There is increasing evidence that occupational stress increases psychosomatic health complaints in the long run. However, the underlying mechanisms are still unclear. The present longitudinal actigraphy field study investigated the role of sleep quality—objectively assessed sleep-onset latency, sleep efficiency and sleep fragmentation, and subjectively assessed sleep quality—as a mediator in the relationship between stressful work conditions at time 1 and psychosomatic health complaints at time 2. A longitudinal hierarchical regression analysis revealed that social stressors at work were positively related to objectively assessed sleep fragmentation and to psychosomatic health complaints. Moreover, objectively assessed sleep fragmentation mediated the effect of social stressors at work on psychosomatic health complaints. Contrary to our expectations, social stressors at work were not related to other sleep quality parameters (i.e. sleep-onset latency, sleep efficiency and subjectively assessed sleep quality) during follow-up. Sleep fragmentation is discussed as an important consequence of social stressors at work that increase the risk of psychosomatic health complaints in the long run. Copyright © 2013 John Wiley & Sons, Ltd.