Ah, the 4-month sleep regression—the developmental curveball that makes even the most chill babies suddenly sleep like newborns again. Just when you thought you had things figured out, bedtime turns into a battle and naps go rogue. But don’t panic—this phase, while challenging, is actually a milestone and not a mistake.

At around 3.5 to 4 months old, your baby's sleep undergoes a major transformation. Until now, newborns tend to fall into deep sleep quickly and cycle through sleep phases without much disruption. But suddenly, your baby’s sleep starts to look more like yours—with lighter stages of sleep, deeper stages, and REM (dream) sleep all in a cycle. These cycles last around 45–50 minutes, and the big change? Your baby now fully wakes at the end of each one.

This means more night wakings, short naps that end before you can sip your coffee, and a lot of “wasn’t she just asleep?” moments. It can feel like sleep has completely fallen apart. But really, your baby’s brain is growing in incredible ways. They’re becoming more alert, more social, and more sensitive to their environment. This leap in development is a great thing—it just happens to mess with sleep while it’s happening.

So how do you cope when it feels like everything’s gone off the rails?

First, lean into routine. Babies this age thrive on consistency. Having a simple, predictable wind-down routine before naps and bedtime gives your little one clear signals that sleep is coming. It doesn’t have to be elaborate—a short book, a song, and a snuggle is plenty.

Next, focus on wake windows. At this age, babies typically do best with wake times between 75 and 120 minutes. Keeping your baby up too long can make it harder for them to settle and stay asleep. On the flip side, putting them down too soon can also backfire, so pay attention to sleepy cues.

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