Does Shower Temperature Affect Brain Speed?

In November I learned about benefits of cold showers. So I tried them. I took cold showers that lasted about 5 minutes. I liked the most obvious effect (less sensitivity to cold).

Maybe a bigger “dose” would produce a bigger effect. Maybe the  mood improvement cold showers were said to cause would be clearer. So I increased the “dose” in two ways: (a) more water flow (I stopped holes in the shower head) and (b) lower water temperature. After a week or so with the stronger dose, I saw I was gaining weight. It could be the cold showers, I thought. Fat acts as insulation and I couldn’t think of another plausible explanation. So I went from cold showers back to warm showers (48 degrees C.) — this time with greater water flow. My warm showers were 5-10 minutes long.

I began to lose weight, suggesting that the cold water did cause weight gain. More surprising was that my arithmetic speed (time to do simple arithmetic, such as 7-3, 8*4) began to decrease. Here is a graph of the results.

2011-01-04 shower temperature and arithmetic speedBefore the cold showers started my arithmetic speed was roughly constant. The mild cold showers had no clear effect. I had noticed the increase during the strong cold shower phase but hadn’t paid it much attention — I suppose because it seemed implausible. These results, however, are excellent evidence for cause and effect: cold showers made me slower, warm showers made me faster. The arithmetic tests weren’t done soon after the shower. There seems to be some sort of brain-speed adjustment that takes place over ten days or more.

I’ve never heard of anything like this, whereas I’ve heard many times is that cold showers are good. There is one complication, which is that December 3rd I stopped eating walnuts. I believe walnuts are bad for the brain, in contrast to the usual belief. I came to believe that because of results from two students of mine who had tried eating them. Improvement due to no longer eating walnuts would explain why line fitted to the strong cold data starts below where the weak cold line ends.  The final days of the strong warm phase may be the same as the weak warm phase when adjusted for the walnut difference.

What explains this? Maybe the weight change. When gaining weight, maybe fat was taken from the blood to be deposited in fat cells, thus lowering the fat content of the blood reaching the brain and thus degrading brain performance. Losing weight, the opposite happens. Eventually the weight loss will stop; this explanation predicts when that happens the warm-water effect will go away.

In a previous post I wondered why I had gotten faster at arithmetic over the previous six months. These data suggest that warm showers may be at least part of the reason. In Berkeley I take baths, not showers.

22 Replies to “Does Shower Temperature Affect Brain Speed?”

  1. Hi Seth,

    If I may, what time did you take the cold showers? Tim Ferris argues that taking a very cold shower an hour before bedtime improves sleep. This timing might also reduce the effect on weight, provided you can resist eating til bedtime.

  2. I took them in the morning and afternoon usually, but no fixed time. The slowness of the temperature effects I observed on arithmetic speed (they took at least 10 days or so to level off) suggests that the time of day doesn’t matter, at least for the arithmetic effects.

  3. I believe the lines were fit using a model but,

    it just looks like there is a lot of noise, and it’d be my hunch that there is room for error in a 40ms range in the recorded data itself.

    I can imagine a lot of other things might be getting stuffed into the error term over a 3-month period.

  4. You’re assuming that temperature was the important controlled variable. But a hot shower exposes you to a lot more chlorine gas, via the lungs, than a cold shower. Depending on how well ventilated your shower is, you might be varying your chlorine exposure substantially, with unknown effect. Teasing apart effects of temperature and chlorine exposure might be difficult, but seems possible.

    Some water systems don’t have dissolved chlorine gas, but instead chloramines. Chloramines would not be released as a gas in hotter water, but any reactions would be speeded up. Many animals are sensitive to chloramines; a subtle effect on humans would not be surprising, although one would expect more effect from ingestion than immersion.

  5. troy, you write “it looks like there is a lot of noise”. To me it looks just the opposite. In statistical terms, I believe the p value associated with the changes is very low (i.e., the probability that the observed changes would happen by chances is very low). In other words, the signal is high relative to the amount of noise. Since you seem to disagree, care to estimate the p value for the difference in slope between the last two phases (which I haven’t yet computed)?

  6. Sounds thyroid-related to me. I need to take thyroid hormone replacement because my own gland doesn’t work anymore. Occasionally the blood test indicates that I need to up the dose, and when I do, I don’t feel the effect for 10-12 days. Others have told me that they’ve had the same experience.

    Once I tried stopping the medication totally to see what would happen. After about 10-12 days, I noticed significant memory problems (along with weight gain and constipation). I would walk into a room and initially forget why I was there…but each time I would remember again after about 7 or 8 seconds. So the memory was there, just ‘delayed.’

    One of the things that causes thyroxine to be used up more quickly is cold exposure. And if I remember correctly, iodine deficiency is a problem in China and adequate iodine intake is critical for thyroid hormone synthesis. So perhaps the cold showers are causing you to use up thyroxine at a higher rate and your thyroid gland isn’t able to quickly keep up with the demand? That would also explain the weight gain (usually it’s due more to excess fluid from decreased metabolism, rather than to fat buildup). Then when you warmed up the showers, your hormone levels were able to catch up.

  7. Starting with a hot shower and ending with a cold shower is possibly a way to improve benefit; I think some health spas alternate exposure to hot and cold temperatures. I often end my showers with 1-2 minutes of cold (well) water which is about 51 degrees fahrenheit. When I have time, I alternate between hot and cold water a few times; I find it easier experiencing the cold water on the 2nd repitition. Part of the benefit/exhilaration may come from the feeling of accomplishment after having the discipline to take a cold shower. Another important variable is how long to expose yourself to the cold; my hunch is that a brief exposure avoids a cortisol stress response which would come with a longer exposure.

  8. There’s no direct answer to your question because thyroid is a complicated, multifaceted issue. Some things to consider:

    Before starting any discussion, you would need to know if your baseline thyroid hormone levels (no cold stress) were normal. Then look at your diet to make sure it contained adequate iodine. If the area’s water supply is deficient, iodine can be supplied by iodized salt, marine fish, sea weed, etc. However, the situation is thorny because too much iodine can cause the thyroid to shut down temporarily…and in some individuals it can result in autoimmune destruction of the gland.

    If thyroid hormone levels decrease, metabolic functions slow down in all tissues of the body, including the adrenal gland. The adrenals are responsible for producing hormones that increase levels of glucose and fatty acids in the blood so that your brain, heart and muscles can react to stress. If you stress the body with a long, cold shower, and your adrenal reserves are lower (secondary to lower thyroid levels), overall response will be slower. Normally, the brain uses glucose for almost 100% of its energy needs. If a person is starving, the liver can produce ketone bodies from fat…and the brain can use these for energy…but only to a limited degree. To maintain blood sugar levels, the body will then break down proteins and convert them to glucose for the brain. But every one of these processes is slowed by decreased thyroid and/or adrenal hormones.

    The hormone circulates primarily bound to protein carriers…only the free fraction is active. When a person is given a dose of radio-labelled thyroid hormone, it takes almost a week for the level to decrease by 50%…so it’s probably binding to the carrier proteins instead of being used up right away, which probably has something to do with that 10-12 day period. Presumably that bound portion is ‘reserve.’ the amount of carrier protein is upped by estrogens, so women have more (especially if pregnant or on the pill). Increased androgens lower the carrier protein levels.

    Low normal levels of thyroid or adrenal hormones from any cause could mean that you’re naturally susceptible to being subpar after stressful activities, while people with higher reserves would bounce back without skipping a beat. Some patients with adrenal insufficiency are only diagnosed after a high stress experience lands them in the ER with hypoglycemia.

    Interestingly, a high carb diet also uses up a lot of thyroid hormone. You’ve written about avoiding bread, rice, pasta in your diet. On the thyroid boards, a lot of folks post about being carb-intolerant.

  9. CTB, my thyroid levels are normal. Now it doesn’t sound plausible. What you describe might explain why a cold shower made me slower, but not why a hot shower made me faster. Whereas I suspect the two effects have the same explanation. And I don’t think either effect is caused by poor health or malnutrition.

  10. So, if you do a two-sample t-test on the strong-cold versus the strong-warm group you get a p-value of .037, so there is strong evidence at least that the samples have different means. The linear model explains about 35% of the variation for the cold-showers and 69% for the warm.

    But, my problem with treating it as very good evidence is that essentially you’re using time to explain the difference in rt, and assuming that the groups are different at the point you switched from very cold to very warm showers, and that the only inconstant variable is the shower temp. You could easily replace the very cold with “shortening days” and very warm with “lengthening days” (the solstice was Dec 21st) and you’d have just as plausible a relationship. Actually, now that I think about it, that would better account for the appearance of the cold-shower effect being cumulative while applied, and fading-out when not.

    I’m only skeptical because I always take cold-showers outside of Dec-Feb, and
    can’t say I’ve ever noted any differences weight, cognitive or other after about 20 minutes
    of stepping outside the shower.

    Thanks for sending the data. Sorry for being annoying. I enjoy the blog.

    R output on pastebin (t1 is the warm group and t2 is the cold):

  11. why is a t-test not appropriate? It seems obvious the slopes are significantly different, but the sample is small, so a difference of means test seems more robust. asking out ignorance, not disagreement.

  12. Seth can you give me some suggestions about how to design an experiment on becoming more tolerant to hot weather?

    I don’t know if anyone has done this before and I don’t know the proper place for me to post this question.

    Please email me back if you can at

    I’d love to know where I can start experimenting on this idea as I’m going to go to Thailand in September where the heat and humidity is very bad. However, the thais are quite able to tolerate the heat and I would like to design an experiment to see if a 4NR can adapt to the heat faster.

    1. If you can figure out a way to measure heat tolerance, you might get somewhere. I don’t know how to do that. That is the first step: find a way to measure your tolerance to heat.

  13. I like the fact you carried out the experiment but the conclusion is quite simple in my eyes. When cold water hits the skin vasoconstriction is the body’s way of reducing heat loss which in turn reduces blood flow. This will cause a reduced blood flow to the brain meaning less glucose and oxygen which in turn lowers brain speed. When your body gets used to this response in regular cold showers the body will eventually use this response at the hands of a stressor. With hot showers vasodilation occurs meaning a higher glucose and oxygen level available for the brain.

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