This project seeks to redefine how the mammalian circadian system operates. The suprachiasmatic nucleus (SCN), the brain’s master clock, coordinates daily physiology by receiving light input and synchronizing time signals throughout the body. Traditionally, glucocorticoids (GCs) are viewed as one-way outputs from the SCN, with the SCN itself considered insensitive to GC feedback. We challenge this view by proposing that the mouse SCN does receive GC feedback through the activation of the GC receptor in astrocytes and that these cells are key integrators of light and internal time cues. We hypothesize that astrocytes might play an essential role in the adaptation to circadian misalignment, and that this feature is acquired during development. By examining the effects of GC and light in adult mice and during critical developmental windows, this project aims to inform strategies to mitigate circadian disruption in lifestyle, pregnancy, and neonatal care settings.