Sleep quantity and quality change dramatically across the lifespan of altricial mammals born with immature brains such as humans and rodents (Logan 2019, Cirelli 2020, Blumberg 2005, Blumberg 2014). Despite a crucial role for healthy development, we still know quite little about developmental sleep – especially when it comes to architecture and oscillatory dynamics, or to the emergence of global inter-regional brain cross-communication. Current methods to analyse immature sleep fail to provide the resolution needed to characterise its longitudinal microstructure. We designed light-weighted chronic devices adapted to freely-behaving rats as young as postnatal day 9 (up to adult age) to simultaneous record muscle activity (EMG), field activity from three brain areas (prefrontal, parietal and hippocampal cortices), as well as multiunit activity from CA1. We then developed a semi-automated scoring algorithm to analyse the longitudinal sleep microstructure inferred from tracking the pup’s position with DeepLabCut and the overmentioned electrophysiological signals. This allowed us to define sleep substages both during REM and non-REM sleep in developing rodents, while mapping global communications between brain regions. We specifically characterize how sleep architecture exhibits temporal segmentation across the lifespan – hinting at different specific sleep needs across developmental stages. Our results also revealed that global cross-region oscillations emerge between the second and the third postnatal week, potentially supporting sensory and memory integration. In summary, with new chronic recording and analytic methods, we determine how sleep micro-architecture in freely-behaving rodents changes across the lifespan, revealing the emergence of sleep substages and their predominance during specific developmental windows.