What is Baby Sleep Regression?

Published on May 4th, 2021

It’s finally happened. Your baby closed her eyes…and kept them closed…all night long. Then she did it again. And again! After your baby gets the hang of sleeping through the night consistently, parents tend to celebrate—with a bottle of sparkling cider, a mini-date night, or even just savoring those nights of perfect uninterrupted sleep. Until…your little one goes back to waking up in the middle of the night. 

Sweetheart, Baby won’t sleep! I thought we figured this out! What’s the deal?  

What is sleep regression in babies, how does it work, when can you expect it, and how long does it last? 

Baby Sleep Regression: The Basics

Quite simply, sleep regression occurs when your infant’s sleep pattern gets worse after a period of improvement or stability. Instead of progressing towards the next stage of development, a baby regresses back to a previous state—one that is likely characterized by restless nights, shorter naps, difficulty falling asleep, and waking more frequently.

The word “regression” seems negative, but sleep regression is a perfectly normal stage in a newborn’s development, so ordinary that baby sleep experts have pinned these regressions to points in a child’s age: 

  • 4 month sleep regression
  • 8 month sleep regression
  • 12 month sleep regression

Each of these periods has its own underlying causes, durations, characteristics, and management tips.

How long do sleep regressions last? 

Sleep regressions are more of a passing phase than a permanent disruption. The duration varies from child to child and from phase to phase. Most regressions last between two to four weeks, sometimes a bit longer. Even though they feel like forever, sleep regressions are temporary (we promise!). Just take heart and know that soon enough, your little one will be a snoozing little angel dreaming through the night again. 

What’s causing my child’s sleep regression? 

If only our children could tell us exactly why they’re having trouble sleeping! Alas, our newborns don’t yet have the capabilities to explain what they’re experiencing. 

Often, each of the common baby sleep regression ages might link to one cause or another (we’ll get into those details in just a bit), but the same triggers and changes affect all of them:

  • Development advances or changes in stimulation 
  • Sudden growth spurts
  • Pain, including from teething or other illness
  • Changes in routine, environment, or lifestyle

Any parent could tell you that change happens fast, especially in newborns—“blink and you’ll miss it,” as they all seem to lament. It’s so important to identify which phase of development your child is in to understand his sleep habits and how best to support him.

Figuring Out the 4 Month Sleep Regression 

The four-month sleep regression hits first (and for that reason, hardest). 

Just as you’ve finally come to understand and the intricacies of newborn sleep schedules, the first sleep regression appears. Happening anywhere between three and five months old, this initial regression phase is usually caused by a permanent shift in your child’s sleep cycles. While you may cause you to lose some zzzz’s now, it’s a sign your baby is moving properly into the next phase of sleep development. 

The Science Behind the Shift in Sleep 

There’s a reason people boast that they’ve just “slept like a baby.” That’s because babies are expert sleepers, racking up almost 18 hours in a 24-hour cycle. But infants don’t “sleep like babies” forever. At around three to four months, their newborn sleep cycle starts to resemble an adult’s sleep cycle. This change leads to significant differences in the baby circadian rhythm:

  • Infant sleep cycle – Baby sleep cycles are shorter than adults, often lasting just under an hour. They only experience two distinct stages of sleep, in which they spend about equal amounts of time:
    • REM sleep – Also called “active sleep,” this phase is characterized by rapid eye movements (hence the title, REM) and slight twitches, jerky gestures, and faster breathing. This directly contradicts the muscle atonia (lack of movement) that adults experience during REM sleep. Babies also differ from adults in that they start their sleep in the REM phase.
    • NREM sleep – Named “quiet sleep” or non-rapid eye movements sleep, NREM is characterized by stillness. This infantile version of deep sleep is much shorter in babies than adults, mainly because the slower breathing and more restful sleep can be dangerous for prolonged periods. 
  • Adult sleep cycle – As newborns age, they gain two additional sleep stages. The four-stage sleep cycle is completely new to them and isn’t always a smooth transition:
    • Stages 1 and 2 – The first two NREM phases are the lightest. At this point, most people can awaken fairly easily. Newborns are now spending more time in this stage and likely more time waking throughout the night.
    • Stage 3 – The last NREM phase is the deepest point in the adult sleep cycle when it’s difficult to wake the person up.
    • Stage 4 – The final stage is characterized by REM sleep. Unlike infants, adults become more or less paralyzed during this phase to avoid physically acting out their dreams, which is typical of stage four sleep.

This massive transition in sleep development is one basis for the four-month regression, along with the general increase in brain activity, physical development, and increased environmental stimulation as they encounter and react to novel experiences.

Easing Through the 8-Month Sleep Regression 

Your baby is going through some of her most significant milestones and periods of development between seven and ten months of age—exactly when the so-called eight- (or seven- or nine- or even ten-) month regression hits.  

If you’re wondering how there can be a four-month window for a supposedly regularly occurring sleep regression, that’s because of how much variation there is in each baby’s development at this early age. Like the four-month phase, the eight-month regression is driven by the occurrence of many developmental and physical changes  in tandem.

The Science Behind the Shift in Sleep 

With so much happening inside your little angel’s brain and body, it’s no surprise that the previously established rhythm is thrown off.

Your kiddo’s regression is likely caused by their progression in a number of other ways:

  • Serious brain development – Congratulations, your baby is experiencing healthy cognitive development at the right stage! However, the more she knows and understands the world around her, the more there is to worry about. This enhanced cognitive ability and awareness lead to:
    • Separation anxiety – As your child develops a sense of object permanence (the realization that things and people exist even when they can’t see them), she’ll likely also develop separation anxiety. The act of separation may lead to crying and fussiness at nap and bedtimes. 
    • Overstimulation – Everything is new and exciting to infants. With an increased awareness of their surroundings, she may also be overwhelmed by all the lights, colors, smells, sounds, and textures. Just like in adults, it’s hard to sleep when we’re overly stimulated, only exacerbated by your baby’s lack of control and inability to communicate her experiences.
  • Teething – Baby teeth start breaking through a child’s gums between six and eight months old. The physical discomfort from teething frequently causes sleep troubles.
  • Greater physical abilities – Your baby’s body is developing alongside her brain. The increased physical activity of sitting up, crawling, and climbing may cause sleep disruption and decreased interest in bedtime rituals. Your child may also wiggle around in bed more or attempt to climb and roll over, rather than laying and sleeping peacefully. 

Tackling the 12-Month Sleep Regression 

Beyond the presents, cake, and adorable home videos, a first birthday may also be a factor to keep in mind for mild baby’s sleep regression.

The 12-month regression is often shorter than some of the other phases, typically lasting just two weeks or less. Many parents get to skip this period altogether, noticing their baby’s sleep pattern instead and don’t suffer from their own sleep deprivation. However, this period is defined by significant developmental changes (and subsequently, challenges) for many.

The Science Behind the Shift in Sleep 

Blowing out candles and opening presents aren’t the only novel experiences that one-year-olds have. These new experiences and skills can, once again, disrupt your child’s nighttime sleep patterns:

  • Learning to cruise – Some children are already walking along on their first birthday, and most are at least taking cautious steps while holding onto nearby tables, couches, and adults’ hands. And once toddlers taste a bit of freedom, they may want to cruise when they’re supposed to snooze
  • Language acquisition – Learning, remembering, and understanding words requires a massive leap in a baby’s brain development. As young children undergo this period of neurological progress, they may have difficulty adjusting in other areas, namely sleep. 
  • Nighttime fears or nightmares – Though not as common, some one-year-olds will have dreams (and therefore, nightmares) as their imagination develops while others might feel scared at bedtime, whether from the separation, darkness, or perceived dangers. 

Just a head’s up, your little one’s sleep regressions don’t end after they stop being babies and become toddlers. Many of the same disruptions occur in toddlers around the 18-month mark because they’re still having new experiences and widening their thoughts. This period is defined by increased independence and sense of self, which empowers toddlers to resist their bedtime and other parental instructions—na-na-na-na-boo-boo, you can’t catch me, Mom! 

As you attempt to soothe your little rug rat (ahem, angel), just remember that this sleep regression will pass, just like the ones that came before it.

Move Forward, Not Back, With SneakPeek’s Traits Test 

There are so many firsts to look forward to, celebrate, and prepare for. You don’t have to enter these periods blindly—you can get the full scoop beforehand with SneakPeek Traits

One of the major insights of this early DNA test is Sleep Behavior. By rubbing the inside of your kiddo’s mouth with a cotton swab and sending it off to SneakPeek Labs, you can find out what to expect with their sleeping habits, including:

  • Difficulty falling or staying asleep
  • Inclination towards becoming either a night owl or early bird 
  • Sleep latency (how long it takes them to fall asleep)
  • Sleep efficiency (how much sleep they actually get)

Each traits report comes with in-depth (yet digestible) explanations of your child’s genetic predisposition, plus tips and tricks tailored to your child’s DNA so the whole family can get a good night’s sleep! Despite the progressions (and regressions) that are inevitable with any newborn, you’ll have more tools at your disposal to navigate these difficult periods with ease. 

Here’s a first you can get excited about: when you first learn about your child’s unique traits!


Healthline Parenthood. Your Guide to Managing the 4-Month Sleep Regression. 


Plos One. On the development of sleep states in the first weeks of life. https://journals.plos.org/plosone/article?id=10.1371/journal.pone.0224521 

Parenting Science. Newborn sleep patterns: A survival guide for the science-minded parent. https://www.parentingscience.com/newborn-sleep.html 

NCBI. Physiology, Sleep Stages. https://www.ncbi.nlm.nih.gov/books/NBK526132/ 

Sleep Foundation. 8-Month Sleep Regression. https://www.sleepfoundation.org/baby-sleep/8-month-sleep-regression 

Grow by WebMD. Baby Development: Your 8-Month-Old. https://www.webmd.com/parenting/baby/baby-development-8-month-old#2 

MedlinePlus. Teething. https://medlineplus.gov/ency/article/002045.htm 

Healthline. The 12-Month Sleep Regression: What You Need to Know. https://www.healthline.com/health/baby/12-month-sleep-regression 

Sleep Foundation. 12-Month Sleep Regression. https://www.sleepfoundation.org/baby-sleep/12-month-sleep-regression 

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SneakPeek aims to provide the most accurate and up-to-date information to help our readers make informed decisions regarding their health before, during, and after pregnancy. This article was written based upon trusted scientific research studies and/or articles. Credible information sources for this article are cited and hyperlinked.

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