A ubiquitous feature of active sleep in mammals and birds is its relative abundance in early development. In rat pups across the first two postnatal weeks, active sleep promotes the expression of synchronized oscillatory activity within and between cortical and subcortical sensorimotor structures. Sensory feedback from self-generated myoclonic twitches – which are produced exclusively during active sleep – also triggers neural oscillations in those structures. We have proposed that one of the functions of active sleep in early infancy is to (...) provide a context for synchronizing developing structures. Specifically, neural oscillations contribute to a variety of neurodevelopmental processes, including synapse formation, neuronal differentiation and migration, apoptosis, and the refinement of topographic maps. In addition, synchronized oscillations promote functional connectivity between distant brain areas. Consequently, any condition or manipulation that restricts active sleep can, in turn, deprive the infant animal of substantial sensory experience, resulting in atypical developmental trajectories. The synchronization of rhythmic neural activity across brain structures provides evidence of communication. In developing sensorimotor networks, such oscillations are triggered most clearly and prominently during REM sleep. We propose that REM sleep provides a context for the emergence and expression of functional connectivity in the developing sensorimotor system. (shrink)