Mental Fatigue – From cloudy to clarity

Whilst physical fatigue is easily identifiable, mental fatigue may be a less obvious yet significant contributing factor to a person’s ability to maintain a positive outlook and achieve better health outcomes. Mental fatigue is a common clinical presentation that may be hard to distinguish from other underlying conditions which present in clinic, as it is a subjective sensation that is difficult to measure. Therefore, many patients may not realise they are mentally exhausted and correcting it can be paramount to helping support their health journey, irrespective of their presenting complaint.

Where is my mind?

Mental fatigue is defined as a reduction in the ability and efficiency of cognitive processing that may be caused by prolonged periods of mental or physical activities.1 It is often associated with negative feelings like anxiety, frustration, and boredom,2 and symptoms include wandering thoughts, tiredness, inability to concentrate and aversion to continue performing mental tasks.2,3 These symptoms can contribute to dysfunctional cognitive and behavioural performance, not only in the workplace, but in social and homelife circumstances.3 Furthermore, mental fatigue results in reduced productivity, lowered reaction times and increased risk of accidents.4

The degree of wakefulness is thought to be related to autonomic and endocrine processes, modulation of neurons by the noradrenergic and cholinergic pathways2 and the suprachiasmatic nucleus within the hypothalamus which controls circadian rhythm. 2 Other influencing factors include an individual’s genetics, health status, age, physical exercise, stress levels and mood. 2

Where it’s at: the prefrontal cortex

The prefrontal cortex (PFC) is highly important in the regulation of the autonomic nervous system (ANS), with abnormalities in this brain region associated with reduced energy.5 The PFC is critically involved in executive function, which are cognitive processes that govern concentration, attention, judgement, self-monitoring and decision-making.6 Mental fatigue is associated with an increase in sympathetic stimulation and decrease in parasympathetic ANS function,7 which over time, can generate a viscous cycle. As the cognitive demands increase, the sympathetic hyperresponsiveness causes a decline in the parasympathetic response which further contributes to the state of mental fatigue, eventually leading to chronic symptoms of physical fatigue.7

Boosting brain power with BDNF

Brain-derived neurotrophic factor (BDNF) is a neurotrophin that is expressed in the brain, primarily in the PFC and hippocampus, where it acts as a key neuroplastic agent that modifies neuronal circuits and brain function,6,8 and helps to protect neurons from damage or death.6 BDNF acts at pre- and post-synaptic areas within the central nervous system (CNS) to regulate our ability to improve learning and form memories.8 Subsequently, BDNF reduces the effects of chronic stress and is found to be low in people experiencing chronic anxiety, insomnia and depression.9 Key herbs that enhance or modulate BDNF include: St John’s wort, Turmeric, Rhodiola, Bacopa, Saffron, Ginkgo, Siberian ginseng, Schisandra, Panax ginseng, and Withania.10,11

Check out part two of our series.

References

1 - Ishii A, Tanaka M, Watanabe Y. Neural mechanisms of mental fatigue. Reviews in the Neurosciences. 2014 Aug 1;25(4):469-79.

2 - Tran Y, Craig A, Craig R, Chai R, Nguyen H. The influence of mental fatigue on brain activity: Evidence from a systematic review with meta-analyses. Psychophysiology. 2020 May;57(5):e13554.

3 - Boksem MA, Tops M. Mental fatigue: costs and benefits. Brain research reviews. 2008 Nov 1;59(1):125-39.

4 - Sun Y, Lim J, Meng J, Kwok K, Thakor N, Bezerianos A. Discriminative analysis of brain functional connectivity patterns for mental fatigue classification. Annals of biomedical engineering. 2014 Oct 1;42(10):2084-94.

5 - Tanaka M, Shigihara Y, Ishii A, Funakura M, Kanai E, Watanabe Y. Effect of mental fatigue on the central nervous system: an electroencephalography study. Behavioral and brain functions. 2012 Dec;8(1):1-8.

6 - Savitz J, Solms M, Ramesar R. The molecular genetics of cognition: dopamine, COMT and BDNF. Genes, Brain and Behavior. 2006 Jun;5(4):311-28.

7 - Mizuno K, Tanaka M, Yamaguti K, Kajimoto O, Kuratsune H, Watanabe Y. Mental fatigue caused by prolonged cognitive load associated with sympathetic hyperactivity. Behavioral and brain functions. 2011 Dec;7(1):1-7.

8 - Anastasia A, Hempstead BL. BDNF function in health and disease. Nat Rev Neurosci. 2014;15.

9 - Schmitt K, Holsboer-Trachsler E, Eckert A. BDNF in sleep, insomnia, and sleep deprivation. Annals of medicine. 2016 Jan 8;48(1-2):42-51.

10 – Sangiovanni E, Brivio P, Dell’Agli M, Calabrese F. Botanicals as modulators of neuroplasticity: focus on BDNF. Neural plasticity. 2017 Oct;2017.

11 - Jang Y, Lee JH, Lee MJ, Kim SJ, Ju X, Cui J et al. Schisandra extract and ascorbic acid synergistically enhance cognition in mice through modulation of mitochondrial respiration. Nutrients. 2020 Apr;12(4):897.