Parents often think kids’ sleep problems are over once their babies slumber through the night, but at least a quarter of school-age children have nighttime troubles, says Gregory Stores, M.D., emeritus professor of developmental neuropsychiatry at Britain’s University of Oxford. It’s more than an issue of parent sanity—when kids don’t get the recommended nine or more hours (depending on their age) of shut-eye, they’re not only tired, they can be cranky, wired, and aggressive.