(Revised and updated from an earlier version)
At Baby Sleep Science we don’t like the term “four-month regression,” because the changes in sleep that occur around 3–4 months old aren’t really a regression at all; they result from a permanent change in sleep architecture.
Sleep is made of several stages. You've probably heard of "deep" sleep and rapid eye movement (REM) sleep, and there is also Stage 1 (the light sleep that makes up the transition from awake to asleep) and Stage 2 (a light sleep that makes up half of a typical night). Each of these stages of sleep is associated with the different benefits that come from a night of sleep, including learning and memory consolidation. When a baby is born, sleep is immature. For the first few months, babies don't enter all of the sleep stages. Instead, sleep is a fairly constant state, alternating between two types of immature sleep (sometimes called "active" sleep and "quiet" sleep).
A healthy newborn (free of colic, reflux, or other medical concerns) will sleep until one of three things happens: 1) hunger, 2) another need (wet/dirty diaper), 3) sleep is no longer needed. This pattern is difficult at first, but for most families sleep unfolds as you expect it to; your baby begins life with fragmented sleep and frequent feedings. Virtually every parent is prepared for disrupted sleep at this time. Over the course of the first few months or so, the interplay between sleep and those waking factors is intuitive. As you get to know your baby, you start to learn when he or she will be sleepy, and you anticipate that need. As your baby needs less frequent feedings, he or she will naturally start to sleep longer stretches during the day and at night. These stretches might start with three hours, but over the course of a few weeks, they grow to four hours, then five hours, and so on. Some lucky parents might even see their baby naturally sleep through the entire night during this time (10–12 hours straight!). This is a beautiful progression that feels very natural and appropriate.
Need More Help?
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SLEEP TRAINING CLASS (for well babies 6-15 months)
PHONE CONSULTATIONS (with one of our sleep experts)
THE SECRET TO NAPS (downloadable e-book)
Unfortunately, somewhere around 3–4 months everything changes. Around this time, sleep matures from being newborn sleep to a dynamic progression between sleep stages. As your baby grows, deep sleep will begin to dominate the beginning of the night, the middle of the night will be dominated by Stage 2, and REM sleep will dominate the last third of the night. However, these stages aren't continuous, they flow together in sleep cycles that contain a little bit of different stages of sleep. Sleep cycles last about 60 minutes during the night and are marked by a brief waking that happens at the end of the cycle. This brief waking is the cause of most “sleep problems.”
The brief waking that happens at the end of a sleep cycle is meant to be protective (adults have it too). It allows your baby to briefly check in with the environment and ask, “Am I ok?” When we fall asleep, we expect our environment to remain constant throughout the night, so if your baby has fallen asleep in your arms and you then transferred him or her to the crib, the crib will feel wrong when s/he hits the checking wake-up. This means that after every sleep cycle, your baby will probably wake up, and you can interpret his or her cries as, “Hey, Mom! Dad! Something is wrong here!”
If you don’t know that this sleep “regression” is coming, then you’ll probably do what you normally do when your baby wakes up. For many families, the default response is to offer a feeding, but for some, it will involve rocking, bouncing, or a pacifier replacement. Since what you do at bedtime is how your baby knows how to go to sleep, s/he will fall back asleep for another hour or two and then wake again at the end of the next sleep cycle. At that point, you’ll feed/rock/bounce/replace the pacifier again. If you have been feeding your baby overnight, after a few nights of this, you’ll convince yourself that this is a growth spurt or teething or that your child getting sick, but the waking usually doesn’t resolve on its own (hence why “regression” is the wrong term). At this point, you’ll have reinforced that feeding/rocking/bouncing/pacifier replacement is needed to get to sleep. If offering a feeding is your default response, then your baby will have started to take in more calories during the night. Offering multiple "extra" feedings per night also means that the wakings now have a dual nature: 1) your baby is waking due to the sleep association, but 2) he or she is also now genuinely hungry at night and probably eating less during the day. Many babies get stuck in this reversed feeding pattern because another change that happens around 4 months is that most babies become very interested in the world around them and become very distracted during daytime feedings. This reduction in daytime feeding paired with increased night feeding means that you’re stuck in a pattern that can be difficult to fix.
During this phase, most babies also become chronic cat-nappers and start to have short naps of 30-45 minutes. We describe more about these nap changes in our series on naps.
How can you prevent or minimize the impact of the four-month regression?
Teach your baby to fall asleep without your help
If your baby is younger than 3–4 months and hasn’t yet had a sleep regression, then the best thing you can do is give your baby practice falling asleep in the place where you would like him or her to sleep all night. Start by creating a little nap and bedtime cue (e.g., singing a specific song, rocking, bouncing, comforting), and when your baby is tired, put him/her in the sleep space awake. Your baby may be confused and frustrated by this, and that’s fine; just pick him/her up and repeat your sleep cue. You can rock/nurse to sleep, or you can try to put him in the crib again, awake. At this point in a baby’s life, you shouldn’t try to force the crib if your baby isn’t ready to fall asleep there on his/her own, but you do want to teach him/her that the crib is a reasonable place to sleep. If your baby never even has the chance to fall asleep there, then there’s no reason h