EHS Management

How Tired Is Too Tired? Gauging Fatigue with a Fatigue/Risk Index

On July 27, 2017, tractor-trailer driver William Jones was traveling eastbound on I-70 through Putnam County, Indiana, when he lost control of his truck, crossed the median and westbound traffic lanes, left the roadway, struck some trees, and rolled the tractor-trailer. Jones was hospitalized with non-life-threatening injuries. Investigators at the scene believe that Jones fell asleep at the wheel of his truck—a victim of fatigue on the job.

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Are your workers dangerously tired? If they are working very long hours or are on a shiftwork schedule, it’s possible. If their jobs are physically demanding, that’s another checked box in the “fatigue” column. But how can you tell when workers are too fatigued to be safe and productive on the job? If you want to put a numeric value on it, there’s an online tool that will help you do that. Developed by Great Britain’s Health and Safety Executive (HSE), it’s called the Fatigue/Risk Index, and it consists of a downloadable spreadsheet and a set of instructions for completing the sheet and interpreting the results.

Quantifying Fatigue and Risk

The index distinguishes between “fatigue”—a measure of how tired the worker feels—and “risk”—the accident risk involved in working, because these two factors peak at different times of day. Accident risk tends to peak around midnight, while fatigue peaks in the early morning, around 5 a.m.

In order to accurately quantify fatigue and risk, using the HSE tool or any other, it is necessary to account for multiple factors that include:

  • Cumulative effects of a worker’s schedule. The risk and fatigue associated with any given shift are affected by the structure of the worker’s other shifts that precede it. In particular, the interval between two successive shifts should be long enough to enable workers to get enough sleep; 16 hours is considered the minimum necessary between successive night shifts. For workers who work long shifts (longer than 8 hours) over more than 7 consecutive days, the amount of time they need to recover appears to be linked to the amount of sleep lost during their extended work schedule. When workers do not get enough restorative sleep, their cognitive performance suffers along a dose-response curve. When workers get less than 7 hours of sleep over a period of days, their decline in cognitive performance is cumulative.
  • Timing of the worker’s shift. Because fatigue and risk vary by time of day, the start time and shift length will affect both fatigue and risk. Risk and fatigue both increase as the amount of time spent on a specific task increases.
  • The job type and breaks. What the worker does, and the breaks provided, affect fatigue and risk, although the relationship between breaks and risk is not well-established. In general, breaks are believed to improve alertness and cognitive performance, but information about optimal timing and length of breaks is limited.

The fatigue/risk index tool can help you evaluate these factors to identify the most serious fatigue risks in your workforce and to identify modifications that can reduce that risk.

Tomorrow we’ll look at how to create a fatigue risk management plan.

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