Circadian rhythms are innate biological processes that cycle over periods of approximately 24 hours and are crucial in regulating many physiological and metabolic functions. At the molecular level in mammals, these cell-autonomous oscillations are controlled by a network of self-regulating feedback loops comprised of transcriptional and translational regulators. Reporter platforms for following promoter activity and/or proteins via detectable fluorescent and bioluminescent entities have facilitated the tracking of circadian elements in cells and organisms in real-time, which is especially important for studying such a dynamic biological system. As a result, reporters have been pivotal in uncovering fundamental aspects of the molecular core clock, changes in individual components in healthy versus various disease models, and screening for synthetic clock modulators. While the initially established tools of in vivo reporters (i.e., Per2::Luc mice) continue to be widely utilized, newer ones have capabilities for evaluating other components and performing simultaneous assessments of multiple circadian components across different levels of expression. Concurrently, in vitro reporter models have been developed to track alterations in rhythms and protein presence in various cellular models, including those of disease. In this review, we discuss various reporter systems, including historical context, and their applications for monitoring rhythms at transcriptional and translational levels of expression, in vitro and in/ex vivo, with a focus on mammalian systems. Furthermore, we assess the advantages, disadvantages, and limitations of employing bioluminescent and fluorescent reporters in monitoring biological rhythms and components. Our goals are to provide an account of available reporter systems and pose considerations for their use and the development of technologies that can be used to expand our tracking and understanding of the molecular clock.
