Description

Shift work disorder (SWD), classified as a circadian rhythm sleep-wake disorder, is characterized by symptoms of insomnia and/or excessive sleepiness when required to work during usual sleep times (1). SWD is caused by a misalignment between the internal circadian rhythm and the required sleep-wake schedule defined by nontraditional work shifts including night shifts, afternoon or evening shifts, early morning shifts, irregular shifts, and rotating shifts (2).

Epidemiology

Prevalence

• In the United States, approximately 20% of employed adults are shift workers, particularly in service-related occupations such as health care, protective services, transportation, and food services (2).
• SWD has an estimated prevalence of approximately 2–5% in the general population of the United States (2).
• Prevalence of SWD rises with age, particularly for adults age >50 years (1).

Etiology and Pathophysiology

• Sleep disturbances in shift workers are explained by disruptions in two biologic processes. The homeostatic sleep drive and circadian rhythm of alertness interact with each other to regulate sleep. The homeostatic sleep drive increases with wakefulness and decreases with sleep. Alertness is regulated by the suprachiasmatic nucleus (SCN) of the anterior hypothalamus and promotes wakefulness during the daytime and sleep at night. As homeostatic sleep drive increases during the day, the circadian rhythm of alertness increases and vice versa (2),(3).
• Circadian rhythms play a role multiple biologic functions, including regulation of body temperature, hormone levels, blood pressure, metabolism, cellular regeneration, sleep-wake cycles, and DNA transcription and translation (4). This internal biologic clock fluctuates with 24-hour cycles and is primarily regulated by external and environmental cues (3).
• The strongest influence on circadian rhythm is light. Light entering the eyes stimulates the retinohypothalamic and retino-geniculo-hypothalamic pathways to the SCN of the hypothalamus, subsequently suppressing melatonin from the pineal gland. During the day, melatonin levels are low, and the circadian rhythm of alertness is high (2).
• Periods of darkness cause the SCN to induce melatonin release from the pineal gland, which can also help to reset the molecular clock (4).
• In shift workers, there is a misalignment between one’s endogenous circadian rhythm of sleep and wakefulness and a sleep-wake schedule based on nontraditional shift work. Dyssynchrony results in excessive sleepiness during the work shift and/or insomnia during desired sleep time (2),(3),(4).

Genetics
Not all shift workers experience SWD and there is variability in symptoms occurring among shifts workers with SWD. One possibly genetically linked trait is a person’s preference for the morning versus the evening. This is partially linked to the polymorphism of the PER3 “clock” gene involved in sleep-wake regulation (2).

Risk Factors

• Age >50 years (2)
• Strong competing social and domestic needs (1)

General Prevention

• Reduce or eliminate shift work.
• Try to rotate shifts forward if shifts must be rotated.
• Use bright light during shifts.
• Improve sleep hygiene.
• Schedule regular sleep including naps (<1 hour) just before a shift or, if possible, during a shift.

Commonly Associated Conditions

• SWD has been associated with functional consequences including impaired immediate free recall, decreased processing speed, and selective attention impairments (4).
• SWD has been associated with higher risk of vehicular accidents, job-related injuries (4).
• SWD has been associated with poor physical health (including increased incidence of gastrointestinal [GI] disorders, cardiovascular disease [CVD], diabetes, and possible increase risk of cancers), as well as poor mental health (including increased incidence of substance use disorders and mood disorders) (4),(5).