And what really happens to your baby’s sleep.

**1. It isn’t a phase...**

The “regression” isn’t a phase — it’s a **permanent neurological shift.**

Around 3.5–5 months, your baby’s **sleep cycles mature** from newborn-style deep sleep to adult-like cycles with light sleep every ~45 minutes.

This is why they suddenly start waking more often — and **won’t just “grow out of it.”**

**2. Sleep Associations**

**What worked before** — rocking, feeding to sleep, contact naps — **now becomes a disruption.**

As your baby cycles into light sleep and wakes, they’re looking for the same conditions they fell asleep with.

If those conditions aren’t there (like your arms), they will likely wake and cry.

This is called a **sleep association**, and it’s the #1 cause of frequent night wakings after 4 months.

**3. Overtiredness**

Poor naps lead to **cortisol buildup**, making bedtime battles and 2 a.m. parties more likely.

Now’s the time to **start tracking wake windows** and aiming for age-appropriate nap timing.

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