I found this excellent article on the Wee Bee Dreaming website. I thought it was worth repeating.
by Pam Edward
The ‘4 month sleep regression’ has celebrity status in my sleep world. I hear this phrase almost every single day; from an exhausted mom of a 5 month old, wondering why her baby’s sleep has gone downhill in the last month, to parents of a toddler who say that “their baby has never slept well since 4 months of age”, or even from a mom of a 3 month old, worried about the upcoming “regression”. I am here to help you realize what goes on around 4 months, and how we can prevent a true ‘regression’ from occurring.