I have noticed several times that my speed of thought is significantly reduced on days after doing a lot of (very tiring) sports. I notice this since I'm working on my master's thesis, thus having to think about a lot of complex stuff all day. On those days, I sometimes find myself simply not capable of solving problems that normally don't really challenge me. Apart of this reduced thinking capability, I feel ok; I don't have a cold or something, and even my muscles are ok due to proper stretching and massage.

For completeness: I do a lot of sports (at least 4-5 times a week) and would consider myself to be quite healthy. I typically sleep about 8 hours after such tiring days, and I eat healthy.

I'm interested in explanations as to why this is the case, as well as hints on how I might prevent this.

  • 1
    Are you eating enough to replenish your carbs?
    – Tyler
    Commented Feb 2, 2015 at 17:07
  • Probably not. I'm talking about whole-day badminton tournaments here, so sometimes 10 hours of intensive physical activity with breaks of maybe half an hour between matches. I can't have large meals between the matches because it would reduce my performance in the following match, so I try to get along with small snacks (fruits, nuts, energy bars, etc.) all day. I would have to eat really excessive amounts in the evening in order to compensate for all this activity. Do you think this may be related?
    – Eike P.
    Commented Feb 2, 2015 at 17:22
  • Just to be clear: I do eat a lot, I just guess it's not enough to compensate for what I burn during the day.
    – Eike P.
    Commented Feb 2, 2015 at 17:30

1 Answer 1


I read Thinking, Fast and Slow last year and it touched on several things you seem to be experiencing. The author is Nobel Prize winning psychologist, and the book is thorough in its citing of studies and empirical data.

Fundamentally it came down to the fact that your brain only has so much energy. Pain tolerance draws from the same pool as self discipline and complex (slow) thought. And by "same pool" the evidence suggests it's not some nebulous concept but rather directly linked to sugars and components that make up neuro transmitters. Quoting directly from the text:

The evidence is persuasive: activities that impose high demands on System 2 require self-control, and the exertion of self-control is depleting and unpleasant. Unlike cognitive load, ego depletion is at least in part a loss of motivation. After exerting self-control in one task, you do not feel like making an effort in another, although you could do it if you really had to. In several experiments, people were able to resist the effects of ego depletion when given a strong incentive to do so. In contrast, increasing effort is not an option when you must keep six digits in short-term memory while performing a task. Ego depletion is not the same mental state as cognitive busyness.

The most surprising discovery made by Baumeister’s group shows, as he puts it, that the idea of mental energy is more than a mere metaphor. The nervous system consumes more glucose than most other parts of the body, and effortful mental activity appears to be especially expensive in the currency of glucose. When you are actively involved in difficult cognitive reasoning or engaged in a task that requires self-control, your blood glucose level drops. The effect is analogous to a runner who draws down glucose stored in her muscles during a sprint. The bold implication of this idea is that the effects of ego depletion could be undone by ingesting glucose, and Baumeister and his colleagues have confirmed this hypothesis n ohypothesiin several experiments.

Several studies showed that physical exertion lead to lower math scores (when the tests quickly followed the exertion), and likewise difficult cognitive challenges resulted in lower pain tolerance and physical capacity.

There's a lot of research to show that exercise increases cognitive function, but those studies tend to be macro-scale, looking at longer term results. In the short term, I think any athlete can attest to the fact that during physical exertion you simply cannot think as much. Personally I actually find it difficult to do the math on barbell plates when I'm lifting heavy, and frequently have to write down simple addition problems that wouldn't be an issue at all during non-strenuous times.

  • Great answer, thanks a lot! I take it thus that a valid strategy to prevent or at least diminish this effect would really be consuming a lot (as in: more than I do now) of carbs after such intense days?
    – Eike P.
    Commented Feb 3, 2015 at 11:42
  • Besides, I really need to get that book...
    – Eike P.
    Commented Feb 3, 2015 at 11:59

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