Structural plasticity of the circadian timing system. An overview from flies to mammals

• Circadian plasticity involves structural changes at cellular and network levels.
• Circadian remodeling at subcellular level subserves functioning of cell oscillators.
• Clock’s photic synchronization involves reorganization of neuronal–glial networks.
• Glial cells are critically involved in the circadian remodeling of neural circuits.

The circadian timing system orchestrates daily variations in physiology and behavior through coordination of multioscillatory cell networks that are highly plastic in responding to environmental changes. Over the last decade, it has become clear that this plasticity involves structural changes and that the changes may be observed not only in central brain regions where the master clock cells reside but also in clock-controlled structures. This review considers experimental data in invertebrate and vertebrate model systems, mainly flies and mammals, illustrating various forms of structural circadian plasticity from cellular to circuit-based levels. It highlights the importance of these plastic events in the functional adaptation of the clock to the changing environment.