Mental energy vs physical energy: why you confuse the two

Feeling low on energy is one of the most common complaints among adults. However, when someone says they have no energy, they rarely distinguish whether this feeling is mental or physical in origin.  

Mental energy and physical energy are distinct phenomena, with different physiological mechanisms, specific causes and their own approaches to intervention. Recent scientific literature shows that confusing these two dimensions can lead to ineffective strategies for nutrition, rest and lifestyle. [1,2] 

What is physical energy 

Physical energy refers to the body's ability to perform muscular work and maintain motor performance over time. It is directly associated with the availability of ATP, muscle and liver glycogen reserves, mitochondrial function, and the efficiency of the cardiorespiratory system. [3] 

Physical fatigue occurs when these systems reach their limits, whether due to energy depletion, prolonged exertion, nutritional deficits, or inadequate recovery. Systematic reviews show that physical fatigue is strongly related to peripheral factors, such as muscle metabolic changes and inflammation induced by exercise or prolonged sedentary behaviour. [4] 

What is mental energy? 

Mental energy corresponds to the ability to sustain cognitive processes such as attention, concentration, working memory, decision-making, and self-control. Its decline manifests itself as mental fatigue, characterised by a feeling of cognitive tiredness, difficulty in maintaining focus, lower motivation, and a greater perception of effort in intellectual tasks. [5] 

From a neurobiological point of view, mental energy depends on the activity of specific neural networks, particularly in the prefrontal cortex, the availability of neurotransmitters such as dopamine and noradrenaline, and the balance of cerebral homeostasis. Recent studies show that mental fatigue is associated with measurable changes in electroencephalographic patterns and neuroendocrine markers related to stress and vigilance. [6] 

Why we confuse mental energy with physical energy 

The term energy is used generically in everyday speech. We say we need energy both when we are physically tired and when we are mentally exhausted. This linguistic imprecision reinforces the misconception that both states have the same origin and solution. 

Reduced performance, feeling heavy, irritability, and difficulty starting tasks are common to both mental and physical fatigue. Studies show that subjective self-reporting does not allow for reliable distinction between the two without structured assessment. [7] 

Furthermore, science shows that prolonged mental fatigue increases the perception of physical effort, even when physiological muscle markers remain normal. Similarly, low levels of physical activity and loss of functional capacity are associated with poorer cognitive performance and reduced mental vitality. This interaction contributes to the confusion between the two types of energy. [8,9] 

One of the most consistent findings in recent literature is that the brain acts as a central regulator of the perception of effort and energy. Studies on the subject show that, after prolonged cognitive tasks, individuals have a lower tolerance for physical effort, even though there is no significant muscle energy depletion. This explains why many people feel physically tired after a day of intense intellectual work, even without significant physical effort. What fails is not muscle energy, but the central nervous system's ability to sustain the effort. [6,8] 

Nutrition and mental versus physical energy 

Energy intake is necessary for both the body and the brain, but the brain does not simply respond to more calories. The FAO and the World Health Organisation clarify that the brain uses glucose continuously, but that abrupt fluctuations in blood glucose are associated with poorer cognitive performance and greater feelings of mental fatigue. [3,10] 

Documents from the Order of Nutritionists and the Portuguese Nutrition Association highlight that unbalanced eating patterns, rich in simple sugars and poor in micronutrients, can generate energy spikes followed by sharp drops, often interpreted as a lack of physical energy when they reflect mental fatigue and metabolic instability. [11,12] 

Sleep and recovery 

Sleep is a key determinant of mental energy. Systematic reviews show that sleep deprivation first affects cognitive performance, emotional regulation and decision-making, even before significantly compromising physical capacity. [13] 

The Directorate-General for Health and the World Health Organisation recognise mental fatigue associated with insufficient sleep as a public health problem, with an impact on work performance, risk of errors and quality of life. [14] 

Practical implications for everyday life 

Confusing mental energy with physical energy often leads to ineffective strategies, such as consuming more food when the problem is lack of sleep, or trying to rest passively when what is lacking is regular physical stimulation. Scientific evidence supports an integrated approach: 

For mental fatigue: adequate sleep, cognitive breaks, stress management, regular and balanced eating. 

For physical fatigue: progressive physical activity, adequate recovery, sufficient energy intake, and correction of nutritional deficiencies. 

For both: consistency in schedules, quality eating patterns, and reduction of stimuli that promote abrupt energy fluctuations. 

Conclusion 

Mental energy and physical energy are not the same thing, although they often manifest themselves in similar ways. Science shows that they have distinct but interconnected mechanisms, and that confusing them leads to less effective choices in lifestyle, diet, and recovery. 

Understanding this difference allows you to make more informed decisions, recognise early signs of imbalance and adopt strategies aligned with what your body and brain really need.  

In a context where mental fatigue is increasingly prevalent, especially in cognitively intense work environments, choosing snacks that promote energy stability and avoid abrupt fluctuations in blood sugar can contribute to better management of attention, performance, and overall well-being. Here is a list of some snacks that can help you combat fatigue: 

→    Cereal bar coconut 

→    Nuts australian mix  

Author:

Sofia Silva 5784N