If your once-predictable sleeper has suddenly started waking every hour, fighting naps, or refusing to settle at all, you're not imagining it — and you're not doing anything wrong. Somewhere around 3–5 months, most babies go through what's commonly called the "4-month sleep regression."
Why It Happens
Around this age, your baby's sleep architecture matures — shifting from the simpler newborn sleep pattern into the same multi-stage sleep cycles adults use, including brief, natural wake-ups between cycles. A newborn could sleep through those transitions unaware; a 4-month-old is developmentally alert enough to notice, and to need help getting back to sleep. That's why we describe it as a permanent shift, not a phase that simply passes on its own.
Signs You're In It
- Frequent night waking after a stretch of longer sleep
- Shorter, harder-to-extend naps
- Fighting sleep at bedtime despite clear tiredness
- Increased fussiness or a shorter fuse during the day
How Long It Lasts
The acute disruption typically eases within 2–6 weeks — but because it reflects a genuine developmental shift, sleep usually doesn't return to "normal" until your baby has learned a few independent sleep skills to carry them through those natural night-time wake-ups.
What Helps
- Consistent, age-appropriate wake windows so your baby isn't over- or under-tired at sleep times
- A short, predictable bedtime routine that cues sleep without becoming a prop your baby can't fall asleep without
- A dark, cool, quiet sleep environment to support longer stretches
- Opportunities to practice falling asleep independently, even briefly, so those natural night wakings don't require full intervention every time
When to Bring in Help
If you're weeks into the regression with no improvement, exhausted, and unsure what to try next, that's exactly the point where a personalized sleep plan tends to make the fastest difference — or, if your baby is still very young, a night nurse can buy your family real rest while you decide on next steps.