Things were going well. Really well. Baby was doing a 4-hour stretch at night, maybe even a 5-hour one. You had started to believe you were going to be okay.
Then somewhere between week 14 and week 20, it collapsed. Baby is waking every hour. Naps that used to last 90 minutes are now 35. The 4-hour stretch is gone. You are back to feeling like a newborn parent, except your baby is supposed to be past this.
You are not imagining it. This is the 4-month sleep regression — and unlike some other so-called regressions, this one is real, it is documented, and it has a cause.
What Actually Happens at 4 Months
Before 4 months, babies sleep differently than adults. Newborn sleep cycles are shorter and simpler — baby goes into deep sleep almost immediately, and waking between cycles is minimal.
Around 3.5 to 5 months, this changes permanently. Baby's sleep architecture matures to resemble adult sleep: alternating cycles of light and deep sleep, roughly 45 minutes per cycle. Every time a cycle ends, baby stirs. Adults do this too — we just learned to put ourselves back to sleep without noticing.
Baby has not learned this yet.
So every 45 minutes, baby surfaces into light sleep. If the conditions that helped them fall asleep the first time are no longer present — you're not holding them, the motion of the car is gone, the breast is not there — they wake fully and cry.
This is not a phase that passes. The mature sleep cycle is here to stay. What changes is baby learning to settle themselves through those transitions — and that takes a few weeks.
Signs You're In It
- Was sleeping 4+ hour stretches at night, now waking every 45–90 minutes
- Short naps — 30 to 45 minutes exactly (one sleep cycle)
- Harder to get to sleep than before, even when clearly tired
- Needs feeding, holding, or motion to fall back asleep when they didn't before
- Fussier than usual during the day from accumulated sleep debt
The "suddenly waking exactly 45 minutes into a nap" pattern is the clearest signal. That is one sleep cycle ending and baby not yet knowing how to start the next one.
How Long It Lasts
Most babies work through the adjustment in 2 to 6 weeks.
Parents who respond consistently — with age-appropriate wake windows and a simple wind-down routine — typically see improvement in 2 to 3 weeks. Without any structure, it can drag longer.
What does not go back to "normal" is the sleep architecture itself. Baby is now a light sleeper permanently. The goal is not to reverse that — it is to help baby learn to handle it.
What Actually Helps
Nail the wake windows. An overtired baby is a harder-to-settle baby. At 4 months, the wake window is roughly 90 minutes to 2 hours — meaning baby should be starting their wind-down about 1 hour 45 minutes after they last woke up. Going past this window consistently makes the regression worse, not better.
The baby wake windows by age guide → has exact windows from newborn through 6 months, including the 4-month range.
Create a consistent pre-sleep routine. Not elaborate — 5 to 10 minutes is enough. The same sequence every time: dim the lights, change into sleep clothes, white noise on, feed if feeding to sleep, a short rock or hold, into the crib. The routine itself becomes a sleep cue. Baby's nervous system starts downshifting when the sequence starts.
Differentiate hunger from comfort waking. This is where tracking pays off. When baby wakes at 3am and you're trying to decide whether to feed or settle without feeding — your log tells you when they last ate and how much. A baby who fed an hour ago and is waking again is likely cycling through sleep stages, not genuinely hungry. A baby who hasn't eaten in 3+ hours probably is.
Mommy's Log tracks feeding times so you're not guessing at 3am. You see the last feed at a glance — that one piece of information changes how you respond to a night waking.
Consider where baby is sleeping. If baby has been sleeping in a swing, bouncer, or on you — the 4-month regression is often when the contact sleep becomes unsustainable. Baby is now waking every cycle and needing the same conditions repeated. This is a natural time to start transitioning toward a flat sleep surface, though it does not need to happen overnight.
Lower your expectations temporarily. The regression is doing something — baby is developing. The disruption is a sign of neurological maturation, not failure. Two to six weeks is a long time when you're not sleeping, but it does end.
About Sleep Training
Sleep training is one option. It is not the only option and it is not required.
Some families formal sleep train at 4 months and find it effective and straightforward. Others wait until 5 or 6 months. Others use more gradual methods. All of these can work.
What does not work is waiting for the regression to resolve on its own without any consistency. Some structure — even just wake windows and a bedtime routine — is the minimum. What level of intervention beyond that is a personal choice that only you can make for your family.
Tracking During the Regression
When you are running on broken sleep and every night feels random, data helps.
Looking at three or four days of feeding and sleep notes tells you things you cannot hold in your head: how long the waking stretches actually are (usually longer than they feel), whether the pattern is improving even slightly, whether night wakings are happening at a consistent interval. That pattern — even a messy one — is more useful than the panicked feeling that everything is broken.
Download Mommy's Log free → — log feeds in two taps, no account required. Your partner sees the same log in real time through Partner Sync, so night duty can be shared without a handoff briefing at 2am.
The 4-month regression is genuinely hard. It hits when you thought the worst was behind you and replaces hard-won progress with a new set of challenges. Most parents come out the other side within a month saying it was the hardest stretch of the fourth trimester — and then, eventually, that it passed.
It will pass.
For the specific wake windows that help the most during this stretch, see the baby wake windows by age guide →. And if you're wondering when all the tracking and logging eventually becomes unnecessary, when to stop tracking baby feedings → covers the natural off-ramp from the newborn tracking phase.
Disclaimer: This article is for informational purposes only and does not replace medical advice. If you have concerns about your baby's sleep or development, please consult with your pediatrician.
