Wonder Weeks Leap 4

Leap 4 - The world turns on.

Why Leap 4 is the “big one” and how to survive the sleep shake-up

Around week 19, give or take (based on your baby’s due date), your baby enters Leap 4, which is a developmental explosion so big it’s often mistaken for something being wrong.

They were sleeping better, now they’re up all night.

They were settled, now they’re cranky. It’s one of the most dramatic leaps in the first year, and one of the hardest for parents.

The science: What’s changing in your baby’s brain

Leap 4 is when your baby begins to process events i.e. the building blocks of sequences, transitions, and cause-and-effect. It’s a leap from sensing the world in simple bursts (light, movement, sound) to understanding patterns in the world around them.

They’re starting to notice the flow of life:

• The mobile spins before the music starts

• Mum walks in after the lights go on

• I cry → someone picks me up

This is a massive cognitive upgrade. According to the Wonder Weeks and backed by neuro-developmental science, this leap is all about predictability and anticipation and it makes your baby more alert, more aware, and yep… more likely to wake up.

What this means for your baby

• They may be fussy during feeds or distracted by every sound

• They could start anticipating your actions (and reacting when things change)

• They may wake more often, especially between sleep cycles

• They might cry more before naps or bedtime

• They could go from chill to clingy in 0.2 seconds

This isn’t regression. It’s reorganisation. Their brain is literally rewiring to understand time.

Why Leap 4 feels so intense

Because it touches everything.

Sleep. Feeding. Attachment. Sensory overload.

It’s the first time your baby can anticipate being separated from you, but they don’t yet know when (or if) you’ll return. They’re learning about continuity, and that can trigger protest and clinginess, especially around sleep.

They also notice more sensory detail now - bright lights, loud noises, even busy prints can become overstimulating.

Big feelings. Tiny body. No idea how to calm down.

That’s Leap 4.

How to support them through Leap 4

No need to overhaul everything. This leap is best met with connection, consistency, and a little extra compassion (for both of you).

Stick to sweet spots

Wake windows are 1.75–2.5 hours at this age.

Use this formula:

Last wake-up time + wake window = ideal next sleep

Set a timer 15 minutes before that window ends so you can start winding them down.

Use a short wind-down ritual

Dim lights, switch on white noise, gentle nappy change, a phrase like “Time for sleep now.” Same cues each time = less guesswork for them.

Name what’s happening

"It's alright, sweetheart, mummy's here"

Language builds safety and emotional literacy, even when they don’t have words yet.

Limit stimulation

Simplify their sleep space and play environment. One toy at a time. Less is calming.

Hold them more, not less

This isn’t manipulation, it’s regulation. They’re learning how the world works, and you are their safe base.

Sleep during Leap 4: Why it unravels (and what helps)

Leap 4 is famous for the so-called “4-month sleep regression”, but it’s really a progression.

Here’s what’s happening:

• Their sleep cycles are shifting from newborn to more adult-like stages

• They start waking fully after one cycle (every 45–50 mins)

• If they haven’t learned how to resettle, they need help

• Night feeds may reduce (or not), depending on weight and feeding

This doesn’t mean your baby is broken.

It means their brain is developing.

What helps:

• Keep the sleep environment dark, quiet (except for white or pink noise), and boring.

• Avoid introducing lots of new sleep crutches unless you’re happy to keep them long-term. The key with Leaps is to try to hold somewhat steady.

• Try “pause, then respond” - wait 1–2 minutes before intervening in case they were just stirring.

• If building sleep skills, practice at one nap per day, not every sleep

  
  

What NOT to worry about

• Short naps

• Needing to be held more

• Falling asleep during feeds

• Waking 2–4 times at night

• Wanting only you

All totally normal. Nothing to fix. Hold steady and they WILL come out the other side. If you need tailored support and can't afford a Sleep Consultant, don't forget Ask Yawn has a free trial. Its whole job is to fix baby sleep.

Common myths during Leap 4

Myth: “You’ve spoiled them by holding them too much.”

Truth: You’re shaping brain architecture. That’s literally your job.

Myth: “If they’re waking more, you’ve gone backwards.”

Truth: It’s not backsliding, it’s upgrading... eventually. This Leap can last about 6 weeks.

If you could ask an expert one thing…

“How do I help my baby link sleep cycles without leaving them to cry?”

👉 Ask Yawn. It is built to personalise the ‘how’ based on your baby’s temperament, your values, and where you’re at right now.