Shift Work Sleep Disorder

Shift work sleep disorder, also called shift work disorder, is a circadian rhythm sleep disorder characterized by symptoms of tiredness, fatigue, difficulty sleeping and excessive daytime sleepiness. Shift work sleep disorder can lead to accidents and injuries at work as well as errors in judgment and absenteeism. Unlike other circadian rhythm sleep disorders, shift work sleep disorder is an occupational hazard and resolves completely with a switch to a regular daytime schedule.


Sleep is a natural state of bodily rest observed in humans and other animals. We have known for quite some time that sleep is vital to health. The reason sleep is necessary for us is, however, not well understood. It does not appear that sleep gives rest to the body, as most biological functions continue just as during wake periods. It is believed that the brain gets the much needed time to process thousands of pieces of information obtained during wake periods (garbage in, garbage out). Other theories suggest that sleep is needed to master certain skills like riding a bike (procedural memory) and memorization (declarative memory)

Humans cycle between two phases of sleep, rapid eye movement (REM) and non-rapid eye movement (NREM), in 90 minute cycles. REM sleep, also known as the dream sleep, is associated with vivid dreams and loss of muscle tone and is concentrated towards the latter part of the sleep. NREM or non dream sleep is further divided into three stages (stage 1, 2 and 3). During the early part of the sleep, humans spend a substantial part of sleep in stage 3, characterized by distinctive low frequency electrical waves. Stage 3 is also one of the most restorative parts of sleep.


Sleep is controlled by two powerful processes: circadian rhythm and homeostasis. During waking hours, the sleep drive gradually increases until it reaches a critical threshold. This drive is referred to as homeostasis. Circadian rhythm, on the other hand, is a signal generated by the master clock in the human brain, the suprachiasmatic nucleus (SCN) located in the anterior hypothalamus. Circadian rhythm, derived from the Latin term “circa diem”, which literally means “approximately one day” is the body’s internal clock. This clock is set at slightly over 24 hours.

It controls sleep as well as most biological processes, including hormone production, metabolism, core body temperature variations and cell regeneration among others. This clock is normally highly synchronized to environmental cues, or zeitgebers (German for “time giver”), the strongest of them being the light-dark cycle. In most humans the sleep pattern shows a biphasic distribution, with a mid-day decrease in alertness around 2-4 pm, followed by an increased alertness during mid to late evening, and finally declining to its lowest levels during the night.

Almost all physiological systems in humans run slightly over a 24-hour cycle. Disturbance of this well-regulated circadian rhythm and homeostatic drive can lead to various sleep disorders collectively known as circadian rhythm sleep disorders. While a host of sleep disorders can impact work performance, the scope of this chapter is limited to sleep-related disorders potentially caused by work and work schedules, including shift work sleep disorder (SWSD, which is also called shift work disorder (SWD). These two terms will be used interchangeably in this chapter, as both appear in the medical literature on this topic. The severity of impact is determined by the type of shift work and frequency of shift rotation.

Shift work is defined as an employment practice designed to make use of the whole 24 hours of the day, rather than a standard “nine to five”-type business hours work schedule. Typically it is either divided into three, eight-hour shifts or two, twelve-hour shifts. Some job categories and professions that often require shift work include:

• Hospital staff (nurses, physicians, nurses assistants)

• Retail cashiers

• Waiters, bartenders, cooks and other food service workers

• Law enforcement and firefighters

• Operators of heavy vehicles and machinery (e.g., truck drivers)

• Public transportation operators

• Airline crews (e.g., pilots, stewards)

• Manufacturing workers

These occupations may involve a significant disruption of sleep pattern and circadian rhythm, resulting in shift work sleep disorder in susceptible individuals. Two examples of major disasters related to sleep deprivation and/or disarray include the 1986 space shuttle Challenger incident, which resulted in a destruction of the space shuttle and its crew members within 80 seconds of its launch, and the Chernobyl nuclear disaster. Many other known and unknown catastrophes have been related to sleep deprivation as a result of sleep disorders. As we increasingly become a 24-hour society and thus expect more workers to perform shift work, the prevalence of shift work disorder is likely to increase in the coming years.

Shift Work and Cardiovascular Disease

Data on the effect of shift work and its impact on cardio-vascular diseases has been expanding. Several studies have pointed out an association with shift work and blood pressure. There is also a greater variation in blood pressure in a 24-hour time period in employees involved in frequently rotating night shift work. A recent retrospective study performed at the University of Michigan targeting nursing staff shows an increased risk of developing a cerebrovascular accident in women with a long history of night shift work along with frequent shift rotation, after adjusting for all other variables. A total of 80,108 subjects were available for analysis from the Nurses Health Study database. Sixty percent reported at least 1 year of rotating night shift work. Each 5 years of rotating night shift work was associated with a 4% increase in the risk of ischemic stroke.

Shift work has also been associated with an increase in the risk of ischemic heart disease. The study was done among Japanese men comparing daytime workers with fixed night shift workers and frequently rotating shift workers. In this prospectively designed study they found an increase in the risk of death due to ischemic heart disease in rotating shift workers, but not in fixed night time workers compared to the control population. The difference remained after adjusting for other confounding variables.

Another prospective study by Tenkanen and colleagues followed over 1,800 male industrial workers for seven years and found that after adjusting for confounding factors the risk of ischemic heart disease was higher for all shift workers compared to all day workers, with a hazard ratio of 1.38. Other studies however have not found similar associations. A meta-analysis looking at 14 relevant studies in shift workers and the risk of ischemic heart disease revealed only a modest positive association. This was relevant for studies analyzing both fatal and non-fatal outcomes. The association was weaker if studies analyzing only fatal outcome were considered.

Women with heart disease tend to have a poorer prognosis than male counterparts. Increased incidence of coronary artery disease has also been found in women who have been shift workers for six years or longer. This study revealed relative risks of 1.19 for fatal cases, 1.34 for non-fatal cases and 1.31 for the combined risk. Studies have also reported higher triglyceride levels and lower HDL levels in shift workers.

Shift Work and Weight Gain

A few studies have shown that shift work leads to a tendency to gain weight while others have not found this association. Most of these were cross-sectional studies. Recently two large longitudinal studies however support the observation that shift work is related to weight gain. A longitudinal cohort study by Suwazono and colleagues followed a large group of factory workers for 14 years. They found that compared to day shift workers, 43% of night shift workers developed a ≥5% increase in body mass index (BMI), 25% had a ≥7.5% increase, and 15% developed a ≥10% increase in BMI (2008).

Shift Work Disorder and Cancer

Several studies have looked at the relationship of shift work and the occurrence of cancer. They have found an increased incidence in prostate, colorectal and endometrial cancers among employees involved in shift work compared to employees who work regular work hours.

Shift Work Disorder and Diabetes

A positive association between years in rotating night shift work and diabetes has been shown in a study done at the University of California. This association was linked with excess body weight. Shift workers may also have increased risk of smoking and reduced ability to quit smoking. Shift work has also been associated with an increase in the risk of developing Parkinson’s disease, depressed immune function and risk of being involved in motor vehicle accidents.

© Copyright 2010 LexisNexis. All rights reserved. This article was excerpted from Occupational Injuries and Illnesses (upcoming 2010 Edition), Brian J. Caveney, MD, JD, MPH, Editor-in-Chief.


  • 11-29-2012

I think the sleep is for dreaming only :) your other life