The relationship between metabolism, physical fatigue, and the energy cost of physical motion is relatively well-understood.

• We know the body's efficiency in harvesting energy from stored sugars and fats.
• We know how energy is transported throughout the body.
• We know what fuel sources different tissues use.
• We have detailed knowledge on scales ranging from the molecular to the gross anatomical to explain how organisms move.
• We have Newtonian mechanics to explain how a certain motion of the body can generate a force on a moving object and what its trajectory will be after impact.
• Physical motion can be directly observed by others, mostly without influencing the motion itself.

Such robust, quantitative models are mostly lacking when we try to study and explain thought.

• We lack many of the connections that would be necessary to explain how brain chemistry generates conscious experience.
• We don't understand the energy costs of thought.
• We don't have comparable models to explain how one thought leads to another, or how thoughts are stimulated and influenced by the physical world. There is no equivalent to Newtonian mechanics for modeling the dynamics of conscious thought.
• We rely entirely on other people's self-reports to observe their conscious experience.
• We are extremely limited in our ability to perform precise, repeatable experiments to test whatever empirical "laws of consciousness" might exist.

Mental fatigue, akrasia, demotivation, and discomfort are common experiences. However, the factors that influence them seem diverse and contextual. The effect of glucose, sleep, drugs, physical fatigue, boredom, sensory stimulation, being watched, having a conversation, talking to yourself vs. thinking in your head, flow states, and many other factors seem to have an effect, but learning how to trigger desirable states, avoid undesirable ones, and make efficient short- vs. longterm tradeoffs and investments is a challenging process of self-discovery.

Placebo effects, behavior compensation, the body's attempt to preserve homeostasis, and the effects of aging and changing life circumstances conspire to give a short life expectancy to knowledge we might hope to gain via self-experimentation.

Yet many people enthusiastically put a lot of effort into trying to optimize their mental state and abilities, even if they don't think about it in those terms. So when is it advantageous to try anyway, despite the difficulty?

• When the scientific literature on the specific topic we are interested in is uncontroversial and has been established long enough that we can trust it.
• When the "information decay" is slow enough, and there's enough ability to re-investigate in the new circumstances, that it's worth "keeping up with the times" for our own brain. For example, a person getting medicated for ADHD who finds that their previous medication isn't working as well anymore might be well-served by investing some time and money to retune it, perhaps by adjusting medication and dose.
• When we can get rapid, clear, repeated feedback on at least some aspects of the question.
• When we have a range of plausible options to try, and can cycle through them in order to try and achieve our desired effect. For example, when I am feeling mentally tired, I've noticed it seems to have a range of potential causes: low blood sugar, dehydration, being undercaffeinated or overcaffeinated, anxiety, neck and back pain, too much sitting or moving around, a stuffy room, being overheated, being overstimulated or understimulated, being sleepy. While I can't usually "prove" which one was the culprit (and sometimes it's more than one), I can simply treat each problem and usually my problem goes away. I'll never have the ability to know exactly which issue is giving me problems on any given occasion, but I can treat the problems pretty consistently by addressing all possible issues (in moderation) - and this is what I really care about.

More than anything, I think it's important to look for robust but common-sense solutions, and to avoid getting bogged down with unknowable answers to noncentral questions. We don't need to settle the debate on whether and when glucose intake, reducing air particulates, or lowering the temperature can improve mental performance. Instead, we can just look for intuitive signals that these needs might be affecting our own cognition at a particular time, and then provide for being able to make the necessary adjustments.

Keep some healthy snacks near your workspace, open a window, get a fan or A/C, keep a water bottle handy. If you're feeling tired midday, take a little caffeine and see if it helps, but not so much that you risk overcaffeinating yourself or ruining your sleep later. Learn to take broad plausible treatments for low mental performance, in moderation.