The two most well-documented infant sleep regressions happen around 4 months and 8-9-months. If your baby's sleep suddenly gets harder around these ages, it's probably a sleep regression.
The term sounds alarming, as if something has gone wrong or backward, but in most cases, the opposite is true. Sleep regressions happen because your baby's brain is making a biological leap, and once that change stabilizes, so does their sleep.
Note: If you're reading this during a rough stretch around your baby's first birthday, there's a note on that at the end.
What is a sleep regression?
A sleep regression is a period — usually 2–6 weeks — when a baby who was sleeping reasonably well suddenly isn't. Night wake-ups increase, naps shorten, and settling takes longer. It typically coincides with a developmental milestone: something significant happening in your baby's brain or body that temporarily disrupts sleep patterns.
The word “regression” is a misnomer. Sleep isn't actually going backward — the underlying biological change is moving forward. What you're seeing is the disruption that comes alongside growth, not a sign that something has broken.
Two things are consistent across all infant sleep regressions:
- The disruption is temporary
- Keeping routines consistent is the best approach for parents and caregivers
4-month sleep regression
When: 3–5 months (the “4-month” label is an average)
How long: 2–6 weeks for most babies
What's happening: A change in how your baby's brain organizes sleep
This is the most significant sleep regression of the first year, and one with a clear neurological cause. Here's what's going on.
Newborns cycle between just two sleep states — active sleep and quiet sleep — and unlike adults, they enter sleep through active sleep (the newborn equivalent of REM), not through the quiet, deep sleep that comes later. This changes around 3–4 months, when the brain reorganizes into adult-patterned sleep with NREM and REM stages — and that shift is exactly what drives the 4-month regression
Your baby now briefly surfaces toward wakefulness at the end of every sleep cycle — just like adults do. The difference is that adults know how to settle themselves back to sleep without fully waking. Babies are still learning that skill.
The disruption this causes is temporary. Your baby's sleep cycles won't go back to how they were before — the underlying maturation is a one-way change — but the harder nights settle for most families within 2–6 weeks as your baby adjusts to these new sleep cycles.
What you might see
- Increased night wake-ups after a period of longer stretches
- Short naps — 30–45 minutes, ending at the close of one sleep cycle
- Difficulty settling at bedtime
- Fussiness and increased feeding that seems unrelated to hunger
What you can do to help
- Keep wake windows age-appropriate. An overtired baby is harder to settle at every sleep. At 3–5 months, wake windows are roughly 75–120 minutes
- Maintain a consistent, short bedtime routine. Predictable cues — dim the lights, feed, a song — signal to your baby's developing brain that sleep is coming.
- Give your baby a moment to try to settle between sleep cycles before going in. They may surprise you and soothe themselves back to sleep (a great skill for them to learn).
- Avoid introducing new sleep habits during the regression that you'll need to undo afterward. Rocking to sleep every time, for example, can create a dependency that outlasts the regression itself.
Read: 3-6 Month Sleep Guide →
8/9-month sleep regression
When: 7–10 months (peaks around 8–9 months for most babies)
How long: 2–6 weeks for most babies
What's happening: Object permanence, one of the biggest cognitive leaps of the first year
Around 6–8 months, babies develop object permanence. This is the understanding that things (and people) continue to exist even when they can't see them. Before this milestone, when you left the room at bedtime, your baby had no framework for where you went. Now they do. They know you're out there, and at 2 am, waking between sleep cycles in a dark room, the pull to call for you is very strong.
The AAP notes that separation anxiety — the emotional counterpart to object permanence — usually peaks between 10 and 18 months, but its first appearance is often right here, in the 8–9 month window. These predictable anxieties are evidence of a healthy attachment relationship, not a sign that something has gone wrong with sleep.
The 8/9-month regression also often coincides with the 3-to-2 nap transition and with major motor milestones — crawling, pulling to stand, increased spatial exploration. Your baby's brain is processing a lot, and sleep is one of the places it shows up.
What you might see
- Increased night wake-ups after a period of more settled nights
- Resistance to going down at bedtime — clinging, crying when you leave the room
- Short or refused naps
- Early morning waking
- More fussiness during the day
What you can do to help
- Consistency at bedtime is the most powerful tool. Same routine, same room, same approximate time — your baby's developing circadian system responds strongly to predictable cues.
- A brief extra connection step before sleep — a few more minutes of calm holding or singing — can ease the transition to independent sleep without creating a long-term dependency.
- Keep goodbye consistent and warm, not prolonged. A lingering farewell tends to increase rather than reduce anxiety.
- Distinguish regression from nap transition. If your baby is consistently resisting the third nap, that's likely a schedule signal rather than a regression — drop the third nap and move bedtime earlier instead.
Read: 6-9 Month Sleep Guide →
Sleep around 12 months
You may notice sleep getting harder again around your baby's first birthday. What's typically happening is that several things collide at once: Major developmental leaps around walking and early language, potentially the beginning of the 2-to-1 nap transition, and sometimes the lingering tail of the 8/9-month separation anxiety wave.
The response is the same: keep routines consistent and wait it out. There's more detail on what to expect in the 9–12 month sleep guide.
Read: 9-12 Month Sleep Guide →
What works across all sleep regressions
Regardless of which regression you're in, the approach is largely the same:
- Keep the routine consistent. Regressions are temporary. A consistent routine gives your baby's developing circadian system the signals it needs to move through this.
- Watch your baby's wake windows. An overtired baby is harder to settle. If sleep is harder, going down a little earlier than usual often helps more than keeping your baby up longer.
- Give it time. Two to six weeks is the typical length of a sleep regression. Weeks of disrupted sleep can feel long, but it will pass.
Call the pediatrician if sleep disruption is accompanied by significant changes in feeding, growth, or behavior — or if it's been more than 6–8 weeks with no improvement.
