The American labor market is increasingly unequal, characterized by extraordinary returns to work at the top of the market but rising precarity and instability at the bottom of the market. Research on precarious work and its consequences has overwhelmingly focused on the economic dimension of precarity, epitomized by low and stagnant wages. But, the rise in precarious work has also involved a major shift in the temporal dimension of work such that many workers now experience routine instability in their work schedules. This temporal instability represents a fundamental and under-appreciated manifestation of the risk shift from firms to workers and their families. To date, a lack of suitable existing data has precluded empirical investigation of how such precarious scheduling practices affect the health and wellbeing of workers. We use an innovative approach to collect survey data from a large and strategically selected segment of the US workforce: hourly workers in the service sector. These data reveal relationships between exposure to routine instability in work schedules and psychological distress, poor sleep quality, and unhappiness. While low wages are also associated with these outcomes, unstable and unpredictable schedules are much more strongly associated. Further, while precarious schedules affect worker wellbeing in part through the mediating influence of household economic insecurity, a much larger proportion of the association is driven by work-life conflict. The temporal dimension of work is central to the experience of precarity and an important social determinant of worker wellbeing.