Circadian misalignment: A biological basis for mood vulnerability in shift work

Abstract Background Approximately one in five workers perform night shift work. Epidemiological studies suggest that night shift workers are at a 25–30% higher risk for mental illnesses, including depression and anxiety, which is an increasingly important socioeconomic burden for the workforce. Thus, it is important to determine how shift work negatively affects mood, as it will help identify mechanisms that underlie a night shift worker's higher risk for developing mood disturbances. Methods This opinion discusses recently identified, potential biological mechanisms—including the role of the circadian system and circadian misalignment—underlying mood vulnerability in shift workers. Studies included are recent epidemiological, human laboratory studies and animal preclinical work on night shift work or circadian misalignment. Target biological mechanisms of interest discussed here include circadian misalignment effects on brain activity and brain–gut axis, essential for mood regulation. Results Circadian misalignment, which corresponds to the misalignment between biological (circadian) system and daily sleep‐wake behaviours, can adversely affect mood levels and cortical activity underlying mood regulation. Furthermore, animal preclinical work shows that the brain–gut axis function is not only implicated in mood regulation but can disrupt specific metabolites essential for mood regulation when animals are exposed to circadian disruption. Conclusions Circadian misalignment is a key mechanism underlying mood in e.g. shift workers. Therefore, understanding its role and applying sleep/circadian behavioural interventions to mitigate the adverse consequences of circadian misalignment on mood have the potential to improve quality of life, which is tightly associated with mood and sleep quality, in shift workers.