The first few months with a baby are unpredictable and, more or less, sleepless. But there is a light at the end of the dark, sleep-deprived tunnel. Once they reach 3 or 4 months old, babies can sleep for longer stretches and may have settled into a more predictable routine. Until, without warning, they're waking frequently again and seem to have lost all of the ground they gained in the previous months. These phases are called sleep regressions and the first is thought to hit at around 4 months old. But do all babies go through a 4 month sleep regression and will it be as horrible as you're expecting it to be? Like most aspects of parenting, the answer looks different for each baby.
According to the Baby Sleep Site, a baby's sleep habits go through a permanent change at 4 months. Up until that point, they sleep like a newborn — frequently waking to eat. After the 4 month mark, however, babies develop more adult-like phases of deep and light sleep. At this point, the same Baby Sleep Site stated that all bets are off. Your baby may easily fall back into a routine and continue to sleep well, or their habits may change completely, (much to your dismay.)
Technically, every baby does go through the 4 month sleep regression, because it's more of a developmental milestone than an actual bump in the bedtime road. That being said, the regression will affect each baby differently, depending on their personality. The regression will be shorter for some babies, and longer for others as they learn how to fall back asleep after waking from a deep sleep phase.
One article from the website for The Today Show noted that this age is the perfect time to start establishing nighttime routines and getting your baby more accustomed to a set schedule, especially if they're having a hard time adjusting.
Whether you co-sleep, breastsleep, bed-share, sleep in separate rooms, or do a little bit of it all, the 4 month sleep regression might throw you for a loop. Take heart though, because it means that your baby's development is right on track and that their bodies are working to understand what "adult" sleep looks like. And that's a skill every parent wants their child to know.