The tragedies of Chernobyl, Three Mile Island and the Exxon Valdez all occurred during the night shift.

Fatigue is a critical occupational safety concern for shift workers, especially workers in the transportation industry. Off the job, being overtired creates a risk for anyone who undertakes an activity that requires concentration and quick response -- from driving, to home repair, to skiing. And exhaustion is one of the most common health complaints for Canadian workers, especially women.

How sleep affects safety

Irregular working hours, monotonous work, long shifts, vibrations and drugs increase drowsiness and reduce alertness. The costs are devastating in human terms, and the economic consequences are enormous. Worldwide, the National Institute for Working Life, a Swedish organization, estimates that sleep-deprived workers cost $350 billion US per year.

Sleep is as basic to survival as food and water. Losing as little as two hours of sleep can negatively affect alertness and performance. Sleep deprivation affects a person\'s carefulness and ability to respond to an emergency. Symptoms can include: decreased judgement, decision-making and memory; slower reaction time; lack of concentration; fixation; and worsened mood.

Studies monitoring brain activity show that one shift worker in five dozes off during the shift. Often, they do not realize afterwards that they have done so. Drowsy drivers, according to sleep researchers, may cause as many crashes as impaired drivers. Regardless of motivation, professionalism, training or pay, an individual who is very sleepy can lapse into sleep at any time, despite the potential consequences of inattention.

The circadian clock

The body\'s processes have peaks and low points during every 24-hour period. These are called circadian rhythms. Time cues -- such as sunlight and work/rest schedules keep the circadian clock \"set.\" Crossing time zones or changing from a day shift to a night shift forces the circadian clock to move to a different schedule. Time is required to adjust to the new schedule. During the transition, symptoms similar to sleep loss can occur.

Disruption of the circadian rhythm when combined with loss of sleep can create a dangerous increase in fatigue.

Factors in the work environment

The environment and nature of the work can further magnify the effects of sleep debt and circadian rhythms. Environments with dim lighting, limited visual acuity (e.g. due to weather), high temperatures, high noise and high comfort tend to enhance fatigue. Also, a worker\'s susceptibility to fatigue is increased by tasks where attention must be sustained for long period, and those which are long, repetitive, paced, difficult, boring and monotonous.

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