The 4-Month Sleep Regression

Why your baby is suddenly sleeping poorly

The 4-month sleep regression is often not a concern for new parents with a newborn baby.

Most new parents can’t complain. Their newborn sleeps a lot and well.

But suddenly, your baby can’t fall asleep anymore. And you wonder why your 4-month-old baby wakes up more often than before, maybe even crying. Why the daytime naps suddenly become shorter? The reason behind it is called the 4 month sleep regression.

Let me explain what it is, what the signs are, and give you tips on how to improve your baby’s sleep.

What is the 4 month sleep regression?

The 4-month sleep regression is the only scientifically proven sleep regression. Your child’s sleep patterns change fundamentally, becoming cyclical and thus more like adult sleep patterns. This also officially marks the end of the newborn phase.

Does every baby experience the 4 -month sleep regression?

Every baby goes through this change in sleep patterns. However, not all babies suddenly start sleeping poorly. This depends on whether parents have, from birth, responded appropriately (a practice called parent-child co-regulation) and taught their child how to fall asleep and stay asleep. These babies are more independent in their sleep habits and more secure in their self-regulation.

Are there other regressions?

Over the next few months, your baby will go through more regressions (developmental phases). Traveling, illness, and teething—all these things can lead to a few bad nights in a row. But as for the four-month „progression“ (because it’s actually progress, not a regression), it is unique. Once you get through this phase, your baby has officially entered the sleep cycle they will essentially follow for the rest of their life.

What are the signs of the 4-month sleep regression?

The regression begins around the 3rd month of life. At this point, the changes in sleep behavior become permanent. The following signs may alert you to this change:

Difficulty falling asleep
Frequent nighttime awakenings (every 1-2 hours)
Your baby demands the breast/bottle more often
Restless sleep
A lot of crying and fussiness when falling asleep or waking up

How long does the 4-month sleep regression last?

The regression itself lasts only a few weeks. However, the challenges that come with it should not be underestimated, as your baby’s sleep pattern has fundamentally changed, and now your child has to learn how to (re)settle to sleep independently. If they can do this, you’ve already made significant progress. It’s important that, as a mom, you feel confident that your baby’s other basic needs—affection and hunger—have been met during the day.

How does a baby's sleep change?

Newborns connect two sleep phases: they spend 50% of their time in deep sleep and 50% in dream sleep. The 4-month sleep regression changes this by making sleep cyclical, and unfortunately, four additional light sleep phases are introduced.

A sleep cycle for a 4-5 month-old baby lasts only about 30-40 minutes, whereas an adult’s cycle lasts 90-120 minutes—our sleep phases are therefore longer.

This is what a sleep cycle looks like:

Phase 1: A light drowsiness, your baby seems awake but has actually already entered the first light sleep phase.
Phase 2: A light sleep, from which your baby can easily wake. If you lay them down now and they’re not used to it, they will wake up.
Phase 3: Deep sleep phase – Non-REM. Finally here. Your baby’s hands relax, breathing and body become calm. This is also called the „slow wave“ sleep phase.
Phase 4: Dream sleep phase – REM phase. This is where the day is processed. Sleep during this phase is very active.
Phase 5: Another light sleep phase, from which your baby easily awakens, makes noises, and moves.
Phase 6: This phase serves to connect and check whether the baby will continue into the next sleep cycle.

And this is where the real issue lies: we all want to continue sleeping the same way we fell asleep.

Why is my 4-month-old baby sleeping so restlessly at night?

In general, baby sleep (and even toddler sleep) is very active and noisier than adult sleep. Your baby squeaks, grunts, groans, and „wiggles“ a lot at night? No need to worry—this is all normal and developmentally appropriate, and it will continue for quite some time.

However, if your baby’s sleep is truly very interrupted, keep in mind that around 4 months, during the 4-month sleep regression, there are some significant developmental milestones. At 4 months, your baby becomes more active and alert. They suddenly notice a lot more, may feed less well during the day as a result, and now have a new sleep cycle with so many light sleep phases.

This often creates a vicious cycle: your baby sleeps too little during the day, becomes overtired, drinks less during the day, feels hungry at night, and you start using a pacifier—then the pacifier falls out during deep sleep and is needed again to continue sleeping.

What helps my baby sleep more peacefully?

Routines, darkness, and an optimal bedtime (use my wake window chart for this) are the basic prerequisites. However, in the long run, it’s about how your little one falls asleep. Many babies are nursed, carried, rocked, bounced, or fed to sleep. If your baby falls asleep exclusively with the help of these methods, even if they are mixed, these external aids become your baby’s sleep strategy. Your baby begins to associate the feeling of tiredness and the subsequent act of falling asleep with these specific forms of support.

They complete their sleep cycle, wake halfway in Phase 6, and check the situation for continuing sleep. But now the situation has changed because you’ve laid them down. Startled, they think: “Where am I? Where is mom, the breast, etc.? They were here when I fell asleep.” Your baby then signals this by crying. It’s a bit understandable, right?

So, it’s important how your baby falls asleep, so they can start connecting the sleep cycles more independently in Phase 6 and not be startled. That’s why it’s so crucial for your baby to fall asleep in their bed while they’re tired but not yet fully asleep. Ideally, you should teach your baby this path to sleep even before the 4-month sleep regression, as it helps foster more independent sleep habits in a preventative way.

And I know from personal experience what a big challenge this can be, but it’s so, so worth it!

Sources:

Flynn-Evans, Casano. Baby Sleep Science Guide: Overcoming The Four-Month Sleep Regression. Marta Sadurni, Marc Pérez Burriel, Frans X Plooij: The temporal relation between regression and transition periods in early infancy. In: Span J Psychol 2010. https://www.ncbi.nlm.nih.gov/pmc/articles/PMC7877162/