What is the difference between Fatigue and Tiredness?

These two terms are closely related and are in some cases interchangeable and in some languages share the same word. Tiredness is used in everyday language and people are more likely to complain of being tired or exhausted than of being fatigued. Not many people will visit their doctor with fatigue; they are more likely to report tiredness, exhaustion, or sleepiness. Adjectives which qualify the extent of the fatigue or tiredness add further confusion.

In terms of severity, it is difficult to say which is worse: very fatigued or extremely tired.

Unlike sleepiness, tiredness and fatigue may encompass physical and mental symptoms together or alone. Indeed, fatigue in a muscle can be measured by the reduction in contraction strength.

Muscle fatigue, or physical fatigue, is the decline in ability of a muscle to generate force. It can be a result of vigorous exercise but abnormal fatigue may be caused by barriers to or interference with the different stages of muscle contraction. There are two main causes of muscle fatigue: The limitations of a nerve’s ability to generate a sustained signal and the reduced ability of calcium (Ca²⁺) to stimulate contraction.

Central Fatigue
/ˈsɛntr(ə)l fəˈtiːɡ/
noun
1. A decrease in neural drive to the muscle by the central nervous system.

Peripheral Fatigue
/pə-ˈrif-(ə-)rəl fəˈtiːɡ/
noun
1. Decline in muscle function due to processes at or near the neuromuscular junction.

Fatigue is a word more commonly used by health professionals in particular to distinguish between the normal experience of tiredness and the extreme tiredness (physical and mental) associated with conditions such as multiple sclerosis. In addition, Chronic Fatigue Syndrome is a debilitating condition which can occur post infection and in extreme cases patients are unable to work and may be confined to bed. There are some papers claiming a difference between fatigue and tiredness but it is in this clinical context.

Some scientists differentiate between sleepiness or drowsiness and fatigue, and it is true that physical fatigue is often unrelated to any reduction in sleep quantity or quality. Mental fatigue also may occur without sleep loss and may be related to high workload, overload or stress. However, in the context of fatigue in the aviation workplace there is likely to be a combination of sleep reduction or increased time since the last sleep and time on task and workload as well as working in the Window of Circadian Low (WOCL).

Causes of Fatigue and Tiredness

Fatigue and tiredness can stem from a wide range of factors, including lifestyle habits, medical conditions, and psychological stressors. Lack of sleep or poor sleep quality is a common culprit, leading to both physical and mental exhaustion. Additionally, physical exertion, prolonged periods of stress, nutritional deficiencies, and certain medical conditions such as anaemia, thyroid disorders, and chronic fatigue syndrome can contribute to feelings of fatigue and tiredness. Previously undiagnosed cancers have been revealed when aircrew have reported fatigue and operational and/or personal causes were excluded.

Another confounding factor is the effect of the circadian rhythm in alertness/sleepiness particularly when working at night or at the low point in the rhythm when time zones are crossed.

Crewmember Fatigue is defined by ICAO as:

A physiological state of reduced mental or physical performance capability resulting from sleep loss or extended wakefulness, circadian phase or workload (mental or physical activity) that can impair a crew member’s alertness and ability to safely operate an aircraft or perform safety related duties. 

The word tiredness could easily be swapped for fatigue, but it sounds less scientific!

Biomathematical Fatigue Models

Biomathematical fatigue models are computational tools used to predict and quantify the effects of fatigue on human performance based on various factors such as sleep patterns, work schedules, circadian rhythms, and task demands. These models integrate principles from biology, physiology, psychology, and mathematics to simulate the dynamic interaction between sleep, wakefulness, and performance.

The more advanced fatigue models also incorporate factors such as sleep inertia, cumulative fatigue and the effects of workload to calculate fatigue scores. SAFE includes a component for number of flights in a duty due to workload effects and the number of days worked, as fatigue increases particularly after working for six consecutive days.

All of these words give no indication of the degree of tiredness or fatigue and this is the reason for the creation of scales such as the Samn-Perelli 7-point fatigue scale, the Karolinska Sleepiness scale or the use of horizontal fatigue scales with extremes at either end to assist in the recording and presentation of fatigue scores across many different industries.

Summary

While fatigue and tiredness are closely related concepts, their nuances and implications extend far beyond mere semantics. Tiredness, commonly used in everyday language, often serves as the go-to descriptor for feelings of exhaustion or lethargy, while fatigue, with its more clinical connotations, is frequently employed by healthcare professionals to delineate extreme states of physical and mental exhaustion, particularly in the context of chronic conditions like Chronic Fatigue Syndrome or multiple sclerosis.

In the context of occupational fatigue, all fatigue reports should be investigated or assessed.

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