I'm sitting here, preparing to write a blog post on thermoregulation. I finished a good run a while ago. The temperatures outside weren't too extreme (50ish degrees F, so comfortable for a good run), and I was sweating freely when I finished. About an hour later, here I am, in fleecey pants, shirt, socks, hoodie...and sleeping bag. And afghan. And cat.

I'm freezing. Really, seriously cold. My nailbeds are almost purple, my hands are like ice, and I've got goosebumps all over. I'm almost too cold to shiver.

This happens every time I run more than about 5 miles. It happens winter or summer (I think winter is worse, usually in summer it's a relief!). I'll go out, run 5 or more miles, come home sweaty and glowing with my happy runner's high, and about 30 minutes later, once all the sweat is dried, I'll descend into what I call the "post-run shivers". They last up to two hours after the run, and are the reason I keep my sleeping bag close to hand.

(What I feel like right now. Source)

When I've asked other runners about it, many of them are mystified. Some of them have only experienced the hot feeling post-run, and tell me they can't shower immediately, or they'll come out still sweating! But a few others know what I mean. And I've always wondered, what is happening to me? Is it normal? Is it ok?

When I learned about how humans regulate their body temperature, I learned that we have a natural temperature "set point" of around 37 degrees Celsius (98.6 degrees Fahrenheit), based in the hypothalamus of our brains, and your body regulates its temperature around that set point. When you get cold, your hypothalamus senses the temperature change by getting information from "cold" and "warm" receptors in the skin, and starts inducing mechanisms to reduce heat loss (shivering to make more heat and constricting the blood vessels near the skin to reduce heat loss, for example). Conversely, when you are hot, the skin temperature rises, the hypothalamus senses the change in body temperature, and induces mechanisms to promote heat loss (such as sweating and dilating blood vessels in the skin).

So what causes my shivering, even though it's perfectly warm inside? I thought that perhaps my hypothalamic "set point" was off. The idea behind this is that there is a "set point" temperature which causes the hypothalamus to induce shivering or sweating, in order to maintain a set core body temperature within a very limited range. So I thought that perhaps, while I was running, my body was hot and giving off heat to maintain its set point of 37 degrees, causing me to sweat. According to this logic, when you stop exercising, your body should quickly stop sweating in an effort not to lose too much heat. My thought was that maybe my hypothalamus was a little slower, not catching on that I'd stopped working out, and continuing to give off heat until my temperature got too low, and I ended up cold as a consequence.

But I wasn't sure that this was really true. So I got in touch with Ollie Jay, who runs an exercise physiology lab focused on temperature management at the University of Ottawa. And it turns out that, while we used to believe that our hypothalamus controlled everything via an adjustable "set point", that may not be the case. While our bodies do maintain a set point, our core temperature fluctuates a bit more than we used to think and there's a much larger variation in body temperature responses than we thought, and it's the dissociation between skin temperature and core temperature that's causing my current shivers.

Let me explain. When I start running, my skin is cool. Once I begin to run, my body begins to produce a lot of heat (running is a terribly inefficient mode of transport). But my SKIN is still cool. So the heat ends up stored inside for a short time, until my skin heats up, the heat loss mechanisms kick in, skin temperature goes up to cause heat to be lost to my environment, and if I continue to produce heat by continuing to run, I will start to sweat (which will evaporate and help me lose heat). However, even though I'm now releasing heat, my core body temperature is still a little elevated from the beginning. My body can now keep up with my current rate of heat production, but it can't get rid of all of it. This means that my core temperature has risen, and is staying at a new, elevated temperature, while I exercise. This happens to everyone, but will vary depending on how big you are and how hard you're working.

But what happens when I stop??

When I stop running, the heat loss and sweating will stop fairly quickly. But remember, my core temperature is still elevated. This means my skin temperature, though I won't be sweating, will remain high and I'll still lose some heat to the air. And because my heat production is down (I'm not running anymore), my heat loss is going to be greater than my heat production. At first, this is good because my core temperature is a little elevated, and this will lower my core temperature. As long as I don't lose heat too quickly, Everything will go back to normal.

But in my case, I may lose heat too quickly. This can happen if you have a smaller mass. In my case, my BMI is 21, which is in the normal range, but I'm also tall. This means I have a relatively high surface area for my mass. Dr. Jay hypothesizes that my relatively high surface area will dissipate heat rapidly, and that I may keep losing heat even after my core temperature has come down to normal, overshooting the mark and ending up at a lower core temperature. This causes my cold temperature mechanisms to kick in, and I get cold, the blood vessels near my skin constrict (hence my purple fingernails) and I start shivering.

You can see that the temperature regulation mechanisms are the same, but what's interesting about this relatively new idea of temperature regulation is that, while the mechanisms are the same, there is much more variability than previously thought. The core temperature of our bodies can rise and then fall a little as our conditions change, and there's more variability in core temperature and in the degree of core temperature changes from person to person. It also means that we can't just do research on things like exercise and body temperature in athletes. We also need to do it in unfit people, normally fit people, exercisers, non-exercisers, young, and old. Each of these groups of people is going to have a temperature response that could vary in a different way, creating different risks for different people. And these are further complicated by differences in body mass and exposed surface area. In my case, this variability means that I go hot and then cold after my run. And while it's nice to have science explain the phenomenon, it'd be nice if it would get rid of the shivers as well!!

Thanks very much to Dr. Jay for giving me so much information and the reference for this post! For more on Ollie Jay and the work going on in his lab, you can visit his website here, and his Facebook: www.facebook.com/thermalphysiology (I have to admit I'd love to participate in a study!).

Romanovsky, A. (2006). Thermoregulation: some concepts have changed. Functional architecture of the thermoregulatory system AJP: Regulatory, Integrative and Comparative Physiology, 292 (1) DOI: 10.1152/ajpregu.00668.2006