The 4-month sleep regression, explained

If sleep that was starting to feel manageable suddenly falls apart around 3–4 months, you're not imagining it — and it's not something you broke.

What's actually happening

Around this age, babies' sleep cycles mature and start to resemble adult sleep architecture — moving through lighter and deeper stages, with brief wake-ups between cycles. Younger babies mostly bypass this; older babies (and adults) wake briefly too, but usually resettle without anyone noticing.

A 4-month-old hasn't yet learned how to resettle on their own, so those normal, brief wake-ups turn into fully-awake, calling-for-help moments — often several times a night.

It's a permanent shift, not a phase to just wait out

Unlike some other rough patches, this one reflects a genuine, permanent change in how sleep works — which is actually good news: it means there's a skill to build (self-resettling), not just time to run out the clock on.

What tends to help

• A consistent, calm bedtime routine (see the routine guide above) — predictability supports the new sleep architecture.
• An age-appropriate wake window — an overtired baby often sleeps worse, not better.
• Putting baby down drowsy but still awake, when you're comfortable with that step, to give them practice falling asleep independently.
• Patience with the process — building a new sleep skill takes consistent practice, typically over a couple of weeks, not one night.

What's normal vs. worth flagging

Multiple wake-ups a night and shorter naps are the norm during this stretch. If sleep doesn't start improving after several weeks of a consistent approach, or you're concerned about something beyond typical sleep disruption, your pediatrician is a good next stop — they can help rule out other causes and talk through sleep-training approaches that fit your family.