[Note: I’ve started a research side project on this question, and it is already obvious to me that this ontology importantly wrong.]

There’s a common phenomenology of “mental energy”. For instance, if I spend a couple of hours thinking hard (maybe doing math), I find it harder to do more mental work afterwards. My thinking may be slower and less productive. And I feel tired, or drained, (mentally, instead of physically).

Mental energy is one of the primary resources that one has to allocate, in doing productive work. In almost all cases, humans have less mental energy than they have time, and therefore effective productivity is a matter of energy management, more than time management. If we want to maximize personal effectiveness, mental energy seems like an extremely important domain to understand. So what is it?

The naive story is that mental energy is an actual energy resource that one expends and then needs to recoup. That is, when one is doing cognitive work, they are burning calories, depleting their bodies energy stores. As they use energy, they have less fuel to burn.

My current understanding is that this story is not physiologically realistic. Thinking hard does consume more of the body’s energy than baseline, but not that much more. And we experience mental fatigue long before we even get close to depleting our calorie stores. It isn’t literal energy that is being consumed. [The Psychology of Fatigue pg.27]

So if not that, what is going on here?

A few hypotheses:

(The first few, are all of a cluster, so I labeled them 1a, 1b, 1c, etc.)

Hypothesis 1a: Mental fatigue is a natural control system that redirects our attention to our other goals.

The explanation that I’ve heard most frequently in recent years (since it became obvious that much of the literature on ego-depletion was off the mark), is the following:

A human mind is composed of a bunch of subsystems that are all pushing for different goals. For a period of time, one of these goal threads might be dominant. For instance, if I spend a few hours doing math, this means that my other goals are temporarily suppressed or on hold: I’m not spending that time seeking a mate, or practicing the piano, or hanging out with friends.

In order to prevent those goals from being neglected entirely, your mind has a natural control system that prevents you from focusing your attention on any one thing at a time: the longer you put your attention on something, the greater the build up of mental fatigue, causing you to do anything else.

Comments and model-predictions: This hypothesis, as stated, seems implausible to me. For one thing, it seems to suggest that that all actives would be equally mentally taxing, which is empirically false: spending several hours doing math is mentally fatiguing, but spending the same amount of time watching TV is not.

This might still be salvaged if we offer some currency other than energy that is being preserved: something like “forceful computations”. But again, it doesn’t seem obvious why the computations of doing math would be more costly than those for watching TV.

Similarly, this model suggests that “a change is as good as a break”: if you switch to a new task, you should be back to full mental energy, until you become fatigued for that task as well.

Hypothesis 1b: Mental fatigue is the phenomenological representation of the loss of support for the winning coalition.

A variation on this hypothesis would be to model the mind as a collection of subsystems. At any given time, there is only one action sequence active, but that action sequence is determined by continuous “voting” by various subsystems.

Overtime, these subsystems get fed up with their goals not being met, and “withdraw support” for the current activity. This manifests as increasing mental fatigue. (Perhaps your thoughts get progressively less effective, because they are interrupted, on the scale of micro-seconds, by bids to think something else).

Comments and model-predictions: This seems like it might suggest that if all of the subsystems have high trust that their goals will be met, that math (or any other cognitively demanding task) would cease to be mentally taxing. Is that the case? (Does doing math mentally exhaust Critch?)

This does have the nice virtue of explaining burnout: when some subset of needs are not satisfied for a long period, the relevant subsystems pull their support for all actions, until those needs are met.

[Is burnout a good paradigm case for studying mental energy in general?]

Hypothesis 1c: The same as 1a or 1b, but some mental operations are painful for some reason.

To answer my question above, one reason why math might be more mentally taxing than watching TV, is that doing math is painful.

If the process of doing math is painful on the micro-level, then even if all of the other needs are met, there is still a fundamental conflict between the subsystem that is aiming to acquire math knowledge, and the subsystem that is trying to avoid micro-pain on the micro-level.

As you keep doing math, the micro pain part votes more and more strongly against doing math, or the overall system biases away from the current activity, and you run out of mental energy.

Comments and model-predictions: This seems plausible for the activity of doing math, which involves many moments of frustration, which might be meaningfully micro-painful. But it seems less consistent with activities like writing, which phenomenologically feel non-painful. This leads to hypothesis 1d…

Hypothesis 1d: The same as 1c, but the key micro-pain is that of processing ambiguity second to second

Maybe the pain comes from many moments of processing ambiguity, which is definitely a thing that is happening in the context of writing. (I’ll sometimes notice myself try to flinch to something easier when I’m not sure which sentence to write.) It seems plausible that mentally taxing activities are taxing to the extent that they involve processing ambiguity, and doing a search for the best template to apply.

Hypothesis 1e: Mental fatigue is the penalty incurred for top down direction of attention.

Maybe consciously deciding to do things is importantly different from the “natural” allocation of cognitive resources. That is, your mind is set up such that the conscious, System 2, long term planning, metacognitive system, doesn’t have free rein. It has a limited budget of “mental energy”, which measures how long it is allowed to call the shots before the visceral, system 1, immediate gratification systems take over again.

Maybe this is an evolutionary adaption? For the monkeys that had “really good” plans for how to achieve their goals, never panned out for them. The monkeys that were impulsive some of the time, actually did better at the reproduction game?

(If this is the case, can the rest of the mind learn to trust S2 more, and thereby offer it a bigger mental energy budget?)

This hypothesis does seem consistent with my observation that rest days are rejuvenating, even when I spend my rest day working on cognitively demanding side projects.

Hypothesis 2: Mental fatigue is the result of the brain temporarily reaching knowledge saturation.

When learning a motor task, there are several phases in which skill improvement occurs. The first, unsurprisingly, is durring practice sessions. However, one also sees automatic improvements in skill in the hours after practice [actually this part is disputed] and following a sleep period (academic link1, 2, 3). That is, there is a period of consolidation following a practice session. This period of consolidation probably involves the literal strengthening of neural connections, and encoding other brain patterns that take more than a few seconds to set.

I speculate, that your brain may reach a saturation point: more practice, more information input, becomes increasingly less effective, because you need to dedicate cognitive resources to consolidation. [Note that this is supposing that there is some tradeoff between consolidation activity and input activity, as opposed to a setup where both can occur simultaneously (does anyone have evidence for such a tradeoff?)].

If so, maybe cognitive fatigue is the phenomenology of needing to extract one’s self from a practice / execution regime, so that your brain can do post-processing and consolidation on what you’ve already done and learned.

Comments and model-predictions: This seems to suggest that all cognitively taxing tasks are learning tasks, or at least tasks in which one is encoding new neural patterns. This seems plausible, at least.

It also seems to naively imply that an activity will become less mentally taxing as you gain expertise with it, and progress along the learning curve. There is (presumably) much more information to process and consolidate in your first hour of doing math than in your 500th.

Hypothesis 3: Mental fatigue is a control system that prevents some kind of damage to the mind or body.

One reason why physical fatigue is useful is that it prevents damage to your body. Getting tired after running for a bit, stops you for running all out for 30 hours at a time, and eroding your fascia.

By simple analogy to physical fatigue, we might guess that mental fatigue is a response to vigorous mental activity that is adaptive in that it prevents us from hurting ourselves.

I have no idea what kind of damage might be caused by thinking too hard.

I note that mania and hypomania involve apparently limitless mental energy reserves, and I think that theses states are bad for your brain.

Hypothesis 4: Mental fatigue is a buffer overflow of peripheral awareness.

Another speculative hypothesis: Human minds have a working memory: a limit of ~4 concepts, or chunks, that can be “activated”, or operated upon in focal attention, at one time. But meditators, at least, also talk a peripheral awareness: a sort of halo of concepts and sense impressions that are “loaded up”, or “near by”, or cognitively available, or “on the fringes of awareness”. These are all the ideas that are “at hand” to your thinking. [Note: is peripheral awareness, as the meditators talk about,  the same thing as “short term memory”?]

Perhaps if there is a functional limit to the amount of content that can be held in working memory, there is a similar, if larger, limit to how much content can be held in peripheral awareness. As you engage with a task, more and more mental content is loaded up, or added to peripheral awareness, where it both influences your focal thought process, and/or is available to be operated on directly in working memory. As you continue the task, and more and more content gets added to peripheral awareness, you begin to overflow its capacity. It gets harder and harder to think, because peripheral awareness is overflowing. Your mind needs space to re-ontologize: to chunk pieces together, so that it can all fit in the same mental space. Perhaps this is what mental fatigue is.

Comments and model-predictions: This does give a nice clear account of why sleep replenishes mental energy (it both causes re-ontologizing, and clears the cache), though perhaps this does not provide evidence over most of the other hypotheses listed here.

Other notes about mental energy:

• In this post, I’m mostly talking about mental energy on the scale of hours. But there is also a similar phenomenon on the scale of days (the rejuvenation one feels after rest days) and on the scale of months (burnout and such). Are these the same basic phenomenon on different timescales?
• On the scale of days, I find that my subjective rest-o-meter is charged up if I take a rest day, even if I spend that rest day working on fairly cognitively intensive side projects.
  • This might be because there’s a kind of new project energy, or new project optimism?
• Mania and hypomania entail limitless mental energy.
• People seem to be able to play video games for hours and hours without depleting mental energy. Does this include problem solving games, or puzzle games?
  • Also, just because they can play indefinitely does not mean that their performance doesn’t drop. Does performance drop, across hours of playing, say, snakebird?
• For that matter, does performance decline on a task correlate with the phenomenological “running out of energy”? Maybe those are separate systems.