Fatigue Risk Glossary

Sleep science & physiology

Core concepts from sleep and circadian science that explain how and why fatigue builds and affects performance.

Acute Fatigue: Short-term fatigue arising from one extended shift, late duty, or single night of restricted sleep.
Chronic Fatigue: A cumulative state of exhaustion resulting from prolonged exposure to sleep restriction or extended wakefulness without adequate recovery, creating a significant sleep debt that—unlike acute fatigue—requires several consecutive nights of unrestricted sleep to fully resolve.
Circadian Rhythm (CR): The roughly 24-hour internal body clock that regulates sleep–wake patterns, alertness, temperature and hormone levels.
Cumulative Fatigue: Fatigue that builds over several days or weeks due to repeated insufficient sleep or demanding duty patterns.
Fatigue: A state of reduced physical or mental performance caused by insufficient sleep, long wakefulness, workload or time-of-day effects.
Light Exposure Schedule: A planned pattern of light and dark exposure used to support circadian adjustment or maintain alertness.
Restorative Sleep (RS): Sleep of sufficient quality and duration to return performance and alertness close to baseline levels.
Shift Lag: Misalignment between the body clock and a new duty schedule, such as moving from days to nights.
Shift Work Tolerance (SWT): Individual differences in how well people cope with shift work and irregular hours without excessive fatigue or health impact.
Sleep Architecture: The structural pattern of sleep phases (REM and non-REM) that occurs in cycles throughout a sleep period.
Sleep Debt (SD): The cumulative gap between the sleep a person needs for normal functioning and the sleep they actually obtain.
Sleep Efficiency: The percentage of time spent asleep relative to the total time spent in bed.
Sleep Hygiene: Practical behaviours and environmental factors that support good quality sleep, such as light, noise, caffeine and screen use.
Sleep Inertia: The temporary period of grogginess and impaired performance immediately after waking, especially from deep sleep.
Sleep Latency (SOL): The length of time required to transition from full wakefulness to sleep once in bed.
Sleep Opportunity (SO): The period in a schedule that is realistically available for sleep, once travel, meals and other demands are considered.
Social Jetlag: The misalignment between an individual’s biological clock and their social schedule, commonly occurring between workdays and rest days.
Time-Zone Desynchronisation: Misalignment between the body clock and local time after rapid travel across time zones (jet lag).
Window of Circadian Low (WOCL): The specific period during the circadian cycle, typically night or early morning, when alertness and performance are at their lowest.
Zeitgeber: An external cue, such as natural light, that synchronises the body’s circadian rhythm to the 24-hour day.

Operational concepts & rostering

Terms used in duty limits, scheduling rules and day-to-day management of staff in safety-critical operations.

Anchor Sleep: A strategy of sleeping during the same time block every 24 hours to maintain circadian stability, often used during split duties.
Backward Rotation: A shift pattern that moves earlier in the day (e.g. Night to Afternoon), often compressing rest periods and increasing fatigue risk.
Commute Time: The time spent travelling between the place of rest and the place of duty, effectively reducing the available sleep opportunity.
Controlled Rest (CR): A planned, short nap taken under defined procedures during a duty period to reduce fatigue risk.
Controlled Rest on the Flight Deck (CRFD): Controlled rest taken by flight crew at the controls under specific aviation procedures and safeguards.
Crewing Optimisation: Balancing staffing levels, skills, legal requirements and fatigue risk when deciding how many people to roster.
Duty Recovery Period: A longer rest break, often following a sequence of duties, intended to allow recovery from accumulated fatigue.
Duty Time Limits (DTL): Regulatory or company limits on how long an individual can be on duty in a given period.
Duty/Rest Harmonisation: Designing duty and rest rules so they work together to control fatigue while allowing operational flexibility.
Fatigue-Critical Task: A task where impaired alertness could have serious safety, operational or clinical consequences.
Forward Rotation: A shift pattern that moves later in the day (e.g. Morning to Afternoon), generally considered less fatiguing than backward rotation.
High-Risk Duty Period: A duty or part of a duty where fatigue risk is judged to be elevated, for example due to timing or sequence.
Hours of Work (HoW): The total time spent on duty, as defined by labour law or sector-specific regulations.
Layover Rest Opportunity: The realistic chance to sleep and recover between duties while away from home base.
Minimum Rest Requirements (MRR): Rules that specify the least amount of rest that must be provided between duty periods.
Non-Rostered Duty: Work that is not planned in advance on the roster, such as call-outs, overtime or emergency tasks.
Operational Pressure: The perceived or actual pressure to maintain output, service levels or on-time performance, sometimes at the expense of rest.
Rostering / Scheduling Optimisation (RSO): Using rules, algorithms or software to build duty rosters that balance fatigue risk, regulation and operational demand.
Split Duty: A duty period divided by a significant break, with rules on how the break counts towards duty and rest requirements.
Work-Roster Variability: The degree to which start times, shift lengths and patterns vary from day to day or week to week.
Workload Management: How tasks are distributed over time and between people to avoid overload or underload that can worsen fatigue effects.

Measurement, modelling & tools

Tools, models and metrics used to estimate fatigue, monitor alertness and track risk over time.

Actigraphy / Actimetry: Wearable movement monitoring used to estimate sleep and wake patterns over days or weeks.
Alertness Curve: A representation of how alertness is expected to rise and fall over time, often based on circadian and sleep factors.
Alertness Monitoring Systems (AMS): Systems that track indicators of alertness or drowsiness in real time, often using cameras or sensors.
Baseline Performance: An individual’s normal level of performance or alertness when fully rested, used as a reference point for calculating impairment.
Biomathematical Models (BMM): Mathematical models that estimate fatigue or alertness over time from duty, sleep and circadian inputs.
Biometrics: Physiological measurements such as heart rate, eye movements or skin response used to infer fatigue or alertness.
Electrodermal Activity (EDA): Changes in skin conductance linked to arousal or stress, occasionally used in fatigue and workload research.
Fatigue Exposure Hours (FEH): An estimate of how many hours are worked under conditions where fatigue risk is elevated.
Fatigue Modelling: The use of biomathematical models or similar tools to predict fatigue risk for individuals or duty rosters.
Fatigue Risk Index (FRI): A composite score produced by some models to represent the relative level of fatigue risk in a duty pattern.
Heart-Rate Variability (HRV): Variation in the time between heart beats, sometimes used as a marker of load, stress or recovery.
Karolinska Sleepiness Scale (KSS): A simple self-rating scale where individuals score their current sleepiness level.
Micro-Sleep: Very brief episodes of sleep or near-sleep, often lasting a few seconds, that can occur without the person realising.
Objective Fatigue: Fatigue measured through observable data such as reaction time errors or eye-closure, independent of how the individual feels.
Oculometrics: Measures of eye behaviour, such as blink rate or gaze patterns, used as indicators of fatigue.
PERCLOS (Percentage of Eyelid Closure): A measure of drowsiness based on the proportion of time the eyelids cover the pupils over a set interval.
Performance Impairment Threshold: The level of fatigue at which performance is considered unacceptably degraded for a given task or role.
Predictive Fatigue Scoring: Assigning scores to duties or rosters based on model predictions of fatigue risk.
Psychomotor Vigilance Test (PVT): A simple reaction-time test used to measure attention and lapses associated with fatigue.
Subjective Fatigue: An individual’s self-reported feeling of tiredness, often assessed using scales like the KSS.
Task-Load Index: A numerical score representing perceived or measured workload during a task or duty.
Wearable Fatigue Monitors (WFM): Wearable devices that estimate sleep, activity or alertness from motion, heart or other signals.

Safety management, reporting & regulation

Terms used in FRMS frameworks, safety management systems, reporting processes and regulatory oversight.

Bio-compatible Rostering: Designing duty schedules that prioritise physiological limitations and sleep needs alongside operational requirements.
Error Management System (EMS): A structured way of identifying, reporting and learning from errors and near-misses.
Fatigue Audit: A review of rosters, processes and data to check whether fatigue risks are being properly managed.
Fatigue Hazard Analysis (FHA): Identifying where fatigue could contribute to errors or incidents within a system or process.
Fatigue Proofing: Designing tasks, procedures or environments to be robust against errors, acknowledging that fatigue cannot be entirely eliminated.
Fatigue Reporting System (FRS): A process that allows staff to report fatigue concerns, high-risk duties or events without fear of blame.
Fatigue Risk Management System (FRMS): A data-driven, risk-based approach to managing fatigue as part of an organisation’s overall safety system.
Fatigue-related Safety Event / Incident: An event or near-miss where fatigue is judged to have contributed to performance or decision-making.
Fitness for Duty (FFD): An individual’s ability to perform their work safely and effectively at a given time.
Hazard Identification (HAZID): Systematic processes for spotting hazards, including those related to fatigue, before they lead to incidents.
Just Culture: An approach that encourages reporting and learning, while distinguishing between human error and wilful violations.
Operational Mitigations: Temporary or permanent changes to operations that compensate for elevated fatigue risk.
Operational Risk Assessment (ORA): A structured review of risk for a specific operation, route, service or change.
Risk Assessment Matrix (RAM): A tool that combines likelihood and consequence to categorise risk levels and support decisions.
Risk Control Measures (RCM): The specific actions, rules or tools used to reduce an identified fatigue risk.
Safety Assurance (SA): Activities that confirm safety processes are effective and that risk remains under control.
Safety Case (SC): A structured argument, supported by evidence, that a system or operation is acceptably safe.
Safety Management System (SMS): The structured management framework that identifies, assesses and controls safety risks across an organisation.
Safety Performance Indicators (SPI): Selected measures used to track how well safety and fatigue controls are working.
Threat and Error Management (TEM): A framework for recognising and managing threats and errors during operations to maintain safety margins.
Voluntary Reporting Programme: A system that allows staff to submit safety or fatigue reports freely, often with protections on confidentiality.