your kidneys were squeezing it outta your (body temperature) blood? And why think that, prior to entering your blood stream, the stuff in your stomach is noticeably colder than your body temperature?


But the fact remains that, empirically, getting the pee out makes for a toastier sleeping bag experience. Why should that be, if my thermodynamic handwaving turns out to be wrongheaded?

My better half theorizes that a full bladder simply heightens the impact of all other irritants. However, as a chick (and one who traveled on Greyhound buses in her youth), I can hold it for two to three days if necessary, so I’m not entirely sold on this explanation.



13 Responses to “Dr. Free-Ride: Peeing At Night”

  1. […] Free-Ride (aka JD “SG” Stemwedel) from Adventures in Ethics and Science posts on Peeing At Night. Janet blogs about ethics, science, sprogging, etc. Posted by Comrade PhysioProf Filed in […]

  2. chezjake Says:

    I expect the physiologists hereabouts (our host CPP and Dr. Isis) may have a better handle on this than I do, but my first thought is that while you are sleeping your body slows down peripheral blood circulation while it’s busy doing routine metabolic maintenance. This would tend to make your appendages (particularly feet) feel cooler. Getting up and moving about to take a pee probably gets the peripheral circulation going enough to make you feel warmer, at least until you can get to sleep again.

    A thought: Does your bundling up for bed include warm socks? If not, you might give them a try.

  3. Although I really doubt that offloading ~300 ml of body fluids is going to make the body warmer, the logic behind Dr. Freeride’s reasoning is sound. It’s just an energy balance. It takes a certain amount of energy per gram bodyweight to keep us warm. By offloading some of that bodyweight, we will have more energy to spend on warming the rest of our bodies.

    I like chezjake’s explanation. It’s probably not an issue of actual body temperature, just perceived body temperature.

  4. JLK Says:

    You know that old folk advice, that if you fall into a frozen pond or other cold water source, to pee in the water to warm yourself up?

    I always thought it was because the pee itself warmed the water around you temporarily, but maybe there’s another reason….?

    To me it just makes sense to empty my bladder before climbing in bed. My husband also likes to keep the thermostat at 60 which drives me insane. I pee before bed because I’ll be damned if I’m getting out of a comfy bed to step on some freezing ass tile.

  5. bob Says:

    Candid Engineer,

    Are you sure about that? The pee isn’t on the surface so it’s not radiating heat away to the surroundings. The relevant thing is the heat radiated away from your body. Unless the pee changes your surface area* or the rate that heat leaves/surface area, I don’t see how it would affect your temperature.

    *it would since you’re fairly incompressible, but it would be pretty darn small.

  6. Jenn, PhD Says:

    What about the insulating properties of said full bladder? Much like the way that coastal communities are insulated from extreme colds in the winter or extreme heat in the summer, wouldn’t the already warm urine actual keep you warmer?!
    But actually, I’m with Dr Freeride’s other half’s suggestion that a full bladder makes nearly every other annoyance unbearable.

  7. What if having a full bladder makes you slightly more awake in the middle of the night, so that you are more likely to notice that you are cold, or more likely to toss and turn and throw off a blanket? This is definitely the case for me when I’m pregnant, especially when I keep trying to fall back asleep and eventually give up to go to the bathroom. I agree with bob that surface area is the most important consideration here.

  8. Definitely on board with the “needing to pee keeps you awake” theorists here. I also think chezjake is right about improving your blood circulation–but it might also be that by getting out into the cold air, your blood vessels constrict a bit, leaving you less vulnerable to the cold–and making the sleeping bag/bed feel that much warmer.

    That said, I actually thought the purpose of marriage was having someone else warm in bed with you for insulatory purposes. I just put my cold feet right on my husband’s warm ones and I, at least, am immediately happier.

  9. Climbupit Says:

    I’ve spent a lot of time in the backcountry in winter and can certainly vouch for the empty-bladder keeps you warmer. I’ve taught people that it is harder for your body to keep a big pot of centralized liquid warm and that that energy can be redirected to other parts of the body once the bladder is empty….
    But I’m not a scientist, so this is an interesting conversation that may change what I tell people…
    For those ginning up the argument that the fresh air or moving around is the actual inciter of warmth, I should tell you that many mountaineers keep an extra one-litre bottle (like a Nalgene, with a different tape design so you can feel which is which in the dark) so that urinating can happen without even opening the sleeping bag or lifting the body…. (women likely have to move a bit more to achieve a spill-proof angle)). This still works to warm you up dramatically! In my experience, it does seem to be about removing that urine from the body….

  10. daedalus2u Says:

    The normal function of the kidney is a major metabolic load. Presumably there is some feed-back inhibition of the kidney when there is a full bladder. When the bladder is emptied, the kidney can resume full metabolic activity. That full metabolic activity will generate heat. That heat may be what is keeping you warm.

    In the case of peeing when one has fallen into ice water, unless one is naked, there is clothing between your skin and the external ice water, and so there is a temperature gradient from the skin to the water. When you pee, you displace that water with pee, which temporarily replaces the water at less than body temperature with pee which is at body temperature. The heat flux from the skin will go down.

    There may also be an evolved mechanism to accelerate metabolism following peeing. In “the wild”, peeing may have ended up with substantial amounts on the skin. That would present a heat load due to evaporative cooling. There may be an evolved compensatory response to accelerate metabolism following peeing to avoid a drop in body temperature from that cooling.

  11. daedalus2u Says:

    In thinking more about my last idea, that there is physiology that accelerates metabolism after peeing to compensate for evaporative cooling; that might have been important for infants living in “the wild”. If so, that would most likely trigger oxidation of lipid in brown fat, kidney metabolism uses carbohydrates. You could distinguish between these two by looking at O2 consumed vs CO2 produced.

    Kidney activity might be accelerated to increase temperature too, so that the waste metabolic heat is actually used to keep warm. In Tibet, people gain weight over the winter, the explanation being that heat generated by digestion is more valuable during the winter.

  12. vcuresearcher Says:

    Why do you refer to yourself as a chick and then publish pics of your dude shoes?

  13. The urine inside you is already at your body temperature. You don’t have to spend any more energy to keep it warm because nothing is cooling it down. Are are two cases to explain it.

    Imagine two hotwater tanks sitting in the snow. Both contain 50 litres of hot water. Now take 10 litres out from of those tanks. Which one will freeze first? The one with 40 because it has less total energy and the snow will absorb all that energy in less time. Now I doubt anyone can pee 10 litres, but the mathematics are the same.

    Identical twins sitting in the snow. Give one 300ml of body temperature water. Which one is going to stay warmer longer? Surely that’s the same as telling one of them to piss out 300ml of body temperature liquid.

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