What Is the 4-Month Sleep Regression and How Long Does It Last?
If your baby’s sleep has suddenly started to feel more broken than it did a few weeks ago, you may have been told you’ve hit the dreaded 4-month sleep regression.
That phrase alone can feel unsettling, especially if things had just started to feel a little easier.
But here’s the truth.
What’s happening isn’t a regression at all. It’s a sleep progression - a completely normal stage of neurological development where your baby’s sleep is maturing.
This shift usually happens somewhere between three and six months, and understanding what’s going on biologically can take a lot of the fear out of this phase.
My blog The One Truth About Baby Sleep That No One Tells You explores why night waking and needing support are part of normal development, not signs that something is wrong.
What is the 4-month sleep regression?
The 4-month sleep regression is the commonly used term for a period when your baby’s sleep changes because their sleep cycles are becoming more mature.
Rather than losing sleep skills or developing a problem, your baby’s brain is becoming more sophisticated. Sleep starts to look more adult-like, but for a while, that also means it can feel lighter and more fragmented.
This change:
doesn’t happen overnight
doesn’t land on an exact date
and doesn’t mean sleep has “broken”
It’s a progression in sleep development, not a setback
Why sleep can feel easier before four months
In the early months, your baby moves through just two stages of sleep.
Active sleep
This is lighter sleep. You might notice twitching, wriggling, grunting, or fluttering eyelids. Breathing can be irregular and noisy. Although it looks unsettled, your baby’s brain is busy forming connections and processing sensory information.
Quiet sleep
This is deeper, stiller sleep. Breathing slows, muscles relax, and growth and physical restoration take place.
At this stage, sleep is disorganised. These two stages don’t follow neat patterns or predictable lengths, and that’s completely normal for an immature nervous system.
Because newborn sleep isn’t yet structured into clear cycles, some babies can drop into long stretches of deeper sleep without fully waking between stages. This is why sleep can sometimes feel surprisingly settled around two or three months.
It isn’t that sleep is meant to stay that way - it’s simply how newborn sleep works before it matures.
What changes during the 4-month sleep progression?
Between around three and six months, your baby’s brain develops enough to support a more complex sleep structure.
Sleep shifts from two stages to four stages, much more like adult sleep. Each sleep cycle now involves moving through all four stages, and every transition creates an opportunity to wake.
Your baby’s sleep cycles are still short - typically around 45 to 50 minutes - which means these transitions happen frequently throughout the night.
Sleep becomes more organised, but also more sensitive to disruption
The four stages of baby sleep explained
After this progression, your baby cycles through four stages of sleep.
Stage 1 - light sleep
This is the lightest stage of sleep and the easiest one to wake from. You might notice small movements or changes in breathing. If something feels different, a change in position, environment, or your presence, your baby is more likely to wake fully here.
Stage 2 - deeper non-REM sleep
Sleep deepens. Breathing becomes regular, muscles relax, and your baby’s nervous system settles. This stage supports physical restoration and a sense of safety.
Stage 3 - very deep restorative sleep
This is the deepest stage of non-REM sleep. Growth hormone is released and the body does its most intensive repair work. This stage is vital, but makes up a smaller proportion of baby sleep than adult sleep.
Stage 4 - REM sleep
REM sleep supports memory, learning, and emotional processing. Babies spend a large proportion of their sleep here, especially in the first year. It’s also a lighter stage, which means waking is more likely if there’s disruption.
Why sleep often feels worse after four months
This sleep progression often overlaps with several other big changes:
a gradual decrease in sleep needs
increased sensory awareness
rapid cognitive and physical development
a growing awareness of separation from you
Where a newborn might have drifted quietly from one sleep cycle to the next, an older baby is far more likely to surface into light sleep and wake fully if something feels different.
This is why you may start noticing:
waking every 45–50 minutes
false starts at bedtime
increased night waking
Even if nothing in your routine has changed.
If frequent waking is leaving you exhausted and wondering whether your baby is actually sleep-deprived, my blog Is Your Baby Really Sleep-Deprived? The Truth About Normal Baby Sleep (and Why You’re So Tired) explains why fragmented sleep can be normal, even when it feels relentless.
How long does the 4-month sleep regression last?
This is usually the question you’re really asking when sleep starts to feel harder.
You may also have come across the idea that this is the moment to “work on sleep” and that if you don’t, sleep will stay all over the place long term.
That can add a huge amount of pressure at a time when you’re already tired and vulnerable.
The truth is, the change in sleep structure is permanent - your baby doesn’t return to newborn sleep after this point. But, and this is the important bit, the disruption isn’t.
For some babies, this unsettled period lasts a couple of weeks. For others, sleep simply looks different for a while as regulation, rhythm, and maturity continue to develop.
Your baby doesn’t get stuck in this phase because you feed them, cuddle them, or support them to sleep. They move through it as their nervous system continues to mature - not because you’ve intervened “correctly” at four months.
There is no closing window. No point of no return.
Just a baby whose sleep is growing up.
A gentler way to look at this phase
This isn’t your baby going backwards.It isn’t caused by bad habits.And it isn’t something you’ve created.
Think of it as your baby not regressing - they’re maturing.
Rather than asking how to fix sleep, it can help to ask what your baby needs from you during this stage, while also thinking about how to protect your own rest and wellbeing.
If this phase has you second-guessing yourself or feeling overwhelmed, my blog Five Things I Wish Someone Had Told Me About Baby Sleep shares the gentle truths that often get lost in sleep advice, and why support matters more than fixing.
Understanding what’s happening biologically can replace fear with clarity. And from there, you can make calm, responsive choices that fit your baby and your family.
Sleep will change again. It always does!
Frequently asked questions about the 4-month sleep regression
Does the 4-month sleep regression pass on its own?
Yes.
The change in sleep structure is permanent, but the unsettled period usually eases as your baby’s nervous system matures and they become more settled within this new sleep pattern.
There isn’t a deadline or a specific approach you need to follow for this phase to pass.
Do I need to change how my baby falls asleep at four months?
Not unless what you’re doing no longer feels sustainable for you.
You may hear that this stage is the moment to “work on sleep” or risk things staying unsettled long term. That pressure isn’t rooted in biology. Babies don’t miss a window if they’re fed, cuddled, or supported to sleep.
If something feels hard, gentle changes can help, but there’s no requirement to change anything simply because your baby has reached four months.
Can responding at night make sleep worse long term?
In a word - no!
Responding to your baby at night doesn’t create long-term sleep problems. Comfort, reassurance, and responsiveness support regulation while sleep is maturing.
Babies move through this phase because their nervous system develops, not because you responded in a particular way overnight.
Why did my baby sleep better before four months?
Before four months, sleep is less structured. Some babies are able to move into longer stretches of deeper sleep without fully waking between cycles.
As sleep matures, babies surface into lighter sleep more often, which makes waking more noticeable. It doesn’t mean sleep has broken or that you’ve done anything wrong - it simply reflects how sleep changes as your baby grows.
When should I get extra support during the 4-month sleep progression?
Support can be helpful if sleep is affecting your wellbeing, your mental health, or your ability to cope day to day.
You don’t need to wait until things feel unbearable, and you don’t need to push through on your own. Seeking reassurance or gentle guidance is about supporting you as much as your baby.
