- When: usually between 3 and 5 months, most often around 4 (corrected age for babies born early).
- Why: sleep matures into adult-like cycles. Between cycles your baby surfaces briefly, and needs whatever helped them fall asleep to get back down.
- How long: the rough patch often lasts 2–6 weeks. The new way of sleeping is permanent, and that's a good thing.
- What helps: a short, same-every-night routine, some chances to fall asleep in the crib, and an early bedtime after short naps.
When does the 4 month sleep regression start?
Most parents notice it somewhere between 3 and 5 months, and around 4 months is the most common time. The calendar above uses a window of 14 to 20 weeks; enter the birth date and it shows the exact dates for your baby. For a baby born more than 3 weeks early, count from the due date: a baby born 6 weeks early usually gets there closer to 5½ months after birth.
The 3 month sleep regression
It's the same thing, arriving early. The changes behind it start around 2 months, and some parents notice the signs from about 3 months. If your 3-month-old suddenly sleeps worse after a good stretch, everything on this page applies.
What's really going on
A newborn's sleep is simple: two states, often called active and quiet sleep, with no regular rhythm between them. In the months that follow, sleep is rebuilt. The AAP puts it plainly: babies don't have regular sleep cycles until about 4 months of age. A small study that recorded 7 babies' sleep at home overnight showed the biggest changes in the sleeping brain between 2 and 6 months, as the share of quiet, deeper sleep grows.
| Newborn sleep | Sleep from about 4 months | |
|---|---|---|
| Kinds of sleep | active and quiet sleep | light and deep stages, like an adult's |
| Falling asleep | often straight into active (REM) sleep, until 2–3 months | into light, quiet sleep first |
| Cycles | no regular cycles | regular cycles, with a brief surfacing between them |
| Day and night | sleep spread around the clock | a day–night rhythm, which starts to show after about 2 months |
The catch is that brief surfacing between cycles. Adults do it too and don't remember. A baby who fell asleep feeding or being rocked wakes into a different situation from the one they fell asleep in, and calls for it back. That's why the waking can come every one or two hours, and why naps often stop at one cycle, 30 to 45 minutes.
Signs of the 4 month sleep regression
- Waking every 1–2 hours at night after weeks of longer stretches.
- Naps that suddenly end at 30–45 minutes.
- Harder to settle at bedtime, fussing or crying when put down.
- More night feeds, or wanting to feed back to sleep.
What it shouldn't look like: a fever, a cough or runny nose, vomiting, fewer wet diapers or a baby who isn't feeding well. Those point to illness, not a regression; call your pediatrician.
How long does the 4 month sleep regression last?
The rough patch often lasts 2 to 6 weeks. Nobody has measured that precisely, so take it as what parents and pediatric sleep advice commonly report. The change itself doesn't go away: your baby now sleeps in cycles for good. What improves is how they handle the moments between cycles, and that's where you can help.
What helps
- A short bedtime routine, the same every night. Feed, change, a song, lights down. Predictable steps tell a baby that sleep is coming; in studies with older babies and toddlers, a nightly routine shortened the time to fall asleep and cut night waking within weeks.
- Some chances to fall asleep in the crib. Put your baby down drowsy but awake now and then, at the start of the night when it's easiest. In a survey of 5,006 families, babies whose parents encouraged self-soothing slept longer and in fewer pieces. It doesn't have to be every time.
- Shorter wake windows after short naps. A baby who napped 30 minutes gets tired sooner. The nap planner shows wake windows by age and re-plans the rest of the day after a short nap.
- An early bedtime on rough days. After a day of short naps, putting your baby down earlier usually helps rather than hurts.
- Feed when hungry. Many 4-month-olds still need night feeds. Not every waking is habit.
- Safe sleep doesn't change. On the back, on a firm flat surface, in an empty crib or bassinet, even at 3 a.m.
What the evidence says
“4 month sleep regression” isn't a medical term; you won't find it in pediatric guidelines or sleep research. The change behind it is one of the best documented things in infant sleep. Recordings show the sleeping brain changing most between 2 and 6 months. Until 2–3 months, babies often fall asleep straight into REM sleep. The day–night rhythm appears after about 2 months. And in a study that followed 75 babies with sleep diaries for a year, the fastest consolidation of sleep happened in the first 4 months: by 3 months, half the babies slept from midnight to 5 a.m. without needing a parent.
What hasn't been measured is the “regression” part: how many babies sleep worse for a while, and for how long. So the timing is solid; the 2–6 weeks is experience, not data.
What's next
Next on the calendar is the 6 month sleep regression, which is mostly a nap change and new skills rather than a brain change. Around the same time as the 4-month change, many babies move from four naps to three; the nap planner shows when. And for the whole picture, the sleep regression calendar puts every age on your baby's dates.
Frequently asked questions
When is the 4 month sleep regression?
Usually between 3 and 5 months, most often around 4 months. For a baby born more than 3 weeks early, count from the due date. Enter the birth date in the calendar on this page to see the dates for your baby.
How long does the 4 month sleep regression last?
The rough patch often lasts 2 to 6 weeks, though nobody has measured it precisely. The change underneath, sleep in adult-like cycles, is permanent; what gets better is how your baby gets through the brief wake-ups between cycles.
Can the sleep regression start at 3 months?
Yes. The changes in sleep start around 2 months, and some parents notice the signs from about 3 months. A 3 month sleep regression is the same thing arriving early.
Does every baby have the 4 month sleep regression?
Every baby's sleep matures around this age, but not every baby sleeps worse because of it. Babies who already fall asleep in the crib often pass through with barely a bump.
Should I sleep train during the 4 month sleep regression?
You don't have to. A consistent routine and some chances to fall asleep in the crib are linked to longer, less broken sleep, and they work at this age. If you're considering a formal method, talk it over with your pediatrician first.
Is it the 4 month regression or is my baby sick?
A regression looks like a well baby who feeds normally but wakes more. Call your pediatrician if the waking comes with a fever, vomiting, breathing trouble, poor feeding or fewer wet diapers.
Sources
- HealthyChildren.org (AAP): Getting Your Baby to Sleep: babies don't have regular sleep cycles until about 4 months
- Jenni O.G., Borbély A.A., Achermann P., Development of the nocturnal sleep electroencephalogram in human infants, Am J Physiol Regul Integr Comp Physiol 2004: largest changes between 2 and 6 months
- Grigg-Damberger M.M., The visual scoring of sleep in infants 0 to 2 months of age, J Clin Sleep Med 2016: sleep onsets more often REM until 2–3 months
- Rivkees S.A., Developing circadian rhythmicity in infants, Pediatrics 2003
- Henderson J.M.T. et al., Sleeping through the night: the consolidation of self-regulated sleep across the first year of life, Pediatrics 2010
- Mindell J.A. et al., A nightly bedtime routine: impact on sleep in young children and maternal mood, Sleep 2009 (children 7–36 months)
- Sadeh A. et al., Sleep and sleep ecology in the first 3 years: a web-based study, J Sleep Res 2009
- HealthyChildren.org (AAP): Corrected Age for Preemies
- Moon R.Y. et al., Sleep-Related Infant Deaths: Updated 2022 Recommendations for Reducing Infant Deaths in the Sleep Environment, Pediatrics 2022
Spotted a mistake or an outdated figure? Write to [email protected].