What are some of the key historical trends in hours of work per worker in US? What economic, social-psychological, organizational and institutional forces determine the length of individuals' working hours? How much of the trend toward longer working hours among so many workers may be attributable to workers' preferences, workplace incentives or employers' constraints? When can work become overwork or workaholism – an unforced addiction to incessant work activity which risk harm to workers, families or even economies? The first part (...) of this article traces the history of the length of working hours and its recent polarization. The second part develops a multi-disciplinary model to identify motivations behind working longer hours. Individuals' desired work hours will stem from the weighted contribution of five sources: (1) current real wage rates; (2) forward-looking, wage trajectories; (3) relative status associated with hours of labor; (4) intrinsic rewards, process benefits or amenities acquired through work; (5) hours demanded by the employer and other structural constraints, to which workers may adapt. Employers and their established conditions of work have influenced the course of long run trends labor supply and in work time structures. The final section suggests policies that might address the persistence of long hours. (shrink)