Overview of Shift Work
Nowhere are the effects of lifestyle on sleep more evident than in shift work disorder, also called shift work change (SWC) and shift lag. As the global marketplace continues to turn the time-to-productivity ratio to its favor, more and more employees are needed to work unconventional shifts. Nearly 20% of employees in industrialized countries are employed in shift work, which requires them to drastically change their sleep habits weekly or even daily.
While there are few statistics for the prevalence of shift work disorder, approximately 20% of shift workers report falling asleep during work, which increases the risk of industrial accidents and decreases productivity. Ironically, shift work can diminish the economic gain it is designed to create.
Causes for Shift Work Disorder
There are two types of shift work. Employees can either (1) work an unconventional nonfluctuating shift, like 11 p.m. to 7 a.m., or can (2) alternate between the three different shifts. Both versions of shift work produce a specific set of effects.
A person can usually adjust to working a new shift, if the change is permanent. Although the worker may have to get used to sleeping during daylight, his or her circadian rhythm can adjust to the body's new sleep-wake routine. It is common for a person who sleeps from 8 a.m. to 4 p.m. consistently to function productively at work from 11 p.m. to 7 a.m. Circadian rhythms operate on a 24-hour cycle. In nonfluctuating shift work, the shift in circadian rhythm remains constant once the body adapts to it. Resynchronization may take a while, but it is possible.
Shift work change affects circadian rhythm, which, similar to delayed sleep phase syndrome and jet lag, desynchronizes the body's sleep-wake schedule. This happens when shift workers toggle betweem the three common shifts, each one-third of the 24-hour day. The first shift usually runs from 7 a.m. to 3 p.m.; the evening or second shift generally lasts from 3 p.m. to 11 p.m.; and the night shift is usually from 11 p.m. to 7 a.m. Many shift workers frequently change shifts, thus intensifying the severity of circadian rhythm disturbance.
The body simply cannot rest and rebuild when circadian rhythms are frequently disrupted. Sleep-wake routines vary with continually changing external cues, known in the sleep medicine world as "zeitgebers," the German word for "timer." In shift work disorder, zeitgebers such as daytime and nighttime are never permanently synchronized with shift end time and shift start time. For SWC patients, predisposition to sleep and wake is governed by consistently mistimed circadian rhythm and alternating external cues.
For example, a person may work the night shift for five nights in a row, followed by two days off. During the two days off, the person resumes a normal daytime (diurnal) activity with family or friends. This disrupts the person's previously adjusted circadian rhythm, and he or she must readjust their sleep-wake pattern when they go back to work. Without a constant pattern, biological rhythms remain out of synch.