sleep regression

sleep regression (plural sleep regressions)

1. (psychology) A period of days or weeks during the first two years of life characterized by a change in an infant's sleep patterns, leading to irregularities such as increased awakenings and fussiness.

   four-month sleep regression

   • 2020 April 15, Jessica Wapner, “Are Sleep Regressions Real?”, in The New York Times‎:

     Between their input and Google’s, Smith surmised that her daughter was experiencing what many parenting books and websites refer to as a “sleep regression” — a stretch of days or even weeks where babies suddenly don’t seem to sleep or nap in the same ways they used to.