© 2021 Condor, All Rights Reserved

Fatigue And Sleep


Sleep is essential for the maintenance and promotion of life, constituting the moment when the body recovers cognitive, physical and metabolic processes (Van Cauter et al., 2007. Circadian and homeostatic factors govern sleep (for more information, see: https://condorinst.com/o-que-e-ritmo-circadiano/so that the greatest propensity to sleep occurs when the accumulation of homeostatic pressure coincides with the moment of circadian propensity to sleep, usually at night and also at noon (Borbély et al., 2016).

Sleep restriction and deprivation are associated with several harms to health and safety, such as poorer cognitive performance (increased reaction time, poorer decision-making ability, affected inhibitory control, among others) (Frey et al., 2004). It also causes hormonal and metabolic damage, such as the deregulation of the hormones ghrelin and leptin, and a greater propensity to cardiometabolic diseases, such as diabetes and hypertension (Bryant et al., 2004). Thus, these conditions increase the probability of accidents and errors in the work environment and in traffic.

The quality and quantity of sleep can be negatively impacted by sleep disorders, increasing levels of fatigue and daytime sleepiness. Much of the population is affected by sleep disorders such as obstructive sleep apnea (for more information, see https://condorinst.com/disturbios-do-sono/(Tufik et al., 2010), which shows us that the level of sleepiness and fatigue can become a problem for society and should be carefully evaluated.


In general, fatigue is a complex and multifactorial process, which can be defined as the reduction of an individual’s physical and mental capacities, resulting in difficulty in performing tasks and efficiency in responding to stimuli. Considering the multifactorial aspect of fatigue, we can classify it as physical, mental and emotional, so that:

  • Emotional fatigue: represents emotional fatigue and reduced ability to engage in emotional activities.
  • Mental fatigue: usually occurs after a prolonged period of maintenance of mental activities or highly stressful activities. It is characterized by reduced ability to engage in cognitive activities
  • Physical fatigue: occurs after a prolonged period of isometric muscle contraction or repeated isotonic muscle contractions, and results in difficulty in performing physical activities.

In addition, some people can also be affected by Chronic Fatigue Syndrome, characterized by an intense feeling of persistent tiredness, muscle pain, mood disorders, sleep disorders and cognitive impairment lasting equal to or greater than 6 months, which leads to reduced capacity to perform physical and cognitive activities. The feeling of fatigue caused by this syndrome is not caused by acute stimuli and does not improve with rest, substantially impacting an individual’s productive capacity. This syndrome can be caused by alterations in immune, neuroendocrine and autonomic functions.

Causes of fatigue

Given the complex and multifactorial component of fatigue, it has several causes that can interact, intensifying or attenuating the fatigue process. We can divide the factors that lead to fatigue into work conditions, individual conditions, and individual performance at work.

Work conditions

It refers to the logistical aspects and organization of the work schedule, such as the duration of the shift, time when the work shift occurs, characteristic of rotation and breaks during the workday and between workdays, in addition to the characteristics inherent to the workplace and workers’ relations.

The length of the work shift significantly impacts the appearance of fatigue, as it can occupy part of the time allocated for rest and leisure. However, the worker’s perception of work also presents great interference, as a work seen as pleasurable and performed consciously has less chance of causing fatigue, even in longer durations. However, it is important to condense all work into less than 60 hours per week, as evidence indicates that, above this workload, the impact on leisure and rest becomes inevitable (Techera et al., 2016). In addition, the risk of accidents increases exponentially in work shifts lasting longer than 8 hours, so that shifts lasting 12 hours have almost double the risk of accidents when compared to 5-hour shifts (Folkard and Tucker, 2003). Thus, breaks should be established during the working day and time off between working days that allow for the complete dissipation of the fatigue process that was accumulated during the working period.

Considering that human beings present circadian rhythmicity in several biological functions, the moment in which work takes place is also related to the appearance of fatigue. Thus, morning and night shifts tend to contribute more to the development of fatigue, as they prevent the recovery process from taking place fully. For example, individuals who need to wake up very early to work (3:00 am or 4:00 am) become deprived of an important part of the night’s sleep, which impacts their fatigue levels. On the other hand, night shift workers need to sleep during the clear cycle of the day, which also prevents them from obtaining full and restorative sleep due to circadian characteristics (high core temperature and absence of melatonin) (Sallinen and Kecklund, 2010).

On the other hand, the intrinsic characteristics of the workplace such as temperature and noise must also be analyzed. Environments with high levels of intermittent or constant noise contribute to the appearance of fatigue (Kjellberg et al., 1998). Not only, environments with extreme temperature, be it hot or cold, lighting characteristics, ergonomics, and pressure conditions exert the same effect (Krause et al., 1997). Furthermore, relationships in the workplace must be as healthy as possible, preventing excessive competition and the presence of bullying from causing unnecessary fatigue.

Individual conditions

            It refers to the personal characteristics and lifestyle habits of each individual, such as the regular practice of physical activities, adherence to a healthy and balanced diet, engagement in leisure activities, use of medication, sleep pattern and presence of sleep disorders, chronotype, tolerance and ways of dealing with stress, and the family environment. All these conditions are related and, thus, the greater the presence of positive variables, as well as family support, the lesser the tendency for fatigue to appear.

Individual performance at work

            It refers to the physical and cognitive characteristics performed by the individual, such as reaction time, decision-making capacity, emotional sensitization, memory capacity and sustained vigilance.

Influence of sleep on the fatigue process

Sleep has been shown to be the main trigger of the fatigue process. The longer the agreed time, the greater the damage observed. Thus, in relation to psychomotor performance, being awake for 17 hours is equivalent to a blood concentration of 0.05% alcohol, while being awake for 24 hours is equivalent to a blood concentration of 0.10% of alcohol (Dawson and Reid, 1997).

In this regard, the total sleep time recommendation for the majority of the population is more than 7 hours per night. However, there are the short sleepers, who feel satisfied sleeping between 4 and 6 hours, while there are the long sleepers, who need to sleep between 10 am and up to 12 hours to feel fully recovered. These are at the extreme of the population, so the vast majority of the population needs between 7 and 8 hours sleep to fully recover. Thus, it is essential that the desired amount of sleep is obtained to prevent fatigue from building up.  

Consequences of Fatigue

The financial costs arising from fatigue are significant. It is estimated that, in Brazil, around 45 thousand people die each year as victims of traffic accidents, which is the main cause of death in the country (Carvalho, 2020). In financial terms, these accidents represent a cost of R$ 50 billion each year, represented by the loss of the victims’ workforce, in addition to hospital admissions. In this aspect, traffic accidents related to fatigue and drowsiness commonly accompany deaths and, when death does not occur, the accident is likely to be more serious (Stahl and Sigua, 2020). For this reason, the treatment of sleep disorders must become a priority for those who suffer from these ailments. Not only, fatigued workers perform worse at work and get sick more often, which generates absenteeism that also entails an important financial cost and which is generally supported by the population (Godet-Cayré, 2006).

            In sports, the pressure for results and performance evolution is constant, which is one of the factors that contribute to poor sleep quality among athletes — in addition to the use of electronic devices at night, travel to compete, muscle pain resulting from physical training, sleeping in unfamiliar rooms, among others. The consequences of sleep debt in this population are represented by a decrease in sports performance, as well as a greater risk of developing injuries, which generates leaves and financial losses for the athlete and club of origin (Hägglund et al., 2013).

            Students also suffer when they find themselves at high levels of fatigue. Given the important function of consolidating sleep memory, students who do not reach the desired total sleep time have worse academic performance (Curcio et al., 2006).      

Fatigue assessment

Given the complexity and multifactorial characteristic of fatigue, this construct is almost impossible to be objectively evaluated. Not only that, the distinction between acute fatigue and chronic fatigue becomes a complicating factor, as acute fatigue has several manifestations, whereas chronic fatigue has well-defined characteristics in the scientific literature. Thus, the level of drowsiness can be used as a parameter to estimate the level of fatigue, and it is often used in the work environment as it is a quick and easy-to-apply assessment (Gates et al., 2018), the focus of fatigue assessment should be on reducing accidents and maintaining performance, while in the sporting environment the focus should be on maintaining performance and reducing the likelihood of injury.

            Portanto, como o sono tem se mostrado o principal fator que interfere no nível de fadiga, pode-se entender que aqueles trabalhadores e atletas que apresentam má qualidade de sono e tempo total de sono insuficiente encontram-se em maior risco de desenvolvimento de fadiga. Assim, torna-se essencial a avaliação da qualidade de sono destas populações. Neste aspecto, a actigrafia é uma técnica útil e não invasiva que pode ser utilizada para avaliação do sono e ritmos biológicos (Sadeh, 2011). O actígrafo é um equipamento semelhante a um relógio de punho e contém sensores de movimento, luz e, em alguns modelos mais modernos, sensores de temperatura. A análise dos dados permite a obtenção de variáveis como o tempo total de sono, eficiência do sono (relação entre o tempo de cama e tempo em que o indivíduo realmente dormiu), latência para início de sono (tempo decorrido entre a intenção de dormir e início do sono), tempo acordado após início do sono e tempo total em vigília. Para maiores informações, veja https://condorinst.com/o-que-e-actigrafia/.


Fatigue is a multifactorial process, complex and difficult to measure, which can cause substantial damage to the work, sports and academic environment. Therefore, it is essential to find ways to mitigate and reduce the occurrence of fatigue. However, objectively evaluating fatigue is almost impossible, given its multifactorial characteristics. Thus, the level of daytime sleepiness and total sleep time can be used as a basis for estimating the level of fatigue in individuals and, for sleep assessment, actigraphy is a useful and valid technique.