Infant sleep positioner

A device for stabilizing the position of an infant while sleeping features a shell assembly including a base panel having two longitudinal sides and two lateral sides, an end wall extending generally upwardly from one of the lateral sides, two side walls extending generally upwardly from the longitudinal sides, a shell cover comprising an outer surface and conforming generally to the shell assembly, and a removable positioner insert disposed atop the base panel and comprising left and right nacelles attached to an insert base and separated by a distance selected to accommodate the width of an infant placed between the nacelles, to inhibit lateral motion of the infant while sleeping.