Siestita Guide
The 4 month sleep regression: what it is, how long it lasts and what to do
If your roughly 4 month old has gone from sleeping reasonably well to waking every 45–90 minutes, you’ve done nothing wrong: their brain has just switched to a new — and permanent — way of sleeping.
What is really happening
Until now, your baby fell almost straight into deep sleep. Around 4 months their sleep reorganises into mature cycles of about 45–50 minutes alternating light and deep sleep, like an adult’s. At the end of each cycle there’s a brief arousal — and that’s the key: if your baby only knows how to fall asleep in arms, with motion or while feeding, they’ll ask for exactly that at every arousal. It’s not hunger or whim: the conditions they fell asleep with have vanished.
That’s why this regression is different from the rest (here’s the full map): it isn’t a phase that “passes”, it’s a permanent change the baby adapts to within weeks.
Typical signs
- Wakings every 45–90 minutes, especially in the second half of the night.
- Naps that systematically break at 30–45 minutes.
- Fighting sleep despite exhaustion, with more crying than usual.
- More distracted feeds and more demand for contact.
What to do, step by step
- Adjust wake windows (1.5–2.5 h). Overtiredness multiplies wakings. The 4 month old sleep guide has the full sample schedule.
- Total darkness and white noise. Melatonin already responds to light at this age: half-dim rooms work against you.
- Move bedtime earlier. 7:00–8:00 pm usually works better than the 9:30 pm inherited from the newborn stage.
- Practise “drowsy but awake” at least at the start of the night: it’s the easiest slot for them to practise settling with less help.
- Keep a short, identical bedtime routine every night: the “sleep is coming” signal their brain does understand.
- Track sleep. Mid-regression, everything feels like “terrible sleep”; data tells you whether it’s 2 wakings or 6, and when improvement starts.
When to check with your paediatrician
If poor sleep comes with fever, feeding refusal, poor weight gain, noisy breathing or crying that doesn’t sound like your baby — or if the chaos stretches past 6–8 weeks with no improvement at all — get it checked. The regression is normal; constant discomfort isn’t.