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Let's say I am studying a subject X. I believe as the time spent on the subject X increases, the marginal rate of knowledge gained per unit time decreases to almost nothing. This reduce is associated with a general feeling of "being tired".

Why does this happen?

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Sympathetic Nervous System (SNS) activity normally increases when you're stressed or in danger through the "fight or flight" response; or when you are physically active (Alshak & Das, 2019). Its effects include increasing your heart rate and breathing ability, improving your eyesight and slowing down processes like digestion. However, mental fatigue induced by prolonged cognitive load is also associated with hyperactivity within the SNS (Mizuno, et al. 2011).

In their study paper, Mizuno et al. highlighted that alterations of autonomic functions have been reported in patients with chronic fatigue syndrome (CFS), multiple sclerosis (MS), and primary biliary cirrhosis (PBC); and, autonomic dysfunction is associated with fatigue with the same conditions. With this study, they found that the same happens with healthy volunteers.

It is no wonder, in this case, that you feel more tired the longer you study something which takes a lot of concentration.

Chronic fatigue is caused by the prolonged accumulation of acute fatigue. Thus, in order to avoid chronic fatigue, it is important to develop effective strategies to recover from and avoid the accumulation of acute fatigue.

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Fatigue is classified into physical and mental types. Physical fatigue, also known as peripheral fatigue, results from repeated muscle actions. In contrast, mental fatigue represents a failure to complete mental tasks that require self-motivation and internal cues in the absence of demonstrable cognitive failure or motor weakness (Chaudhuri, & Behan, 2000). Thus, mental fatigue decreases sufferers' work or study efficiency in daily life.

The Parasympathetic Nervous System (PNS) works in opposite to the SNS (Tindle & Tadi, 2021), and the main purpose of the PNS is to conserve energy to be used later and to regulate bodily functions like digestion and urination.

The findings of Mizuno et al. (2011) suggest that

mental load causes reduced parasympathetic activity, and that prolonged fatigue-inducing mental load reduces parasympathetic activity even further. Thus, reduced parasympathetic activity seems to be a characteristic feature of mental fatigue.

References

Alshak, M. N. & Das, J. M. (2019). Neuroanatomy, sympathetic nervous system. In StatPearls [Internet]. StatPearls Publishing. https://www.ncbi.nlm.nih.gov/books/NBK542195/

Chaudhuri, A., & Behan, P. O. (2000). Fatigue and basal ganglia. Journal of the neurological sciences, 179(1-2), 34-42. https://doi.org/10.1016/S0022-510X(00)00411-1

Mizuno, K., Tanaka, M., Yamaguti, K., Kajimoto, O., Kuratsune, H., & Watanabe, Y. (2011). Mental fatigue caused by prolonged cognitive load associated with sympathetic hyperactivity. Behavioral and brain functions, 7(1), 1-7. https://doi.org/10.1186/1744-9081-7-17

Tindle, J., & Tadi, P. (2021). Neuroanatomy, parasympathetic nervous system. In StatPearls [Internet]. StatPearls Publishing. https://www.ncbi.nlm.nih.gov/books/NBK553141/

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