What is fatigue ?

Ga HIER naar de Nederlandse versie van deze pagina

Everyone feels tired or fatigued from time to time. In fact, next to headache, fatigue is one of the most frequently heard complaints in  general practitioners offices. But what exactly is fatigue?

Two kinds of fatigue

Two types of fatigue can be roughly distinguished: physical or mental fatigue. However, I am not saying here that we have a "mental" state separate from a physical one. In my opinion we only have one body, but because this distinction is made a lot and with physical mainly 'do- things' (e.g., walking, gardening, cycling) are meant, I have left it that way for now.

The physical form of fatigue is easy to detect. When you do things with your body, whether it's gardening, cycling, playing football, you simply get tired after a while. Because you are simply burning your energy supply in your body (glucose and oxygen), in all kinds of cells. That's where your body temperature comes from when you're moving. This can really rise to 40-42 degrees Celsius during vigorous exercise, for example when you are running. It is not for nothing that you sweat a lot, just to cool down a bit.

After this intense energy consumption, your body has to refuel with energy. Usually by drinking and/or eating something. Your muscles also need to rest in order to recover and become stronger afterwards. Of course you also have to rest yourself to recover from the efforts and to build up energy again.

This kind of fatigue after physical exertion is known, everyone knows it. It's also a nice form of fatigue because you always feel like you've done something. Special substances are also produced, endorphins, which also make you feel better after physical exertion. Such a 'high' can feel very good that's why running, for example, can be so addictive if you do it often enough.

The mental form of fatigue is that you feel tired but have done little or nothing physically. For example, if you have been watching TV or listening to the radio for several hours, you may be so tired that you would like to turn off the TV or radio so as not to get irritated. It then seems as if you can no longer tolerate too many sounds or images at once. This is also the most frequently heard complaint from people with, for example, brain injury: they can no longer handle the hustle and bustle around them. It is also the most heard complaint from people when they are overworked, or worse in a burnout. It is also one of the least understood complaints. I hope to be able to provide some clarity on this.

Mental fatigue explained ?

A nice and useful definition of fatigue that I could find in the scientific literature is that of Aaronson et al. (1999): “the realization of a diminished capacity for physical and/or mental work, caused by an imbalance in the presence, use and/or recovery of energy necessary to perform activities”. So fatigue is connected to doing things and the energy resources that are required to do them. Fatigue is therefore a problem in having and recovering from these energy sources.

Mental or also cognitive fatigue is the problem you have with energy resources that are needed to perform mental activities, such as sober and logical thinking, problem solving, arithmetic, understanding images, statements and behavior of other people. Every cognitive activity, all activity that has to do with processing information, that takes place in your brain, takes energy.

The comparison with a computer is very useful here. Now I am typing this text on my Windows 10 laptop, which has 3Gb of RAM and a 500Gb hard drive. My processor is a 2.4 GHz dual Intel Core, which means nothing more than that it can perform many calculations in a short time.

The advantage is that the computer is fast. However, all this work costs the computer energy and I notice that because the battery is running out. The battery now only has 1 hour left. If I keep typing so much, this will continue to cost energy (power) and in 1 hour the power will be gone. The battery will then be empty.

Something similar is going on in the human brain when we perform mental work. Mental work, processing information, also costs our brain energy. Especially if these are activities that really require more thinking, or a complicated task. This energy is also electricity and chemistry because brain cells communicate with each other through electricity and chemical processes.

Only the voltage of this electricity is low in the brain: -70mV. It is generated by the constant movement of certain molecules in the nerve cells (Na+, K+, Ca2+, CL-). These movements in and out of the nerve cell form an action potential that is conducted along the nerve cell and its axons. Ultimately, this leads to the release of chemicals (neurotransmitters) in a synapse (gap between 2 nerve cell ends) so that another nerve cell can also be activated again and the action potential (electric current) can be passed on.

The picture below borrowed from www.biologielessen.nl shows a synapse in detail.


Now the whole point is that this kind of emission and uptake of chemicals costs nerve cells a lot of energy. Cells must have energy in them to be able to generate these kinds of action potentials. They get this energy from taking in glucose and oxygen, which are brought back through the blood. Because we get oxygen every minute through our breathing and lungs, there will not be a lack of oxygen quickly.

The problem of our energy management is therefore not oxygen but glucose. A simple word for blood sugar. Glucose is formed in the liver through the processing of glycogen (a form of glucose that can be easily stored in the liver and muscles). The basis of the glycogen is of course our food: carbohydrates.

Normally we have a storage of glycogen in our liver and muscles, which can be converted into glucose if we need it more. If we do more work mentally (or physically) then more glucose is needed. This can be visualized by a PET scan (which measures the regional uptake consumption of nerve cells). There comes a point that our body has too little sugar and that the body then starts burning fat cells to get energy.

That is also the reason that you can lose weight by studying a lot and not eating well. However, when we have too little glucose, your brain cells will be one of the first to notice and function less. Very likely this also has something to do with your perceived fatigue. But scientists are not sure about this yet.

The condition of the mitochondria also plays an important role in the energy management of a (nerve) cell. These are cell parts, also called organelles, that convert energy from our food into an ATP molecule. This ATP molecule provides energy.

Wikipedia has the following rather technical explanation of ATP:

"Adenosine triphosphate, better known as ATP, is the carrier of chemical energy in all living cells. ATP is an organic compound consisting of the nucleobase adenine, the monosaccharide ribose and three phosphate groups. A lot of energy is stored in the bonds between the phosphate groups, and this energy will be released when a phosphate group is split off. The released energy can be used to drive various cellular processes, such as protein synthesis, active transport, signal transduction and muscle contraction.

ATP is present in all known forms of life, suggesting that it was important as an energy carrier very early in the evolution of life. A large part of the metabolism is aimed at generating enough ATP. In plants, animals and many bacteria, this occurs primarily during a process called oxidative phosphorylation."

I will not go into detail about the mitochondria. More about this can be found on the Internet (especially on Wikipedia).

Brain damage and fatigue

What we now know is that brain injury leads to an increase in more brain cells that are working. Instead of working more efficiently and sparingly with their capacity, more nerve cells start to show activities in information processing. It seems that they are trying to make more new connections with each other in order to cope with a task.

This simple fact, that many more parts of the brain are involved in an activity that showed very little brain activity before the injury, may explain why brain-injured patients are getting tired much faster. They are actually burning more energy when they are doing mental activities. They therefore need more glucose and their glucose level has of course fallen much faster or has even been used up.

If this is really true, and I have not heard any other serious other hypotheses so far, how is it that simply taking extra glucose (dextrose, for example) does not help brain injury patients? Well, it does help…but only in the short term. In the slightly longer term they will only become more exhausted! So the blood sugar, glucose, cannot be the whole story of mental fatigue.

Apparently it is also not good if there is a clear increase in more brain cells that are somewhat active. However, nobody knows exactly how this works. One thing is certain, however: the brain does not like all areas to be constantly active.

Just imagine how you react to sudden heavy traffic around you when you are driving and it also starts to rain heavily. After driving for half an hour in such intense conditions, you will arrive home exhausted. Overloading your brain in this way takes a lot of attention capacity: a lot of concentration and divided attention.

We know that this creates a lot of tension, certain basic emotions become more intense. Mostly fear. Too much build-up of fear leads to a defensive response: either flight or fight. In other words: either panic or an outburst of anger. However, no one knows exactly why this happens, but I can speculate about it in the next paragraph.

Overstimulated or overloaded brain

It is reasonable to assume that our brain's main purpose is to help us (our body) to survive. As Antonio Damasio describes in his book “the feeling of what happens” , the brain is busy maintaining a balance, a homeostasis for the most important bodily functions such as blood pressure, body temperature and heart rate.

It does this by constantly monitoring our environment through our senses (touch, smell, sight, hearing, taste) and constantly checking our bodily functions and predicting our environment. Especially this prediction has recently been seen as very important in the scientific world. After all, that way the brain can prepare for actions necessary for survival.

A new scientific model says in so many words that the brain is constantly trying to predict the environment and adapt it to our actions and bodily functions. In this way it tries to live in harmony with its environment, with the least chance of conflicts and life-threatening situations.

From an evolutionary point of view, the brain is really not 'happy' when it is overloaded or overstimulated. After all, when you are overloaded you can no longer keep a close eye on your surroundings and you run a greater risk of falling prey to deadly situations. Fear, of course, is designed to get you out of such situations as quickly as possible: either run away in panic or aggressively end the situation.

In fact, such emotional outbursts can simply be explained as a self-defense mechanism against information overload. They are meant to stop the information overload.

For example, brain injury patients, but also people who are overworked, often hear that they are unable to control their emotions sufficiently. Perhaps that is partly true (there are also indications for this). But it may also be the case that their information overload occurs much faster than in healthy people.

So this self-defense mechanism will also be used more quickly by them: so also more quickly leading up to irritations or fear. And no one is trained to counteract such a rapid build-up of anxiety (due to information overload).

Some evidence for this "theory" comes from practice and family accounts. It often turns out that if information overload is recognized and prevented much earlier, then these patients are much less likely to erupt emotionally. Of course, what also plays a role is that emotion regulation (the brake on emotions) works less well when fatigue plays a role. Possibly because the frontal lobe in particular consumes a lot of energy for braking and regulating the more subconscious emotional brain.

In a recently published book by Abe Wood, How Overstimulation Is Destroying Your Life, he explains quite clearly how cognitive overload in these modern times is the main culprit of a bad working attentional system and increasing fatigue, less motivational drive and unproductivity. 


Fatigue can also be explained by an excessive use of several parts of the brain, especially if there is a brain injury. Glucose and oxygen are used much faster than average. Despite this, extra glucose does not really help. Possibly because the brain is developed not to quickly end up in an overload. It therefore reacts with more intense basic emotions such as fear and aggression if it remains in an overload situation for too long. Fatigue can be a subjective experience that warns of such an overload situation and in this sense, really is not a terrible thing.

Have A Great Story About This Topic?

Do you have a great story, remarks or any additions to or about this? One that could help other people as well and above all is constructive? Then please share it!

[ ? ]

Upload 1-4 Pictures or Graphics (optional)[ ? ]


Click here to upload more images (optional)

Author Information (optional)

To receive credit as the author, enter your information below.

(first or full name)

(e.g., City, State, Country)

Submit Your Contribution

  •  submission guidelines.

(You can preview and edit on the next page)


I will not take any responsibility for how the information on this website will affect you. It always remains your responsibility to handle all information with care and in case of medical or mental problems you should ALWAYS consult a professional in your neighbourhood!

Ik neem geen enkele verantwoordelijkheid voor hoe de informatie op deze site u zal beïnvloeden. Het blijft altijd uw verantwoordelijkheid om al deze informatie zorgvuldig te bekijken. In het geval van lichamelijke en/of mentale problemen dient u ALTIJD een professional in uw directe omgeving te waarschuwen!