According to the Bureau of Labor Statistics almost 15 million American workers or roughly 16% of workers work on night or rotating shifts. This occupational accommodation is not always well tolerated and may result in misalignment of the circadian influenced physiology with the environment producing the symptoms of shift work disorder. This chapter will explain the pathophysiology and treatment of this common circadian rhythm disorder and discuss the implications of the voluntary desynchronization of the natural world to the artificial occupational world. Individuals who go through this voluntary dissociation of their sleep-wake cycle from other circadian rhythms may suffer sleep disturbance and insufficient sleep accompanied by sleepiness during working hours. They also incur greater health risks and increased risk of accidents. The review incorporates the basic understanding of the circadian clock biology and its interactions with the sleep-wake cycle with the aim of improving the adaptation to shift work based on circadian principles, thus, reducing morbidity and increasing shift work tolerance.
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• The circadian rhythm evolved as a mechanism to prepare the body systems for upcoming predictable tasks and environment to promote alertness during the day and effective sleep at night.
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• The circadian adaptation to night shift work is only partial, at best, due to other time clues and activities giving opposing signals, creating a mismatch of circadian rhythms and behavioral activities.
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• Night shift work is associated with sleepiness during work hours and poor sleep during off-work hours.
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• Shift work tolerance varies for each individual and may be genetic and age-dependent.
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• Night shift work may be associated with increased risk of metabolic syndrome, cardiovascular disease, and possibly cancer.
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• Careful planning and appropriate use of time clues may offset some health effects of night shift work.
Historical note and terminology
Shift work is not a new phenomenon. The history of shift work can be traced back to early nomadic tribes that required camp guards and shepherds to be awake and vigilant during normal sleeping hours. The ancient Greeks and Romans used candles and flaming torches to provide light at night to help people to stay awake during the night. Soldiers guarded military camps to ensure protection against surprise attacks by enemies. Sailors worked a night watch to make sure the ship did not run aground. As civilizations progressed and artificial lighting became more prominent, communication and transportation of goods during the night expanded, resulting in “24/7” societies.
The 19th century Industrial Revolution, followed by urbanization, demanded the expansion of shift work. Gas and electric lamps made shift work more attainable, and large factories took advantage of the economics of continuous processing to make production more profitable. In the 20th century, social pressures forced companies to reduce the work shift to an 8- to 12-hour day, resulting in an influx of more workers on the job, and more workers exposed to shift work. Additionally, the globalization of many companies and industries demanded the ability for 24/7 communication and availability of goods, resulting in the further growth of shift work.