"My newborn baby was sleeping so well...what happened??" Sound familiar? We hear this all. the. time. Our answers vary, depending on the unique baby in question. In our experience, the answer can be anything from teething to illness to temperament to the birth of a new sibling to a nap transition. But often, our answer is - sleep regression. Common Sleep Regressions Generally, we offer up a diagnosis of sleep regression for babies and toddlers who are the following ages: 4