What is fatigue?
Fatigue can manifest in different ways: drowsiness, muscle heaviness, or loss of motivation. According to some research, it corresponds to a change in motivational state leading to disengagement from a task when perceived costs outweigh expected benefits.
Physical vs mental fatigue
Mental fatigue
Mental fatigue, also known as nervous fatigue, is often the result of prolonged exposure to stress, overwork, or difficulty balancing different areas of life, such as work and personal life. Its sources are numerous and often accumulate over time.
While it may sometimes resemble physical fatigue in certain symptoms, it differs in its intensity and especially its duration. Mental fatigue tends to persist despite a good night’s sleep, develop gradually, and affect multiple aspects of daily life, including concentration, motivation, decision-making, interpersonal relationships, and overall functioning.
Physical fatigue
In contrast, physical fatigue is more closely related to the body and how it functions. It can occur following a lack of sleep, significant physical effort, clearly identified emotional stress, or factors such as illness, a virus, a deficiency, or an unbalanced diet.
Unlike mental fatigue, it is generally more temporary and tends to decrease after one or more nights of sleep and adequate physical rest.
What is temporary fatigue?
In the afternoon, it is completely normal to experience a drop in energy. This phenomenon is mainly explained by the circadian rhythm, our internal biological clock, which naturally leads to a decrease in alertness between approximately 1 p.m. and 3 p.m.
This is compounded by the gradual build-up of adenosine in the brain, a molecule that promotes the feeling of fatigue throughout the day. After lunch, digestion also uses energy, which can increase drowsiness.
Finally, a slight drop in body temperature also contributes to reduced attention and concentration. Together, these mechanisms explain why the well-known “afternoon slump” is a completely normal physiological phenomenon.
Factors that influence fatigue
Fatigue is not caused by a single factor. It is influenced by:
- Sleep–wake cycles
- Exposure to natural light
- Seasonal variations
- Hormonal adjustments
Light plays a central role in regulating the circadian rhythm and level of alertness.
Fatigue and pain: a real connection
Persistent pain is strongly associated with increased fatigue. Nearly 64% of people living with persistent pain report constant fatigue, which can affect adherence to treatment and the ability to maintain daily activities.
What can you do to reduce fatigue?
Physical fatigue
Certain habits can help improve daily energy levels:
- Get daily exposure to natural light or use light therapy
- Engage in regular physical activity for 30 minutes every day
- Manage pain with the help of a healthcare professional
These strategies support better regulation of sleep and energy.
Mental fatigue
- Try to “unload” your mind by writing down what is on your mind in a notebook or taking notes
- Take regular breaks to help regulate your thoughts and avoid processing too much information at once. The brain cannot be continuously stimulated. Take this time to practice deep breathing and relaxation exercises
- Choose activities you enjoy: find what helps you let go and rest your mind
Fatigue is never trivial
When it persists or is associated with pain, an evaluation by a healthcare professional can make all the difference. Combined care in physiotherapy and occupational therapy can help. Our team can support you in identifying the causes and help you regain your energy.
Sources
- Massar, S. A. A., Csathó, Á., & Van der Linden, D. (2018). Quantifying the motivational effects of cognitive fatigue through effort-based decision making. Frontiers in Psychology, 9:843. https://doi.org/10.3389/fpsyg.2018.00843
- Sleep Foundation. Lack of sleep and cognitive impairment. https://www.sleepfoundation.org/sleep-deprivation/lack-of-sleep-and-cognitive-impairment
- Merck Manual Professional Edition. (2025). Fatigue. https://www.merckmanuals.com/fr-ca/professional/sujets-sp%C3%A9ciaux/sympt%C3%B4mes-non-sp%C3%A9cifiques/fatigue
- Adan, A., Archer, S. N., Hidalgo, M. P., Di Milia, L., Natale, V., & Randler, C. (2012). Circadian typology: A comprehensive review. Chronobiology International.
- Basheer, R., Strecker, R. E., & McCarley, R. W. (2013). Adenosine and sleep-wake regulation. Progress in Neurobiology.
- Akhter, N., et al. (2015). Post-lunch dip in performance and alertness. Physiology & Behavior.
- Kräuchi, K. (2010). Thermoregulation, circadian rhythms, and sleep. Sleep Medicine Reviews.
- OPMED. (2025). Fatigue mentale: comment retrouver l’équilibre?. https://www.copmed.fr/fr/content/429-fatigue-mentale-comment-retrouver-l-equilibre
- Métro Média. (2019). Do you suffer from mental fatigue?. Journal Métro. https://journalmetro.com/entrepreneuriat/formation-et-emplois/1789939/souffrez-vous-de-fatigue-mentale/
