Shiftwork is a term used to describe an arrangement of working hours that differs from the standard daylight working hours (i.e., 8:00 a.m. to 5:00 p.m.). Organizations that adopt shiftwork schedules extend their normal working hours beyond the traditional eight-hour shifts by using successive teams of workers. Notable examples of organizations that adopt shiftwork schedules include hospitals, fire stations, and police stations. However, forces such as industrialization, new technologies, and the increasing global economy have contributed to the creation of a society that operates 24 hours a day. This 24-hour society has led to an increase in the need for shiftwork. In fact, it is currently estimated that 15% to 30% of all workers in industrialized societies are involved in some type of shiftwork. Although shiftwork remains more common in certain occupations (e.g., process-control industries, emergency services, transport), the growth of shiftwork systems is expected to continue at a rapid pace.

The types of shiftwork systems that organizations adopt differ on a wide array of characteristics, such as the number and length of shifts. For example, one organization may adopt two 12-hour shifts, whereas another may adopt three 8-hour shifts. Shiftwork systems can also differ in the direction and speed of shift rotation. Shift systems that rotate employee schedules from morning shifts to evening shifts to night shifts have a forward rotation, whereas shifts that rotate counterclockwise (i.e., night to evening to morning) have a backward rotation. With regard to the speed of rotation, shift systems fall into three major categories: (a) permanent shift systems (e.g., permanent night shift); (b) slowly rotating shift systems (e.g., weekly rotating); and (c) rapidly rotating shift systems (e.g., an employee works the morning shift on Monday, the evening shift on Tuesday and Wednesday, and the night shift on Thursday and Friday).

A recent review of shift systems produced five general recommendations regarding the design of shiftwork systems. First, it seems that night work should be reduced as much as possible; however, if this is not possible, an organization should adopt a rapidly rotating system. Second, long shifts (e.g., 9 to 12 hours) should be avoided. Third, flexible work arrangements should be integrated with shift systems. Fourth, shift changes within the same day should be avoided, and the number of consecutive days worked should be limited. The final recommendation suggests that forward rotation is most preferable.

Although shift systems remain highly popular with employees on the front end because they seem to provide a degree of flexibility, research investigating shiftwork has found that such schedules have primarily negative effects for both individuals and organizations. The problems associated with shift systems fall into three broad categories: disturbance of circadian rhythms, physical and psychological ill health, and social and domestic disruption.

Disturbance of Circadian Rhythms

A great deal of research has investigated the impact of shiftwork on individual circadian rhythms. In general, humans have evolved over thousands of years as a species that habitually sleeps during the night and is awake during the day. The rotation of the earth around the sun creates a 24-hour cycle of light and dark, which is internalized by humans and forms a natural internal body clock. All human circadian rhythms normally show a fixed-phase relationship. For example, body temperature peaks around 8:00 p.m., and all other circadian rhythms reach their maximum at the appropriate time, allowing us to eventually fall asleep at night.

Problems occur for shiftworkers as a result of the mismatch between environmental time cues and the internal timing system. Although the natural light-dark cycle, the clock time, and other social cues may remain the same, the timing of shiftworkers’ work and sleep is delayed. Evidence suggests that adjustments to the shiftworkers’ body clock are slow, if they occur at all. This mismatch between the environment and the internal body clock has been linked to negative outcomes such as sleep deprivation.

Psychological and Physical Ill Health

Most of the early work on the psychological outcomes of shiftwork focused on the exploration of shiftwork-ers’ attitudes, such as job satisfaction. This research suggests that, in general, although workers welcome the idea of shift systems up front, they are typically less satisfied with their work than nonshiftworkers. Additionally, the research generally shows that psychological and emotional distress accompanies shiftwork; however, these effects are often small. Some studies failed to find any psychological differences between shiftworkers and nonshiftworkers. For example, two recent studies found no differences between shiftworkers and nonshiftworkers in variation of mood and depressive symptoms. Thus, in general, though evidence suggests that shiftworkers are generally less satisfied with their jobs, other emotional and psychological outcomes, such as depression, are hardly affected.

Much more research has explored the physical consequences of shiftwork. Research has found sleep to be extremely disrupted by shiftwork. In general, many bodily functions are at their highest level of activity during the day. Thus, it is often difficult for individuals to sleep during the day because they are attempting to sleep at a time that is not natural for their circadian rhythm. The most prominent outcome of this lack of quality sleep is chronic fatigue.

Chronic fatigue is linked with greater incidence of physical injury. In general, a greater number of serious job-related injuries occur among employees who work night shifts. Additionally, night shift workers are more likely to be involved in automobile accidents on the drive home from work than day shift workers. Thus, the increased risk of injury seems clear. However, several potential confounds must be considered— for example, night shift workers are often less experienced and work with less supervision.

By far, the most prevalent health complaint associated with shiftwork is gastrointestinal problems. According to a recent study, 20% to 75% of night and shiftworkers complain of gastrointestinal problems such as irregular bowel movements and constipation, compared with 10% to 25% of nonshiftworkers. Although some research has found no difference between day and shiftworkers in gastrointestinal disease, the consensus is that these types of disorders are more prevalent in shift- and night-working populations. One explanation for the increase is that shift-workers have less regular eating schedules and may have less access to healthful foods.

The relationship between cardiovascular disease and shiftwork has also been explored. Though there has been much debate, recent studies all seem to support a relationship between shiftwork and cardiovascular disease. Many characteristics of shiftworkers are considered predictors of cardiovascular disease (e.g., poor eating habits, gastrointestinal disorders, sleeping disorders, less favorable working conditions). Thus, the risk of cardiovascular disease should be a concern for shiftworkers.

Aside from chronic fatigue, injury, digestive disorders, and cardiovascular disease, shiftwork has additionally been shown to have negative effects on the reproductive cycle of women (e.g., increased menstrual pain and lower rates of pregnancy) and to influence drug activity and effectiveness. The latter point suggests that persistent shift or night work may be incompatible with the efficacious treatment of disease.

Social and Domestic Disruption

In addition to the psychological and physical effects, shiftwork is related to several social and domestic variables. For example, although organizations may believe that it is advantageous to operate on a 24-hour schedule, estimates place the cost of shiftwork among U.S. companies at $70 billion per year. Research has shown higher rates of absenteeism among shiftwork-ing populations. Thus, the $70 billion cost results in part from lost productivity because of absenteeism and higher medical bills because of increased injury and accidents. Not only are many of these job-related accidents harmful to the company and dangerous for the worker, but also these careless accidents can have detrimental societal consequences. Additionally, shiftwork is associated with a decreased ability to balance work and nonwork responsibilities. In fact, divorce rates for shiftworkers are up to 60% higher than those for day workers.

Individual Differences and Social Support

Several individual difference variables have been shown to be important to the relationship between shiftwork schedules and outcomes. Several of these individual difference variables involve individual cir-cadian types. For example, morningness, or a preference for going to bed early and rising early in the morning, is moderately associated with difficulty adjusting to night work. Additionally, sleep flexibility (i.e., the ability to sleep at unusual times) and vigor (i.e., the ability to overcome drowsiness) predict an individual’s level of tolerance for shiftwork.

In addition to differences in circadian type, age and personality are frequently investigated individual differences. With regard to age, the older an employee is, the less tolerance he or she will have for shiftwork. Over the age of 50, it becomes increasingly difficult for individuals to alter their sleep-wake cycles. In addition, many physical ailments increase with advancing age, and this increase in physical problems affects older individuals’ ability to adjust to shiftwork. In general, it is recommended that shiftwork be voluntary after the age of 40. With regard to personality, it has been found that introverts are generally more morning oriented than extroverts, making it more difficult for them to adjust to shiftwork. Neuroticism has also been linked to lower levels of shiftwork tolerance. However, some evidence suggests that neuroti-cism is an outcome of prolonged shiftwork exposure. Thus, the exact role that neuroticism plays in shiftwork tolerance is not yet understood.

Another individual difference variable that has been explored is the amount of social support an individual experiences. In general, results suggest that supervisor support is extremely important in buffering the negative effects of work stress, and the positive effects of support seem to be particularly important for shiftworkers. Thus, it is extremely important to encourage supervisors to take an active interest in the well-being of their shiftworkers.


Research suggests that shiftwork has negative effects for individuals, organizations, and society. These effects are many and serious. However, this does not mean that shiftwork should be abandoned. For many organizations, shiftwork is a necessity. These organizations need to understand not only how individual differences affect shiftwork tolerance but also, perhaps more importantly, how to design a shiftwork system that is minimally detrimental to employees. Although some research has been conducted, researchers should focus their attention on designing optimal shiftwork systems.


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