Biological rhythms enable living organisms to adapt and live with periodical environmental changes, such as variation in the relative position of the earth and the sun. Internal rhythms, like body temperature and sleep-wake cycle, are driven by numerous biological processes and can be maintained even in the absence of external environmental cues. These rhythms affect how we feel, think, and act. They are profoundly important for our health, quality of sleep, and mood. Yet the digital devices we use are ignorant of our biology. They respond uniformly to our touch and click. Recently there has been a considerable increase of research within the HCI community to support behavior change, personal insight, and increase productivity. This workshop will bring together researchers in sleep, well-being, and circadian rhythms to discuss the possibility of rhythm systems: technologies that play to the strengths of our biology. It will investigate how HCI can complement our biological rhythms and will focus on two areas: measurement and intervention.