Fatigue, shift work and hours of work

Fatigue is a state of reduced mental or physical capacity caused by inadequate sleep, extended wakefulness, work at times when the body expects to be asleep, or prolonged mental or physical effort. It is both a physical safety hazard and a psychological health hazard. Fatigued workers make more errors, react more slowly and are more likely to be injured or to injure others; over time, chronic fatigue and disrupted sleep are linked to anxiety, depression, burnout and physical illness. This lesson explains the science of sleep and circadian rhythms, the risks of shift work and long hours, the legal limits on hours in both countries and how to assess and control fatigue risk.

Sleep and the body clock

Two systems govern when we feel alert or sleepy. The first is sleep pressure, sometimes called the homeostatic drive. From the moment we wake, a chemical signal builds up in the brain that makes us progressively sleepier the longer we stay awake; sleep clears it. The second is the circadian rhythm, an internal clock of about 24 hours set mainly by light. It makes most people naturally most alert in the late morning and early evening, with a smaller dip in the early afternoon and a deep low between roughly 2 a.m. and 6 a.m. During that early-morning window, alertness, reaction time and judgment are at their worst, even for experienced night workers.

Most adults need about seven to nine hours of sleep a night. Getting less creates a sleep debt that accumulates over consecutive nights. A person who needs 7.5 hours but sleeps 5.5 hours for five nights carries a debt of 10 hours, and their performance can decline to levels similar to someone who has missed a full night of sleep, often without them realizing how impaired they are. Recovery usually takes more than one long sleep.

Extended wakefulness has effects similar to alcohol. Research published in the journal Nature in 1997 by Drew Dawson and Kathryn Reid found that after about 17 hours awake, performance on a tracking task fell to a level similar to that at a blood alcohol concentration of 0.05 percent, and after about 24 hours awake, to a level similar to 0.10 percent. A worker who wakes at 6 a.m. and is still working at 11 p.m. has been awake for 17 hours.

Shift work and night work

Shift work, especially night work and rotating shifts, forces people to work when the body clock expects sleep and sleep when it expects wakefulness. Daytime sleep is typically shorter and lighter than night-time sleep because of light, noise, family life and the circadian push to be awake. Most night workers never fully adapt, because they return to a daytime pattern on days off. The results include chronic sleep loss, fatigue, digestive problems, and higher risks of cardiovascular disease and metabolic disorders; the International Agency for Research on Cancer classifies night shift work as probably carcinogenic to humans. Shift work also strains family and social life, which affects psychological well-being.

Shift schedules can be designed to reduce these harms. Evidence-informed principles include:

• Rotate forward (day, then evening, then night) rather than backward, because the body clock adjusts more easily to later times.
• Limit consecutive night shifts, commonly to three or four, and consecutive 12-hour shifts, to limit the build-up of sleep debt.
• Avoid quick returns: allow at least 11 hours between the end of one shift and the start of the next, so workers can travel, eat and get a full sleep.
• Provide at least two full nights of rest after a block of night shifts.
• Keep shift start times sensible: very early starts, before about 6 a.m., cut into sleep, and very late finishes affect commuting safety.
• Give schedules well in advance and minimize last-minute changes, so workers can plan sleep and family life.
• Schedule the most demanding or safety-critical tasks away from the circadian low where possible, and allow breaks and, where policy permits, planned short naps.

Long hours and overtime

Long hours add to fatigue whether or not shift work is involved. Risk of error and injury tends to rise with shift length, especially beyond about 10 to 12 hours, and with consecutive long days. Mandatory overtime, on-call duty that disrupts sleep, long commutes and second jobs all reduce the time available for recovery. Remote and hybrid work can blur boundaries so that working hours quietly expand into evenings. As Lesson 5 noted, the WHO and ILO have linked working 55 or more hours a week with higher risks of stroke and heart disease.

Legal limits on hours

In the US, the Fair Labor Standards Act requires overtime pay for non-exempt employees beyond 40 hours a week but sets no maximum hours for adults. A few states restrict mandatory overtime in specific occupations, most commonly nursing, and some cities and states have predictable scheduling laws for retail and food service workers. Safety-critical sectors have federal hours-of-service rules. The Federal Motor Carrier Safety Administration's rules for property-carrying commercial drivers allow up to 11 hours of driving after 10 consecutive hours off duty, prohibit driving beyond the 14th hour after coming on duty, require a 30-minute break after 8 cumulative hours of driving, and limit drivers to 60 hours on duty in 7 days or 70 hours in 8 days, with a 34-hour restart option; electronic logging devices are required for most drivers. The Federal Aviation Administration's Part 117 sets flight and duty limits and rest requirements for passenger airline pilots, and the Federal Railroad Administration enforces statutory hours-of-service limits for railroad workers.

Complying with legal limits does not by itself mean fatigue is controlled. A worker can be within the hours limits and still be dangerously tired because of poor-quality sleep, a sick child, a long commute or a second job. That is why many safety-critical industries now use fatigue risk management systems.

Fatigue risk management

A fatigue risk management system (FRMS) is a data-driven, risk-based approach to managing fatigue, used in aviation, rail, mining, oil and gas, health care and other industries. Instead of relying only on prescriptive hours limits, an FRMS combines several layers of defense: scheduling that allows adequate sleep opportunity; checking whether workers actually obtained enough sleep; recognizing signs of fatigue on the job; reporting and responding to fatigue-related errors and incidents; and learning from data. It requires a policy, clear responsibilities, training for workers and supervisors, a reporting system that does not punish people for reporting fatigue, and regular review.

One widely used check in fatigue management comes from Australian research by Drew Dawson and Kirsty McCulloch. It compares the sleep a worker has had with the time they will be awake. A common version uses three rules: at least 5 hours of sleep in the previous 24 hours; at least 12 hours of sleep in the previous 48 hours; and the hours awake by the end of the shift should not exceed the hours slept in the previous 48 hours. A worker who does not meet these thresholds is at higher risk, and the response might be to discuss it with a supervisor, assign less safety-critical tasks, increase supervision, allow a nap or, in safety-critical roles, stand the worker down.

Recognizing fatigue

Signs of fatigue include yawning, heavy eyelids, rubbing eyes, head nodding, staring blankly, slow reactions, forgetfulness, poor concentration, irritability, reduced communication and microsleeps, brief episodes of sleep lasting a few seconds that a person may not notice. Fatigue impairs self-assessment, so a tired worker often underestimates how impaired they are. Workplaces should make it easy and safe to say "I'm too tired to do this safely" and should treat fatigue reports as useful safety information, not a disciplinary matter. Supervisors should also watch for patterns: repeated overtime, back-to-back shifts and workers who seem to be holding second jobs.

Controls for fatigue

Using the hierarchy of controls, the most effective fatigue controls are in the design of work: adequate staffing so that overtime is not routine; schedules that follow the principles above; limits on consecutive shifts and total hours; and avoiding safety-critical tasks at the circadian low. Environmental controls include good lighting, comfortable temperatures, places for rest breaks and, where appropriate, napping facilities. Administrative controls include fatigue policies, training, reporting systems, task rotation and fatigue checks. Individual measures include good sleep habits, managing caffeine, avoiding alcohol before sleep, darkening the bedroom for daytime sleep and seeking help for sleep disorders such as sleep apnea, which is common and treatable. Safe travel home after night shifts, such as transport or a place to rest, can prevent fatigue-related crashes.