Peeking into the sleeping brain: Using in vivo imaging in rodents to understand the relationship between sleep and cognition.

•It is not well understood how cognition benefits from sleep.•Sleep involves cellular and network activity beyond the reach of electrophysiology.•In vivo imaging allows access to defined cell types and subcellular structures.•Some limits of imaging must be considered when applied to sleep research.•Imaging in rodents has revealed insights into the function of sleep.