Depression has significant economic impact: According to the World Health Organization, the estimated cost to the global economy is US$1 trillion per year in lost productivity. Shift work is prevalent across the world — approximately 15-20% of employees perform shift work in industrialized countries. With so many people engaged in shift work, it becomes important to determine its possible health-related effects. Growing evidence from large population-based studies indicate that shift work is significantly associated with mental illness, including depression and anxiety. Given that shift work will not disappear anytime soon, it is important to determine how it affects mood, with the goal of identifying the factors underlying workers’ mood disturbances.
My neuro-opinion piece, published in the European Journal of Neuroscience, discusses recently identified circadian mechanisms potentially underlying mood vulnerability in shift workers, with data based on epidemiological surveys, human laboratory studies, and animal models of night shift work. Circadian rhythm disruption is prevalent in modern society, but particularly among night shift workers. This appears to result from their tendency to stay awake during the day, even though they are working a night, a pattern distinctly at odds with the natural day-night rhythm. This circadian disruption leads to a plethora of adverse health effects, including, but not limited to type 2 diabetes, cardiovascular disease, and breast cancer.
Clearly, mood may also be affected, with heightened risk of developing depression. Indeed, recent studies indicate that disruption of circadian rhythms can alter mood levels and brain activity underlying mood regulation [1,2,3]. Furthermore, in an animal model, disruption of circadian rhythms in mice leads to the modification of specific gut molecules essential for mood regulation. Thus, the brain-gut axis is intimately involved in mood state. While human studies are still pending, the animal work suggests that brain and gut mechanisms underlying mood state are disrupted, and/or less efficient, as a consequence of circadian rhythm disruption.
The bridge to night shift work is plausible, even likely. Research on sleep and circadian behavioral interventions in shift workers — including targeted light therapy and appropriate meal timing (necessary for metabolic health) — may lead to effective countermeasures for circadian disruption, with improved mood, sleep, and quality of life.
Sarah L. Chellappa M.D. Ph.D., is a chronobiologist and sleep researcher looking for optimum ways to make shift work physiologically bearable.