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'Protection' might be the goal of physical fatigue, but it doesn't work for intellectual effort - what are you being 'protected from breaking' by not being able to write another line of code? Carpal tunnel? In that case, it looks more like it's about conserving energy or rotating to another task after having spent so much time on one task: "A Meta-Analysis of Blood Glucose Effects on Human Decision Making" http://www.gwern.net/docs/2016-orquin.pdf , Orquin & Kurzban 2016 "An opportunity cost model of subjective effort and task performance" https://www.ncbi.nlm.nih.gov/pmc/articles/PMC3856320/ , Kurzban et al 2013

What about mental fatigue? The more you workout your brain the more it needs to rest. How would you like your brain to tell you that it needs to rest other than feeling fatigue?

I find myself consistently productive over long spans without a burnout effect when I am sleeping at consistent hours, and working for consistent hours a day. When I overwork my productivity becomes inconsistent so I stopped doing that. I learnt it the hard way that late night coding is a short-term investment with negative return in the long run.

My personal theory is that our brains like consistency and adapt/tune to it over time. Just like when you jog every day, if you run for too long one day and too short the other day it cannot adapt/tune itself to a semi-random pattern.

There are two reasons for mental fatigue as best as is known/we can infer. The first is stress from the task (by which I mean the immediate thing that is correlated with cortisol levels). The second is that maintaining attention on a task, where there is not much reward or feedback on the task's progress makes maintaining attention harder and harder to do as time progresses. Imagine a very heavy door that wants to swing back shut the longer you hold it open. Resisting this too is linked with stress inducing mechanisms.

A good theory should explain why watching a television serial, a very complex task involving long term memory retrieval, short term memory of details, inferring various motivations, theory of mind (on a meta level too via genre savviness), agent modeling and prediction, language, visual, audio is less taxing than solving 1000 simple arithmetic problems. The resource management and attention based theories have better explanations for why that may be.

I find I can be highly productive with a highly inconsistent sleep schedule as long as I get significant amounts of sleep every few days. I don't think a consistent schedule is a requirement.

I heard different story: there is no such thing as "mental fatigue". The reason why we get tired when doing office work is:

- our muscles need to support our body while sitting. - most mental work includes some dose of stress, which is a real reason of fatigue.

Buy yourself best chair you can find plus do stress-free, enjoyable work and you can go 16 hours straight, without fatigue. At least that's how I felt about all-weekend-long Quake matches back then when I was living in dormitory.

I think it depends on the task. Playing Quake doesn't require significant amounts of creativity or learning, in the same way that programming or learning a new language would. We know that the brain needs sleep in order to process new memories, so that may be one factor in mental fatigue.

This is anecdotal but I have often noticed that I become more thoroughly fatigued and thus need more sleep when working on really hard programming problems vs less mentally taxing tasks. This implies that it's more than a physical phenomenon.

Anecdotally, more difficult mental work causes me to mentally fatigue faster which feels like good evidence against the claim

Dude i get fatigued by games, u have a point, but there is just less fatigue not 0 fatigue.

Why does your brain need to rest? What exactly 'builds up' and why don't regular tasks like the visual perception of the world cause this? And so on.

More specifically, mental fatigue is a result of dissonance. Working on your dissonance builds a strong and quiet mind.

Perhaps from bad decisions? As I stay awake, the quality of my code drops pretty fast, social decisions get worse, I say the wrong thing to people, et cetera.

This is question-begging. 'We get fatigued because we get fatigued.'

That's conflating two different senses of fatigue. "We feel tired to prevent us from making mistakes when we have been awake a long time."

That separation of the different senses of fatigue is the core of the article. Fatigue is both a feeling and a state, which we confuse because they are connected. But you can give someone drugs, like diphenhydramine or caffeine, which change the feeling of fatigue without changing the underlying state which that feeling normally reflects.

There could be another, simpler explanation: the _mechanism_ used to enforce protection has side effects that inhibit intellectual effort.

Well, maybe for the same reason we need to sleep every day. The brain also experiences fatigue.

"The brain makes up 2% of a person's weight. Despite this, even at rest, the brain consumes 20% of the body's energy. The brain consumes energy at 10 times the rate of the rest of the body per gram of tissue. The average power consumption of a typical adult is 100 Watts and the brain consumes 20% of this making the power of the brain 20 W."


The resting metabolism is very high, because the brain is expensive to build and run. But it's not much more expensive to run on one task rather than another. This is one of the major flaws in the blood glucose theory of willpower as discussed by Kurzban: some additional sugar can't be 'gasoline' for the brain because the brain doesn't use up that much more energy working on something than not working on something. (As one might guess from the basal rate of 20%! Not much room left. Or from thermodynamics: when you walk or do physical exercise, you can feel your body heat up from how much energy you are burning. Your head doesn't heat up while doing arithmetic.)

This assumes that fatigue is merely a thermodynamics problem. Fatigue could also be caused by the depletion of chemicals which take time to regenerate. Such effects would not be captured when looking at energy usage alone.

You assume merely logical work in the pre-frontal cortex. As soon as emotions come to play the body get's involved because heart-rate, body-temperature etc. is influenced.

And I don't mean emotions like anger, but less visible ones like disgust, which seem to be triggered during procrastinating.

Similar to the opportunity cost model, Minsky proposed that boredom with tedious tasks is a guard against getting stuck in an infinite loop: http://aurellem.org/society-of-mind/som-6.13.html

It might be protecting you from becoming so brain-fuzzed you can't respond appropriately to "legitimate" threats. Conserving spare capacity for when the predator shows up.

Having taken neuroscience courses for interest in the subject, the holes in our knowledge far exceed the knowledge - even if the introductory book is over a thousand pages (but that's how it is in most fields these days, and yet you only learn basics).

For example, only very recently we found that the brain literally cleans itself (by flushing) during sleep (http://www.ncbi.nlm.nih.gov/pmc/articles/PMC3880190/). You can bet your Tesla (if you have one) there are lots of mechanisms for physically cleaning up and regeneration that we have no idea yet that they even exist. It's also recent that we find that the various glial cell types in the brain actually help in computation and are not just "helpers" you can leave out when considering brain activity.

I know for myself that when I lie down even when I am not tired (and I rarely am during the day) my brain after a while starts doing some really weird stuff. It's literally like day-dreaming. I have no conscious control, I just let it flow - but I'm not asleep, not one bit. That lasts for 5 to 20 minutes. Afterwards I feel incredibly refreshed. Again, there is no sleep involved, not even half-sleep, so it's a very different mechanism.

All processes are physical in the brain in the end: Even electrical activity is performed by ions, not by electrons, after all. So even electricity is physical movement of atoms, unlike in human-made electrical equipment where it's just electrons for the most part (you see the effect when it's ions in batteries, if you open them when they are old, while a copper wire doesn't change even after years of current flow).

In addition, part of electrical activity performed with ions is the transmission to another cell (within the brain mostly other neurons, of course), which is purely chemical through various transmitters. When I first heard about that I thought "couldn't this be optimized in a human-made system to stick to using only electrical signals, it would be sooo much faster". However, I quickly abandoned that thought, those transmission being 100% chemical is a major part of the information processing. Human-made computers show you can do make something "purely electrical" - but then it works following completely different principles. The chemical vector adds a huge flexible component to the system, it enables the majority of what the brain is and can do. But it means that a huge chemical factory has to be maintained: A gigantic amount of molecules needs to be synthesized and broken up continuously, and vast amounts of it. And while you don't need to do much cleanup except for dust in a computer because it's all just electrons, the brain is less electrical than a chemical factory.

Even the electrical activity which uses just simple ions, so they are always there and don't have to be manufactured, uses a lot of energy, because the gradient has to be maintained by physically moving ions back out of the cells to establish the resting membrane potential. That energy comes from ATP, which first has to be produced. In a human-made system we simply provide the electrical field to the entire system from the outside, and the dirty chemical processes have been outsourced to power plants somewhere far away. In the brain you have all that waste-producing activity right in each cell!

Add to that that there is so much activity in the brain, much more than in most other tissue in the body, and you can see a lot of "house-keeping" has to be performed. And that is an area that we still know precious little about. So it seems plausible that occasionally heavily used parts of the brain may want a little rest - during which they don't really rest, they just do cleanup and maintenance.

Fascinating stuff.

What's a relatively recent write-up of this stuff for a non-specialist that doesn't skimp on the details? I'm not asking for Cognitive Neuroscience for Dummies but I'm not asking for "think of the brain like a CPU" either. If there isn't a decent write-up, could you write it? I'd buy a copy ;-)

I'd really be interested in this too.

I've seen the "brain flushing" research, but this was the most in depth I've heard it explained.

Of course, this goes back to the "1000 page introduction" he mentioned.

Brings to mind the claim that we have X hours of focused mental effort we can spend each day. Go beyond that and we start dropping back to fight or flight instincts etc.

However, I don't think that is actually a real theory in neuroscience, it's more a made-up everyday rule. I doubt it is valid, except in the most general kind of way.

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