fMRI biomarkers of length of working week: A UK Biobank study

Working longer hours is associated with negative outcomes in health and wellbeing. While effects of occupational stress on the brain have been observed, there have been no investigations into the effect of amount of time spent at work on brain function to date. We examined the effect of length of working week (in hours) on task-based and resting-state functional magnetic resonance imaging (fMRI) in 8,528 and 7,589 participants, respectively, from the UK Biobank. Analyses were conducted to treat working hours as both a continuous and a categorical (lower versus higher hours groups) variable. Covariates relevant to length of working week were selected using random forest machine learning and were systematically adjusted for in a sequential manner. No association with length of working week was found for task-based fMRI, as reflected by activation in the amygdala, fusiform face area, and insula during an emotional face matching task. For resting-state fMRI, greater continuous length of working week was found to be positively associated with within-network connectivity in the somatomotor network, but did not predict connectivity within or between any other network. The observed association may suggest compensatory or adaptive somatomotor connectivity changes in the face of increased job demands. All other within- and between-network connectivity metrics showed no significant relationship with working hours, following optimal model selection and correction for multiple statistical tests. When looking at uncorrected p-values, significant statistical associations with resting-state fMRI tended to become non-significant once demographic factors were adjusted for, suggesting that associations between length of working week and brain function are frequently accounted for by these covariates, with the sole exception of somatomotor connectivity. Overall, this suggests that length of working week, as just one employment factor, does not show strong associations with fMRI metrics, when related variables are adjusted for. Interventional studies, and analysis of additional factors such as burnout and shift work may shed further light on neural markers of occupational wellbeing.