Actigraphy / Actimetry
Wearable movement monitoring used to estimate sleep and wake patterns over days or weeks.
Acute Fatigue
Short-term fatigue arising from one extended shift, late duty, or single night of restricted sleep.
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.
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.
Baseline Performance
An individual’s normal level of performance or alertness when fully rested, used as a reference point for calculating impairment.
Bio-compatible Rostering
Designing duty schedules that prioritise physiological limitations and sleep needs alongside operational requirements.
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.
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 requires several consecutive nights of unrestricted sleep to resolve.
Circadian Phase
The current position of an individual’s body clock within the 24-hour cycle, which strongly influences alertness, performance and sleep propensity.
Circadian Rhythm (CR)
The roughly 24-hour internal body clock that regulates sleep–wake patterns, alertness, temperature and hormone levels.
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.
Cumulative Fatigue
Fatigue that builds over several days or weeks due to repeated insufficient sleep or demanding duty patterns.
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.
Error Management System (EMS)
A structured way of identifying, reporting and learning from errors and near-misses.
Fatigue
A state of reduced physical or mental performance caused by insufficient sleep, long wakefulness, workload or time-of-day effects.
Fatigue Audit
A review of rosters, processes and data to check whether fatigue risks are being properly managed.
Fatigue Exposure Hours (FEH)
An estimate of how many hours are worked under conditions where fatigue risk is elevated.
Fatigue Hazard Analysis (FHA)
Identifying where fatigue could contribute to errors or incidents within a system or process.
Fatigue Liability
The inherent fatigue risk associated with a duty pattern before mitigations are applied.
Fatigue Modelling
The use of biomathematical models or similar tools to predict fatigue risk for individuals or duty rosters.
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 Index (FRI)
A composite score produced by some models to represent the relative level of fatigue risk in a duty pattern.
Fatigue Risk Management System (FRMS)
A data-driven, risk-based approach to managing fatigue as part of an organisation’s overall safety system.
Fatigue-Critical Task
A task where impaired alertness could have serious safety, operational or clinical consequences.
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.
Fixed Shifts
A work pattern in which employees consistently work the same shift type (e.g. permanent nights or permanent early mornings). Fixed shifts can allow better circadian adaptation than rotating patterns but may still carry long-term health and fatigue risks.
Forward Rotation
A shift pattern that moves later in the day (e.g. Morning to Afternoon), generally considered less fatiguing than backward rotation.
Hazard Identification (HAZID)
Systematic processes for spotting hazards, including those related to fatigue, before they lead to incidents.
Heart-Rate Variability (HRV)
Variation in the time between heart beats, sometimes used as a marker of load, stress or recovery.
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.
Homeostatic Recovery
The biological process of restoring alertness and performance through sleep and rest following accumulated fatigue.
Hours of Work (HoW)
The total time spent on duty, as defined by labour law or sector-specific regulations.
Impairment Risk
The overall risk of degraded performance due to fatigue, illness, medication, alcohol, stress or other physiological factors.
Just Culture
An approach that encourages reporting and learning, while distinguishing between human error and wilful violations.
Karolinska Sleepiness Scale (KSS)
A simple self-rating scale where individuals score their current sleepiness level.
Layover Rest Opportunity
The realistic chance to sleep and recover between duties while away from home base.
Light Exposure Schedule
A planned pattern of light and dark exposure used to support circadian adjustment or maintain alertness.
Micro-Sleep
Very brief episodes of sleep or near-sleep, often lasting a few seconds, that can occur without the person realising.
Minimum Rest Requirements (MRR)
Rules that specify the least amount of rest that must be provided between duty periods.
Night Shift / Night Work
Work performed during the biological night, typically between 23:00 and 06:00, when circadian alertness is at its lowest and sleep pressure is high. Night work is consistently associated with elevated fatigue risk, reduced performance and increased error
Non-Rostered Duty
Work that is not planned in advance on the roster, such as call-outs, overtime or emergency tasks.
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.
Operational Mitigations
Temporary or permanent changes to operations that compensate for elevated fatigue risk.
Operational Pressure
The perceived or actual pressure to maintain output, service levels or on-time performance, sometimes at the expense of rest.
Operational Risk Assessment (ORA)
A structured review of risk for a specific operation, route, service or change.
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.
Presenteeism
The practice of attending work despite being unfit to perform safely or effectively due to fatigue, illness or stress.
Psychomotor Vigilance Test (PVT)
A simple reaction-time test used to measure attention and lapses associated with fatigue.
Readiness to Work (RTW)
An assessment of whether an individual is fit to begin a duty safely, taking into account fatigue, sleep history, health, workload and time-of-day factors.
Restorative Sleep (RS)
Sleep of sufficient quality and duration to return performance and alertness close to baseline levels.
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.
Rostering / Scheduling Optimisation (RSO)
Using rules, algorithms or software to build duty rosters that balance fatigue risk, regulation and operational demand.
Rotating Shifts
A work pattern in which employees cycle through different shift times (e.g. mornings, afternoons, nights) according to a roster. Rotating shifts can increase fatigue by repeatedly disrupting sleep timing and circadian adaptation.
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.
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.
Shiftwork
Any work schedule that falls outside traditional daytime hours and requires individuals to work evenings, nights, early mornings or irregular patterns. Shiftwork is a primary driver of fatigue risk due to its disruption of normal sleep timing, circadian a
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 Fragmentation
Repeated awakenings or disturbances that reduce sleep quality even when total sleep time appears adequate.
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.
Sleep Pressure (Homeostatic Sleep Drive)
The biological drive for sleep that builds with time awake and dissipates during sleep. Alongside the circadian rhythm, sleep pressure is one of the two main processes governing alertness and performance.
Split Duty
A duty period divided by a significant break, with rules on how the break counts towards duty and rest requirements.
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.
Threat and Error Management (TEM)
A framework for recognising and managing threats and errors during operations to maintain safety margins.
Time-Zone Desynchronisation
Misalignment between the body clock and local time after rapid travel across time zones (jet lag).
Voluntary Reporting Programme
A system that allows staff to submit safety or fatigue reports freely, often with protections on confidentiality.
Wearable Fatigue Monitors (WFM)
Wearable devices that estimate sleep, activity or alertness from motion, heart or other signals.
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.
Workload Management
How tasks are distributed over time and between people to avoid overload or underload that can worsen fatigue effects.
Work-Roster Variability
The degree to which start times, shift lengths and patterns vary from day to day or week to week.
