What gets fatigued when your brain gets tired?
October 31, 2010 4:18 PM   Subscribe

When you read or think a lot or otherwise concentrate, and you get fatigued or sleepy, what exactly is happening biologically? Is it a matter of blood sugar depletion? Something else?
posted by shivohum to Health & Fitness (14 answers total) 17 users marked this as a favorite
I think fatigue would be a sign of liver glycogen depletion (the brain doesn't store its own fuel), but sleepiness would just be a sign of either time-for-bed, or possibly sleep deprivation.
posted by facetious at 4:22 PM on October 31, 2010 [1 favorite]

This happens to me. My doctor said it is due to low blood sugar.
posted by wandering_not_lost at 4:25 PM on October 31, 2010

There's a hormone called Melatonin which is secreted by the pineal gland, which makes you sleepy.
posted by Chocolate Pickle at 5:10 PM on October 31, 2010

Does it occur in the morning when you are well rested (solid 8 for a week or two), well nourished and in sunlight? If so I'd suggest discussing it with a doctor. Late in the evening, well sleep.
posted by sammyo at 5:38 PM on October 31, 2010

Not that I have a suggestion, but merely to chime in: This exact thing happens to me as well. Say I want to brush up on a programming language, or try something that I've been meaning to investigate for a while, and the minute I start to read or concentrate, I start yawning. There are times that I can power through it and get useful stuff done, other times I just have to give up.

No specific time trigger ( morning vs evening, say ), nor would I say that I was particularly more or less rested than usual. I had always assumed it was psychosomatic : my brain's way of trying to get out of doing anything useful.

Next time it occurs, I'll eat a candy bar and see what happens. :-)
posted by HannoverFist at 5:57 PM on October 31, 2010

My experience is that it can be psychsomatic: ie, getting sleepy as an avoidance mechanism. But I think it can also be physical; sustained mental energy, as required by, say, standing and giving a formal lecture, sometimes makes me *exhausted* afterwards. I think it's the level of focus required, since casually talking on the phone wouldn't tire me out, even if I did most of the talking and stayed on the line for much longer.
posted by pseudostrabismus at 6:05 PM on October 31, 2010

There's a lot of evidence that sleep enhances memory retention. That is, if you're learning something new, then sleeping after you study the subject can improve your ability to remember what you learned.

Now at this point I'm speculating, but it seems possible that feeling sleepy after reading or learning is a natural response to aid memorization.
posted by fremen at 6:09 PM on October 31, 2010

I'm actually not talking about it as some kind of abnormal symptom -- I just want to know what garden-variety mental fatigue means.

I mean, doesn't everyone get mentally tired at some point after strenuous bouts of concentration? After taking some long exam haven't you been tired? When trying to plan something complex out, or solve some problem, or engage in some creative task, isn't there a point when focusing becomes harder? What does that mean in the body? That's my question.
posted by shivohum at 6:44 PM on October 31, 2010

I get exhausted from mental exertion all the time. (I don't know what that says about me...)

I have no idea what is happening but I can tell you I sleep better from physical tiredness than mental. A day spent thesis writing would result in a combination of emotional and mental exhaustion + some tired burning eyes and usually a headache. But obviously I couldn't go straight to sleep. I'd need a good hour of walking and some normal human conversation beofre I could sleep, and even then the odds of Cthulhu dreams were high.

my 2 cents.
posted by ServSci at 7:04 PM on October 31, 2010 [2 favorites]

I think it's really simple: you've got to take a break, get up, stretch a bit and get your blood moving and get some more oxygen into your brain. At work sometimes I just get up and go get a glass of water for the sake of moving around for a minute, and the rehydration doesn't hurt either.

Of course caffeine can also help.
posted by zadcat at 7:08 PM on October 31, 2010 [1 favorite]

I read somewhere that your brain uses more glucogen (blood sugar) than any other organ. Use your brain hard, and it needs more fuel. It takes that from your blood stream, which makes you tired.

Whenever I have to do something that demands a lot of brain work, I lay in a supply of small sweets. A few Jolly Ranchers, or a glass of fruit juice will top up your blood sugar nicely.

(Or maybe it's just a placebo. But hey, candy!)
posted by ErikaB at 7:18 PM on October 31, 2010 [1 favorite]

Recent evidence from Baumeister and colleagues suggests that self control is a limited resource, and that blood glucose may be this resource (or at least is very closely related).

The ways that they have assessed self control include tasks such as "look at this plate of cookies" but also tasks where you have to exert conscious control over your mental processes, such as the Stroop Task.

A more recent study (last month) by Job, Dweck and Walton suggests that willpower tasks deplete a limited resource... but only when you believe that willpower is a limited resource. Dweck also has other interesting studies suggesting that if you think intelligence is flexible... you get smarter (than those who think it is fixed).

Other findings that might be sort of related:
REM sleep seems to be more important for psychological effects (learning and memory) whereas Stage 3 and 4 seem to be more important for physical rejuvenation.

But ultimately, I don't think you are going to find a definitive answer (it's your dopamine! or your neurons just need to recharge their potassium ion channels!). Sometimes in psychology, it is best to just leave the level of analysis at "you feel tired" rather than assume this feeling has a distinct biological correlate.
posted by cogpsychprof at 7:30 PM on October 31, 2010 [7 favorites]

This is not a simple question to answer. Mental fatigue is a subjective/behavioral response to many different parallel and interrelated physical processes. For now, I will address one main part of your question.

Low blood sugar can cause fatigue. I would guess that this is mediated either directly, in that a drop in intracellular ATP triggers inhibitory feedback on neuronal firing, or indirectly, in that glucose-sensing neurons in the hypothalamus modulate activity in the brain as a whole in response to a drop in blood sugar. In any case, fatigue in response to hypoglycemia is real enough.

The question is whether hypoglycemia can result from sustained concentration. I would think not. While it's true that your brain is the most metabolically demanding organ in your body (it represents about 2% of your body weight yet uses about 20% of the calories you consume), as I understand it the vast majority of that energy is consumed via the maintenance of membrane potentials. The variations in potential that occur during depolarization is a relatively small percentage of the membrane potential as a whole for a given neuron. In other words, I don't believe an active neuron uses substantially more energy than an idle one, and in fact even "idle" neurons still fire randomly on a regular basis. There's also an implicit assumption here that sustained mental concentration must require more effort for your brain simply because it seems to require more effort for your mind. Although this is a very tempting assumption, I honestly don't think there's much beyond intuition to support it. The fact is that your brain is always active, even when you're daydreaming.

In short, I don't think that concentrating or thinking hard will necessarily burn that much more glucose than twiddling your thumbs. But there are many other ways independent of blood sugar levels that fatigue can occur. I may come back and expand a little more on that later.
posted by dephlogisticated at 8:41 PM on October 31, 2010 [3 favorites]

Um yeah, I meant glucose, not glucogen. Is "glucogen" even a word? I don't know. I plead "having a degree in English Lit instead of something useful."

Anyway, the same thought occurred to me that dephlogisticated mentions. DOES your brain actually work harder when you're learning stuff?

According to Popular Science the answer is yes.

I also found an article from Mind Performance Hacks which indicates that short-term memory can be improved by increasing your blood glucose levels. [PDF]
posted by ErikaB at 2:11 PM on November 1, 2010 [1 favorite]

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