In phototrophic cyanobacteria, the signaling molecule bis-(3'-5')-cyclic dimeric-guanosine monophosphate (c-di-GMP) plays important roles in a wide variety of functions associated with environmental conditions, including biofilm formation, motility, heterocyst development, cell size control, phototaxis, and flocculation. However, its role in circadian rhythms, which contribute to fitness under diel light conditions, remains unexplored. In this study, we investigated the impact of changes in intr