Numerous studies of working hours have drawn important conclusions from cross-sectional surveys. For example, the share of individuals working long hours is quite large at any given point in time. Moreover, this appears to have increased over the past two decades, raising the call for policies designed to alleviate working hours discrepancies among workers, or reduce working time overall. However, if work hours vary substantially at the individual level over time, then conclusions based upon studies of cross-sectional data may be incomplete. Using longitudinal data from the Canadian Survey of Labour and Income Dynamics, we find that there is substantial variation in annual working hours at the individual level. In fact, as much as half of the cross-sectional inequality in annual work hours can be explained by individual-level instability in hours. Moreover, very few individuals work chronically long hours. Instability in work hours is shown to be related to low-job quality, non-standard work, low-income levels, stress and bad health. This indicates that working variable work hours is not likely done by choice; rather, it is more likely that these workers are unable to secure more stable employment. The lack of persistence in long work hours, plus the high level of individual work hours instability undermines the equity based arguments behind working time reduction policies. Furthermore, this research points out that policies designed to reduce hours instability could benefit workers.