Types of fatigue and their causes
Physical fatigue occurs when muscle demands exceed the body’s recovery capacity: intense or repetitive exertion, prolonged static contractions (sustained postures), heavy lifting, working in hot conditions, and a lack of breaks. It manifests as muscle pain and heaviness, tremors, loss of precision, and, in the medium term, musculoskeletal disorders. Mental fatigue results from cognitive load: the quantity and complexity of information to be processed, sustained attention, time pressure, interruptions, multitasking, decision-making with significant consequences, and the emotional demands of interacting with people. It manifests as difficulty concentrating, errors, irritability, slowness, and a feeling of being overwhelmed, and can lead to stress and burnout.
Fatigue related to work schedules is linked to long working hours, overtime, rotating shifts, night work, insufficient rest between shifts, and disruptions to circadian rhythms. Night work reduces the quantity and quality of sleep, increases sleepiness and the risk of accidents, especially in the early morning hours, and is associated with long-term health effects. The Epworth Sleepiness Scale allows for screening for excessive daytime sleepiness, but it does not replace medical evaluation. INSST Technical Note 455 outlines organizational criteria to reduce its effects: short and forward shift rotations, limits on consecutive nights, sufficient rest periods, and employee participation in shift scheduling.
Fatigue is also a contributing factor to accidents: it reduces vigilance, risk perception, and reaction time. Managing fatigue is an integral part of ergonomic and psychosocial risk assessment and work organization design, and should not be treated as an individual problem of stamina or motivation.
Risk factors for fatigue at work
- Physical demands. Effort, forced or sustained postures, repetitive movements, handling of loads and adverse environmental conditions (heat, noise, vibrations).
- Mental demands. Quantity and complexity of information, sustained attention, time pressure, interruptions, and responsibility for people or facilities.
- Emotional demands. Continuous interaction with the public, patients, or conflict situations.
- Working hours. Long days, overtime, shifts, night work, insufficient rest periods, and lack of predictability in schedules.
- Lack of breaks and recovery time. Absence of adequate breaks during the workday and rest time between shifts.
- Workstation design. Poor lighting, noise, monotony, inadequate tools, and poorly configured display screens.
- Individual factors. Age, health status, sleep quality, dual presence and prolonged commutes, which modulate the response to the demands of work.
- Constant connectivity. Using devices outside of normal working hours prolongs exposure and reduces recovery (digital fatigue).
Assessment and preventive measures
- Evaluation. Analysis of job requirements and working time, methods for evaluating physical workload (e.g., postural and load handling methods) and mental workload (subjective scales, NASA-TLX, performance indicators), fatigue and sleepiness questionnaires, and data on errors, incidents, and absenteeism.
- Workstation design. Reduce strain and awkward postures, provide mechanical aids, appropriate furniture and tools, and correct lighting and environment.
- Task organization. Alternation of tasks of varying difficulty, autonomy to regulate the pace, reduction of interruptions and time pressure, adequate staffing.
- Breaks. Short and frequent breaks, especially in tasks requiring sustained attention, physical exertion, or viewing screens, with the possibility of real rest.
- Working time. Limit the duration of working days and overtime, design shifts with ergonomic criteria, limit consecutive nights and guarantee rest periods between shifts.
- Training and information. On sleep hygiene, workload management, signs of fatigue, and the use of breaks.
- Health surveillance. Detection of chronic fatigue, sleep disorders and effects of night work, with the necessary adjustments.
Organizational application: how to prevent work fatigue
- Identify in the risk assessment the positions with high physical, mental, emotional or working time demands, and the indicators of fatigue (errors, incidents, absenteeism, complaints).
- Assess the physical and mental workload using appropriate methods and consult with affected individuals using fatigue and sleepiness questionnaires.
- Act on the design of the workstation: reduce effort and forced postures, improve lighting and environment, adapt tools and screens.
- Reorganize the task: alternation, autonomy over the pace, reduction of interruptions and time pressure, sufficient staffing.
- Establish a system of breaks and working time with ergonomic criteria: limit overtime and extra hours, design shifts with adequate rotation, limit consecutive nights and guarantee rest periods between shifts.
- Provide training in sleep hygiene and the detection of signs of fatigue, and implement the right to disconnect policy .
- Integrate fatigue into health surveillance and review measures with indicators.
Preventive management software allows you to link physical and mental workload assessments to jobs, plan and monitor organizational and design measures, record training, and analyze indicators of errors, incidents, and absenteeism with traceability.
Limits and common mistakes
- Attributing fatigue to a lack of stamina or motivation in the person rather than to the demands of the job.
- Compensating for fatigue with overtime or more pressure instead of recovery and reorganization.
- Designing shifts without ergonomic criteria or participation of the people affected.
- Eliminate or shorten breaks during periods of high workload, precisely when they are most needed.
- Ignoring the mental and emotional burden and only evaluating the physical burden.
- Not considering fatigue as a cause in the investigation of accidents and incidents.
The methods for assessing physical and mental workload are described in the INSST technical notes and in the ergonomics technical standards; this sheet is for informational purposes.
Practical example
Situation: A control center of an electric distribution company works in rotating twelve-hour shifts with consecutive nights and registers an increase in operating errors in the last hours of the night shift.
- Evaluation. The prevention service analyzes the error record by hour and shift, applies a fatigue and drowsiness questionnaire and evaluates the mental workload of surveillance and maneuvering tasks; the concentration of errors between three and six in the morning and after the third consecutive night is confirmed.
- Organizational measures. Shifts are redesigned, limiting consecutive nights to two with forward rotation, minimum rest periods between shifts are guaranteed, scheduled breaks with a change of personnel are established, and a second person is introduced during the highest risk periods.
- Workstation design. Screen lighting and contrast are improved, non-priority alarms are reduced, and procedures for the most complex maneuvers are reviewed.
- Follow-up. Staff are trained in sleep hygiene, follow-up is integrated into health surveillance, and after six months, nighttime errors are clearly reduced and questionnaires show less sleepiness.
Regulatory and reference framework
- Law 31/1995, of November 8. Law on the Prevention of Occupational Risks; adaptation of work to the person and mitigation of monotonous and repetitive work (article 15).
- Royal Legislative Decree 2/2015, of October 23. Workers ‘ Statute; working hours, overtime, night work and shifts, rest periods (articles 34 to 37).
- Directive 2003/88/EC . Organization of working time; breaks, maximum duration and night work.
- Royal Decree 488/1997, of April 14. Working with display screens ; organization of work and breaks.
- NTP 445. Mental workload: fatigue . Concept, factors and prevention of mental fatigue.
- NTP 455. Shift and night work: organizational aspects . Organizational criteria to reduce the effects of shifts and night work.
- NTP 544. Estimation of mental workload: the NASA-TLX method . Subjective assessment method of mental workload.
Organic Law 3/2018 refers to computer fatigue in relation to the right to digital disconnection, and ergonomic standards (ISO 6385, UNE-EN ISO 10075 series on mental workload) complement the technical criteria.
