Mental Energy

Epistemic status: Rough, non-mechanistic, ad hoc model that’s good enough be helpful for me, and which I suspect is roughly true of other people, with a lot of idiosyncratic variation.

Evidence base: Primarily n-of-1 phenomenological observation, supplemented with results from academic psychology

Last major update: 2020-04-30

Previous versions:

My semi-technical term “mental energy” refers to the common folk psychological concept of energy, or energy levels.

As in, if a person feels tired from working hard on a task that doesn’t involve physical labor, or if a person is depressed, and can’t find the motivation to get out of bed, we might say that they are “low energy.” Or if a person is having really good week, and they’re churning through tasks like with ease, we might say that their “energy levels are high”.

Mental energy is a crucial to personal work-efficiency.

A number of “productivity people” (perhaps starting with Jim Loehr and Tony Schwartz in the The Power of Full Engagement), suggest treating energy, not time, as the crucial resource to manage, budget, and allocate, in order to be effective. After all, the number of high energy hours in the day is smaller than the number of hours. So, for most people, energy is their bottleneck resource.

Personally, maintaining high levels of mental energy is a cornerstone of my approach to work-efficiency, because high levels of mental energy enable everything else. When I have ample energy, many tasks that would otherwise be an onerous slog are instead, if not easy, then unhindered. So instead of trying to force myself to do those hard things, I instead / first focus on cultivating my energy levels.

In this document, I’m going to discuss my current theory of subjective mental energy. What it is, how it works, and how to engage with it effectively.

The folk metaphor of “energy”

The term “energy” conveys the naive conception of how mental energy works. Namely, it is a resource that is spent or burnt in order to do work.

It’s like a battery: You start with some of this resource, over the course of the day, you do work of various kinds, which depletes the resource, which causes your performance to drop, and for you to become tired. You rest, which regenerates this resource, until your energy is back to where it started.

The Psychology of Fatigue posits that this metaphor is a consequence of the industrial revolution, and the mechanisms of steam contraptions: The common understanding of mental energy is analogous to that of a machine that runs on fuel, and when it runs out of fuel it stops functioning.

This metaphor is completely wrong.

As near as I can tell, there’s no physical resource that is depleted by mental effort. Yes, the brain runs on calories, but the metabolic difference between effortful thinking and idle day dreaming is nil. And in any case, you never get anywhere near deleting your calorie stores [even local calorie stores?] when thinking hard. [Follow up on this]

Also, it seems that fatigue does not usually impair performance on a task. In many cases, performance is constant, while fatigue increases overtime. The more we work, the more tired we become, independently of whether we can keep working.

It seems that mental energy is almost entirely psychological. It isn’t that you have energy, spend, and run out of energy. Rather, it seems that there there are a number processes that act as regulators on your use of cognitive effort, and their collective action is a limiter on how much cognitive effort you exert in a given time period.

[For more on this see The Psychology of Fatigue]

Semi-Technical Definition

My models of mental energy are based off of phenomenological observation, not on any technical definition. But, precision is useful, so I want to offer a semi-rigorous definition:

Mental energy, is a pseudo-measure of a person’s global propensity to exert cognitive effort, at a given time.

This is a pseudo-measurement, not intended for any actual use. But by way of illustration, I’ll gesture at an incomplete operationalization:

We recruit a mechanical turk (who is used to doing small paid tasks for hours at a time), and give them a series of puzzles of different types, offering to pay him/her a dollar for each successfully solved puzzle. We also randomly mix in a much harder puzzle, which pays out at a higher rate, and which the subject is free to try or skip. The difficulty of this harder puzzles are calibrated so that he/she sometimes skips it and sometimes tries to solve it (instead of always trying, or always skipping. The prices for the harder puzzles are calibrated so that the subject earns somewhat but not hugely more (maybe 10%), solving a harder puzzle, than if they spent the same length of time solving easy puzzles.

Day-to-day, and hour-to-hour, we measure the rate at which the subject tries to solve or skip the harder puzzles. In periods when he/she skips more hard puzzles, we’ll say that he she has low energy. In periods when he/she tries to solve more hard puzzles, we’ll say he has high energy. (We might also measure the extent, frequency, and duration of his/her pupil dilation, to asses how much mental effort he/she is exerting.)

In this “experiment”, we are holding constant the motivational appeal of doing a harder puzzle, since on any given day, the payoff for solving one is the same. (This hardly applies to the real world, where we will feel differently motivated for different tasks.)

We are likewise standardizing the effort needed to solve the harder problem.

So the day-to-day or hour-to-hour fluctuation of the subjects hard-problem-skip rate is intended to be a measure of the degree to which the subject shys away, or not, from cognitive effort.

(I can think of issues in this setup and ones like it, but this is just to gesture at the basic idea.)

More practically, in your own day to day life, you can calibrate your mental energy by considering going to do some task of ~constant difficulty, like sitting down to meditate, or going to exercise. When I contemplate moving in a particular direction like that, I’ll often have an immediate sense of  resistance, or “ugh” (which I might overcome via an exertion of will power or I might not), and othertimes there’s no resistance, the action feels free and available. That difference what I’m pointing at with the term “mental energy.”

I claim that this definition captures what we are referring to, and what we care about, with the folk concept of “mental energy”. When my mental energy is high, I will feel motivated to, for instance, go exercise, but when I feel tired or drained, I will be substantially less motivated.

Or when I’m low energy, I might try to do complicated research, but I find that I keep slipping off, and doing easier, negligibly-valuable research tasks, instead of actually parsing the sentences of the paper I’m trying to read.

Note that this is a behavioral definition. It doesn’t account for how tired or energized a person feels, only their actual cognitive exertion in response to stimuli. I would call that second quality “subjective mental energy.”

The inputs to mental energy

Mental energy can vary from day-to-day and hour-to-hour. Why? What causes that variation? What is happening “under the hood”?

Here I can only give rough models from personal observation. I’ve noticed a number of factors that seems to have an impact on my energy level, but have little more of a model than that.

I currently don’t know the weightings of these factors, or how they interact. As a very rough generalization,  it seems they’re structured in something like an “or” or a “min” function, whereby each factor, when in the “depressive state”, can independently lower mental energy, but all (or approximately all?) of them need to be in the “activation state” for high mental energy.

But this is just a generalization, and some of these factors can certainly override others in some circumstances.

Ideally, I would be able to draw an approximately complete causal diagram, showing which nodes effect others, and which nodes mediate those effects. But this list of factors, will have to do for the time being.

I’m going to dived these factors into two broad classes: physiological factors and psychological factors.

Physiological factors

Health vs. sickness

The first factor is one’s heath status.

This is hardly a new insight: You feel tired when you are sick. Presumably this is because your body’s immune response is mobilizing resources for fighting of the disease, and withholding resources from other behaviors?

[I bet I can find papers on the effect of illness on energy levels]

Being well-slept vs. sleep deprivation

Not having slept tends to reduce one’s energy levels.

[Sleep drive is the phenomenological correlate of adenosine buildup in the basal forebrain. Caffeine competes with adenosine for receptors in that area, which is why caffeine reduces (some of) the effects of sleep deprivation.]

Personally, my mental energy is particularly sensitive to lost sleep, but I know others who seem to be able to function effectively after multiple nights of only a few hours of sleep.

Importantly, the effect of sleep deprivation can be masked, or overcome, by the effects of circadian rhythms or of physiological activation.

Circadian effects

If sleepiness was only controlled by the D-process, we would wake up alert and energized, and get increasingly, monotonically more sleepy over the course of the day. Instead, we usually find that even if we wake up tired and groggy, we usually become alert after and hour or two, and that we often feel most energized in the evening.

The reason for this is that sleepiness/ alertness is controlled by two processes: the D-process, which is slowly building up as the day wears on, and the circadian alerting signal, which (mostly) also goes up over the course of the day, until it “turns of” around bedtime. The net effect is balancing out so that, ideally, you maintain about constant levels of alertness and energy over the course of the day.

This is why, for instance, a person might be sleep deprived, but can feel alert and high-energy, particularly in the late afternoon and evening: the circadian effect is dominating the the effect of the D process.

Similarly, many people experience a dip in mental energy in the mid afternoon, somewhere between 1:00 and 4:00 pm. For poorly understood reasons, there’s a “circadian dip” around that that time, in which the circadian effect is weakened.

Physiological Arousal

Experiments have indicated that the detrimental effects of sleep deprivation on the preference of various tasks can be compensated for by heightened physiological arousal.

For instance,



Cognitive effort always (?) cooccurs with arousal, and anything that increases arousal seems increase mental energy.

I think this is why, for instance, a person might be sleepy and sleep deprived, and tottally unable to work, but if they get news of an urgent emergency, they will suddenly be full of (panicked) energy. (Which is not to say that they won’t still be suffering from the other cognitive effects of sleep deprivation.)

Note: it seems plausible to me that some of the other factors, both physiological and psychological, are mediated by physiological arousal. That is, that they influence energy levels indirectly by influencing level of arousal. I think that this might also be the mechanism by which various stimulants increase alertness and mental energy [verify this?]

Question: Is this also why eating tends to temporarily depress energy levels? Because digestion triggers PSNS over SNS activation?

Activity vs. sedentariness

In my personal experience, regular moderately-intense exercise seems to produce higher baseline energy levels, on the scale of days. I have not tested it with any rigor, but when I haven’t exercised in 3 to 4 days, my energy will falter. But if I do go exercise, it will pick up again pretty much immediately. (For this reason, intense exercise is a good thing to start with, when I’m bootstrapping from an ineffective state.)

Aside from my personal experience, this seems to be common folk wisdom: you’ll feel better and have more energy, if you exercise.

I don’t know why this works. Maybe because of something about endorphins?

I don’t have a very clear sense of what counts as “intense” enough to have his energy boosting effect. This it would be helpful to know, so that I can optimize for the minimum viable dose: I want to spend enough time exercising to maintain my energy levels, and not much more.

One rule of thumb is if you are exercising hard enough that it causes you to ache, at least slightly, either immediately, or the day after. In particular, I find that when I walk enough in a day that my feet and legs are little sore afterwards, that it is sufficient for maintaining my energy, but if I walk for less than that, it doesn’t. Similarly, after a good strength training session, some of my muscle groups will feel at least a little achy, as I am waking up, even if it is unnoticeable over the course of the day.

Psychological factors


[This one is really important, but not yet well defined]

I’m somewhat inclined to call this the “main factor.” For a person who regularly exercises and gets enough sleep, most of the variance in their energy levels will be due to outlook.

I’m not exactly sure how this one is formally defined. I’ve sometimes expressed it as “everything is handled or meta-handled.” That is all parts of you trust that all of your major goals and concerns are currently in an ok state, or in are tracked by some process that will ensure that they get into an ok state. When everything is handled or meta-handled high mental energy results. But each unhanded concern saps some portion of your energy.

But it might be simpler and more parsimonious to define it as optimism: to the extent that you feel like things are going well for, that you expect good things, you have mental energy.

[It’s unclear to me, at this time, if those are just different expressions of the same basic principle, or if they make different experimental predictions.]

The extreme version (on one end of the spectrum) is depression, in which things feel so hopeless that one lacks the energy to even move. Possibly the other end of the spectrum is mania, in which a person might be described as hyper-optomistic (though I admit that “optimism” is not the defining feature of mania. Seeing patterns in noise is the defining feature of maina), and also has “manic energy.”

In any case, some observations:

Very low energy states seem to “contain” “compressed” fear or despair.

When I’m feeling very low, I can often do Focusing and bring up exactly the concern that feels hopeless or threatened.

When I feel into fear “underneath” the low energy state, the fear (which I was ignoring or repressing a moment ago), sort of inverts, and comes blaring into body-awareness as panic or anxiety, and my energy comes back. Usually, from there, I can act from the anxiety, moving to take action on the relevant underhanded concern.

[Example in my logs on April 10]

When I feel into the low energy, and there’s despair underneath, usually the thing thing that needs to happen is move that is like “letting reality in” (this has a visual of a vesicle, with some substance inside of it, which when the membrane is popped defuses and equalizes with the surrounding environment) or grieving. Usually after I do that, my energy returns.

(Notably there seems to be an element of hiding from or ignoring or repressing what’s true in each of these.)

In both cases, it sort of feels like the low energy state is the compacted from of the fear or despair, like snow that has been crushed solid. And then in doing the Focusing, I allow it to decompress.

Expectation of reprieve from stress

Another thing that I think I have noticed (though I’m less confident in this one) is that my energy picks up when I have a reprieve coming up. When I know that I have a rest day soon, I’ll have higher energy than when I don’t have one that I can see coming, even if I ultimately decide that I need one.

This seems like an obvious case of adaptive fatigue: I need a break and I’m becoming tried, as a way to force myself to take a break. But if I’m going to rest tomorrow, then there’s no need to reduce my energy to put on the breaks; there’s a scheduled stop.

Recent effort

Finally, there maybe (I’m not sure about this one) an effect by which mental energy is lower after intense cognitive effort.

I’m a little confused about though, since some evidence suggests that people don’t need breaks when they are engaged in an intrinsically rewarding task.

In any case, for me at least, these low energy periods usually pass within an hour. (Though I did once have a major project, at the completion of which, my energy was so low that I couldn’t even read. The best I could do was just lay on the carpet semi-napping, for a day.)

I wonder if this is just the direct consequence of physiological arousal. i.e. The reason why you can’t maintain cognitive effort indefinitely is that being in an aroused mode is taxing on your body,  and you need to disengage to a PSNS dominate state intermittent to recover.

Unknown idiosyncratic energy-level baseline factors

All of the above factors were derived from observation of my own energy levels, and I don’t know how well it generalizes. I work a lot, and it seems highly likely to me that I am blessed with an unusually high baseline of mental energy. I don’t know if that’s because of physiological factors (like my genes and brain chemistry) or psychological ones (like my motivation/goals/belief structures).

If you’re not similarly blessed, sorry about that? : /

Systems and methods for maintaining high mental energy

Sleep interventions

Because sleep is so important for maintaining high energy (for me, at least), I’ve developed (or am developing) systems and procedures for effective sleep.

In particular, I am currently experimenting with  methods for dropping my active thought loops and bodily activation, so that I can fall asleep quickly (as opposed to lying in bed for hours, not being asleep).

However, long nights are occasionally more-or-less unavoidable. So, additionally, I’m attempting to learn to nap during the day, so that I can make up for short sleep the night before.

I’ve also found that sometimes, but not universally, I can overcome the dragging effect of sleep deprivation via momentum and narrative. Instead of leaning into how fuzzy and awful I feel, I’ll decide to work hard anyway. In particular, I’ll start on some relatively easy task that has some rhythm to it, like processing on of my inboxes. This usually jumpstarts me into a mode that is decently effective, despite my sleep deprivation.

I don’t have a good sense of why it sometimes works and sometimes doesn’t.

Exercise habit

I likewise optimize hard for a reliable exercise habit. It is the first item in the body of my daily checklist, as well as the first item on my rebooting checklist. I ruthlessly clear away trivial inconveniences that might get in the way of being active every day.

I do some brief intense exercise almost every day. Usually for me that takes the form of strength training, but sometimes it is hiking, or jumping rope, or something else instead.

One of the most valuable pieces of advice I ever heard about exercises was “The optimal exercise is whatever you will actually do.” The difference between exercise and not is vast compared to the effect of different kinds of workouts, so individuals should do whatever they like doing enough that they’re likely to do it. Rock climbing and swing dancing are popular in my local circles.

Focusing and dialog

As described above, I can almost always transition out of a low energy state by clarifying which goal(s) or concern(s) is unhandled, owning that goal, and then taking action as needed.