As a professional sleep consultant, I hear the term “regression” used in regards to just about every imaginable circumstance. Essentially, if baby doesn’t sleep well for a couple of nights, parents start dropping the ‘R’ word. Some people subscribe to the idea that there’s an eight month regression, a 9 month regression, a 1 year regression, as well as teething regressions, growth spurt regressions, and so on. Others see these as simple hiccups caused by extenuating circumstances.

But the four-month regression, everybody agrees on, and for good reason. It’s the real deal, and it’s permanent.

So in order to understand what’s happening to your baby during this stage, first you need to know a few things about sleep in general. So here’s the science-y part, told in plain English.​