If your baby's sleep suddenly fell apart around 4 months, you've likely hit the 4-month sleep PRO-gression. During the newborn months, babies' sleep cycles aren't well defined, which means they can easily pass from one sleep cycle to the next without fully waking. But at around 4 months babies go through a huge shift in sleep biology. Their sleep cycles are now separated into four phases and there's a distinct line between the end of one sleep cycle and the next. Which means they now start to wa