Why does night shift work cause shift work disorder?

Why Night Shift Work Causes Shift Work Disorder

Night shift work causes shift work disorder primarily through disruption of the circadian rhythm, leading to misalignment between the body's internal clock and the external environment, resulting in excessive sleepiness during work hours and insomnia during allotted sleep times. 1, 2

Mechanisms of Circadian Disruption

• Night shift work fundamentally alters the light-dark schedule, which disrupts normal circadian gene expression and melatonin production 1, 3
• Exposure to light during night shifts suppresses natural melatonin production, which normally increases in darkness to promote sleep 1
• The alteration in light-dark schedule is immunosuppressive, induces chronic inflammation, and alters cell proliferation, cell death, and nutrient supply 4
• In female night shift workers, there is evidence of altered estrogen levels, which may contribute to both sleep disruption and other health effects 4

Physiological Consequences

• Both humans and experimental animals show altered serum melatonin levels and disrupted expression of circadian genes when subjected to altered light-dark schedules 4
• The body's natural sleep-wake cycle becomes desynchronized from environmental cues, leading to internal desynchrony between various physiological processes 2
• Working during the body's biological night (when core body temperature is lowest and sleep propensity is highest) forces alertness when the body is programmed for sleep 5
• Attempting to sleep during the biological day, when the body is programmed for wakefulness, results in shorter, fragmented, and less restorative sleep 5

Environmental and Behavioral Factors

• Daytime sleep is often disrupted by environmental factors such as light, noise, and social obligations 1
• Inconsistent sleep-wake schedules, especially on days off, prevent proper circadian adaptation and worsen symptoms 1
• Inadequate light exposure during night shifts and excessive light exposure before daytime sleep exacerbate circadian misalignment 1
• The need to adapt to normal circadian rhythms for social activities on days off creates a perpetual state of circadian disruption 2

Individual Susceptibility Factors

• Not all shift workers develop shift work disorder (approximately 30% are affected), suggesting individual differences in susceptibility 6
• Age, chronotype (morning vs. evening preference), genetic factors, and pre-existing sleep disorders influence vulnerability to shift work disorder 2
• Comorbid conditions such as sleep apnea, restless legs syndrome, and mood disorders can exacerbate symptoms of shift work disorder 7

Health and Performance Consequences

• Shift work disorder is associated with higher risks of occupational and motor vehicle accidents due to excessive sleepiness 8
• Chronic circadian disruption increases risks of cardiovascular disease, gastrointestinal problems, and metabolic disorders 5
• Night shift work has been classified as probably carcinogenic (Group 2A) by the International Agency for Research on Cancer, with associations observed between night shift work and cancers of the breast, prostate, colon, and rectum 4
• The disorder significantly impacts quality of life, social functioning, and mental health 8

Diagnostic Considerations

• Diagnosis requires documentation of excessive sleepiness during work hours or insomnia during allotted sleep times, along with a disruption of the circadian rhythm 8
• Wrist actigraphy for 7-14 days with concurrent sleep diaries helps objectively document sleep-wake patterns and distinguish shift work disorder from other sleep disorders 3
• Self-reported sleep logs tend to overestimate sleep time by approximately 1.5 hours per night, making objective measurement important 3

Understanding these mechanisms helps guide appropriate treatment strategies, including timed light exposure, melatonin supplementation, sleep hygiene practices, and in some cases, pharmacological interventions to promote either sleep or wakefulness 1, 8.