How much energy does it take to think?
53 comments
·June 6, 2025mellow_observer
stevesimmons
At the risk of asking a dumb question, what really is mental fatigue and do all people experience it the same way?
On one hand, I understand -- and feel very directly -- physical fatigue, and the metabolic limitations if I try to say run slowly versus push hard up to my lactate threshold. I am currently training for a marathon, and know to train by following progressively heavier loads of long distance runs, interval training, stretches and rest periods to develop my speed and endurance.
But mental fatigue really just isn't a phenomenon that I personally relate to. I know some people say they can perhaps work 4-6 focused hours in a work day, and that's it. Whereas my brain seems to be able to work at essentially the same intensity for as long as I want it to, up to 18 hours a day, and then I need a bit of sleep to recover. So I don't quite comprehend mental fatigue, or what a cure for it would be. I don't even know how I would increase my ability to avoid mental fatigue other than minimising distractions (like HN!) and just keep thinking more for longer.
How do other people here experience mental fatigue (or not)?
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jakub_g
For me the mental fatigue is mostly related to context switching, being disturbed (slacks, meetings etc but also side quests to the original problem), and working on a fuzzy problem.
Working a few hours in such environment is very fatiguing.
On the other hand, when I work on a single thing, no disturbance, clear problem definition, having all necessary skills to do the thing, I can work 10h and it's not fatiguing.
2wrist
Mental fatigue is something that can manifest in different ways.
To keep it short, for me, It is like I can think down a path, but slowly, it is like I have this plodding speed, if I try to think 'quicker' (or more reactive/agile) it feels like a lot of effort, like I have to focus and push myself. The more effort I apply the more energy I use. The more energy I use the longer this state lasts for. The longer this state lasts for the more chance I develop physical issues. When I am in this state, I can't mentally fit pieces together. It is like I am wearing oven mits and trying to build lego. It just doesn't fit together. oh and I get really clumsy, my movement becomes really uncoordinated.
So it is like I have a smaller pool of energy, and I can spend it slowly over a longer period. Or faster over a shorter period. When I go over my limits, then see above.
The only cure, is rest, and that is usually about 3 days of not pushing myself mentally too hard, to get back to a reasonable baseline. It is improving, if we had had this conversation three years ago...
I have seen this in other devs, a friend of mine has MS and she needs to meter her energy levels like this. My neighbour came out of hospital after a serious illness and she has some of these symptoms. It is more common than you would think.
Kaijo
It depends on the kind of work. If it's routine stuff, past seven hours or so, I can keep going and not feel tired, but I increasingly don't want to, and the feeling that I'd rather be doing something else becomes very distracting. If the work is technical and intellectually rewarding, I might feel inspired to continue, but I start making mistakes and past a certain point, it becomes counterproductive. If the work requires conceptual or creative insights, my brain stops delivering them for free and my backup methods for squeezing them out start failing too. If I'm speaking or writing, I lose the thread and my words lose their punch and personality. My occasional bouts of insomnia bring a different kind of all-encompassing fatigue. I become overemotional. At my most sleep-deprived, I struggle to operate a kettle. Things were different when I was younger. Writing up my PhD, I essentially slept every other night for months, yet stayed sharp, productive.
gavinray
Mental fatigue is when your brain is tired, but your body is not.
In other words, you don't want to sleep or lie down, you want to stop thinking.
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winternewt
My layman observation from tons of pop sci consumption is that the rate of metabolism seems to correlate greatly with life expectancy. Running at 100% all the time would likely wear the system out much sooner, e.g. due to higher oxidation and increased cancer risk.
krzat
Cocaine?
card_zero
If you could come up with a ray-gun that give cokeheads mental fatigue, you'd make the world a more peaceful place.
username135
its a helluva drug
alex_duf
I'd be concerned about the side effects. Sounds like burning the candle by both ends
piva00
Perhaps fatigue is not limited by energy but by the byproducts of energy usage to do work. It becomes much more complex to tackle if that's the case, instead of finding ways to allow the brain to use more energy when available we would have to mess around with the cleanup processes which are much less well understood.
I don't think we will have a way to "cure" mental fatigue until we more completely understand both the mechanisms behind thinking, as well as resting, and at the moment we barely scratched the surface of them.
pmayrgundter
Main points are that brain takes ~20% of body's Base Metabolic Rate, and that active thinking takes 5% more than BMR.
Whole body context: - Base Metabolic Rate (awake): ~1.0-1.1 kcal/kg/hr - Deep Sleep Metabolic Rate: 0.8-0.9 kcal/kg/hr
CG says brain follows this as well.
So it suggests brain power use varies from 0.8 minimum to 1.1, with an extra 0.05 for thinking. This supports ideas that thinking is a relatively minor brain function, at least energetically.
rkagerer
...the majority of the brain’s function goes to maintenance
Makes sense. Same for large software projects, my boat, house, etc.
malux85
Yeah we're fighting the universe right, entropy.
jonplackett
> Theoretically, the top speed for a neuron to feasibly fire and send information to its neighbor is 500 hertz. However, if neurons actually fired at 500 hertz, the system would become completely overwhelmed.
> Our neurons, however, have an average firing rate of 4 hertz, 50 to 60 times less than what is optimal for information transmission.
Could this explain the time dilation you can get when under high stress - is this your brain just firing as fast as it can?
memming
This is a vast simplification. Every neuron has a different preferred baseline firing rate. Not all neurons can fire at 500 Hz. In fact, most neurons in neocortex fire at a much lower firing rate. (most neurons are in the cerebellum!)
As for the internal sense of time, there isn't a consensus as to how it is kept, but for the cognitive time scale, it seems that it is a distributed time keeping mechanism rather than some sort of central unified clock mechanism.
gleenn
Anecdata, but I remember writing software for a startup and getting thirsty. We had a fridge that had all sugar-free sodas because one of the founders was diabetic. I remember drinking a soda and still feeling really thirsty. Drank another and still didn't solve it. It finally dawned on me that my brain just really wanted the sugar for the easy calories, and the sugar-free soda obviously wasn't fixing this at all. Thinking hard definitely burns calories faster than not.
chvid
Maybe start by asking what does it mean to think?
From the article: "Jamadar’s analysis showed that a brain performing active tasks consumes just 5% more energy compared to a resting brain. When we are engaged in an effortful, goal-directed task, such as studying a bus schedule in a new city, neuronal firing rates increase in the relevant brain regions or networks — in that example, visual and language processing regions. This accounts for that extra 5%; the remaining 95% goes to the brain’s base metabolic load."
To me it is fairly obvious that those tasks are not what creates the highest loads on the brain. The "thinking load" from active, in-person, social interactions is much higher.
bmacho
Fun fact: when it is cold, and you need more heat inside you, it doesn't take any energy at all. In fact nothing takes energy, except physical work.
The same is true for households, if it's winter, and you have the heating on, washing clothes, cooking, mining bitcoin, they are all free, since every Joule metered in the electricity meter converts to heat.
fside
If our brains are energy misers, maybe the real supercomputers are the ones that can do more with less—not the ones with the most flops. This could reframe how we design efficient algorithms and even AI: sometimes, the best strategy isn’t processing power, but predictive efficiency.
Maybe the next breakthrough in cloud computing isn’t more cores or larger GPUs, but better energy allocation and anticipation, just like the brain.
wvh
Isn't that what is happening on the whole, going from soccer field energy guzzling hardware to laptop to mobile and server farm processors like ARM that are reasonably energy-efficient? There's only that much energy you can squeeze into a small space, so efficiency becomes a bottleneck.
What good is having all data and knowledge somewhere else than in your pocket when and where you need it, so having computing devices in form factors convenient for human beings must be a major driving factor.
Remains to be seen if everything will still revolve around data centres or if devices will start talking to each other in the future, which might be a more democratic way to go.
Perz1val
Is there a drug that can overclock the brain from 4Hz to like a 100Hz? Since up to 250Hz is fine, that's well within, right?. Sure that'd make me burn at least 100/4*20%=5 as many calories (probably in sugar), but that's something I could provide with soda and cookies for a day.
mike_hearn
Not sure why you're being voted down. That's a very sensible and obvious line of thought given the claims in the article. If it's true that our brains are optimized for very low calorie environments but could work much faster, then if and only if there aren't other good reasons to run at low clock rates, "overclocking" the brain could unleash enormous new levels of human intelligence.
Obviously, there are a huge range of possible problems there. Overclocking CPUs is dangerous for heat reasons and the brain generates a lot of heat. Without a doubt, lots of things in the body and mind are evolved around the assumption of a 5-10Hz clock rate. But even just a doubling, or an increase in the efficiency of neuronal transmission ... well, the mind boggles even just trying to imagine what could be done if you can optimize neuronal transmission.
voidUpdate
Overclocking the brain is called ADHD, and you probably don't want it (I do and its not exactly pleasant)
lukaslalinsky
Anything that can increase the speed of your brain will have negative impact on you overall. Many drugs can make your brain more capable, but at what cost.
debuggerson
What about how much energy is needed to not to think? Some people that struggle with overthinking, the approach of thinking definitely is different from how we think when working on something. So is the energy used is different as well?
xyzzy123
A weird thing I've noticed that I've never seen discussed anywhere is that when kids are concentrating / thinking hard, they start breathing heavily. I wonder what is going on with this.
krzat
Random thought: what LLMs do may be just a tiny fraction of what real brains do, so artificial reasoning is probably a much easier problem than an artifical brain.
solarwindy
Then again...
Our brains only developed their abstract reasoning capabilities after we already possessed a cognitive ‘platform’ for learning in a very physical context. Or rather, our cognitive capabilities for things that may seem totally extraneous to reasoning in fact developed in tandem with it. Think, fine motor control over our fingers and opposable thumbs, and the corresponding development of spatial and physical reasoning and hierarchical planning, that allows us to analyse a problem and build a tool with our hands to solve it.
The ‘bitter lesson’ [0] seems to be that we humans are not very good at designing the algorithmic machinery for cognition—better we let the machine discover its own mechanisms. Take the case of AlphaGo, the performance of which greatly improved when the human data was thrown away.
So, perhaps there is a pathway to artificial reasoning that shortcuts past many functions of an artificial brain, as you put it, but it also looks quite like we’re not ourselves going to be able to architect it, and that reasoning has not emerged from the current LLM paradigm of digesting the written knowledge of all humanity.
Where human general-purpose abstract reasoning naturally arose from concrete, goal-directed interaction with the physical world, perhaps we need to recreate that environment for machines to learn to learn and reason themselves.
That need not necessarily be physically embodied, which would surely be heavily constrained in numbers of learning trials. Quickly searching turned up this [1] example of reinforcement learning to walk in a sim, which is then transferred to physical.
I wonder how far the approach can go... Could agents learn to talk to each other? Looks like there’s some recent research in that direction too [2].
[0]: https://www.cs.utexas.edu/~eunsol/courses/data/bitter_lesson...
[1]: https://www.figure.ai/news/reinforcement-learning-walking
[2]: https://www.ifaamas.org/Proceedings/aamas2024/pdfs/p2725.pdf
Since the brain is optimized for low energy environments, and we have now reached a high energy availability era, it makes you wonder if there's a way to get around the mental fatigue problem somehow. Fatigue is incredibly intellectually debilitating and if we could find a way to be fully on all of the time if we so wish, that should come with a great increase in quality of life.
Now available energy is almost certainly not the only reason we have fatigue, so maybe there's other barriers to overcome, but I'm shocked at how little attention this topic gets. In hackernews spirit, if someone could sell a real cure for mental fatigue, you'd change the world