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How Misconceptions about Sleep Training and Infant Attachment Arose from a Study on Cortisol Levels


If you are scrolling on social media for long enough, then you are bound to hear the following summarized claim about sleep training:

During a sleep training study, it was found that a mother’s and child’s cortisol levels were equally raised during the first and second days of sleep training. During the third night, the child stopped signaling to the mother, which caused the mother’s cortisol levels to drop, but the infant’s cortisol levels remained elevated. (Middlemiss et al., 2012)

This claim correctly summarizes the outcome of this study, and at a glance, it sounds terrifying! However, this is one of the many reasons that we cannot pull sentences out of context when reading research. We always have to look at the study as a whole to truly understand it.

I’ve read this study (about twelve times now!), and I am excited to provide you with some clarity and stress relief regarding this information…hopefully without boring you too much!

Here’s the deal, this blog is going to be denser than my norm! It’s a lot of information, and my goal is to make sure you understand the study, not just my opinion of the study. My biases will still come through (I’m human after all) but the core of this blog will be breaking down the study as best I can for you. If you want to skip ahead to my takeaways on the study, then you can click HERE.


Behavioral Synchrony

Behavioral Synchrony is the idea that as you and your baby grow together, your behaviors synchronize. It’s really a beautiful phenomenon to study (and will give you all the good feels as a parent!).

One research article describes synchrony as “the intricate dance that occurs during short, intense, playful interactions (Bell, 2020).” You may be familiar with describing a relationship or interaction as being “really in tune or in sync” and the same is true here! These are some examples of what this might look like:

· You go to get your baby up from a nap. They give you a big, gummy smile and you give them a big smile and some friendly “parentese” talk.

· Your baby stacks a block correctly, looks at you, and claps their hands at the same time that you start clapping your hands together.

Synchronous interactions become more complex as your baby grows! It might just start with eye contact with a newborn, then exchanging smiles around 8 weeks, up to more verbal interactions as your baby grows. This synchrony develops as you respond appropriately to your infant and your bond continues to grow (Bell 2020).

The cool thing about behavioral synchrony? It is foundational for the development of your baby’s cognitive, social-emotional, and self-regulatory skills! This is truly a case where those little moments where you connect throughout your day matter, and behavioral synchrony is correlated with an increased likelihood of a secure attachment (Middlemiss et al., 2012).


What is the Purpose of the Cortisol Study?

The purpose of this study was to look at the physiological synchrony between mothers and infants. While behavioral synchrony is looking at how Mom and Baby’s actions are mirroring each other, physiological synchrony is going to look at how their bodies are mirroring each other. With physiology, we might be looking at things like melatonin production or, in the case of this study, cortisol: the “stress hormone.”

The authors point out that behavioral synchrony has been well-studied in humans, but “less attention has been directed toward the potential role of physiological synchrony between mothers and infants” (Middlemiss et al., 2012). There’s a lot that we don’t know yet. The authors state that “it is tempting to assume that physiological synchrony between mothers and infants is adaptive and has positive effects on children’s development” (Middlemiss et al., 2012). In other words, we don’t have evidence to support the idea that there is a correlation between physiological synchrony and positive outcomes on a child development; this study can be used as a starting point to start exploring this research.

But what do we know about physiological synchrony? Previous research has indicated that this physiological synchrony doesn’t just happen intuitively, While you know your baby better than anyone, you are not a mind reader! Instead, your baby’s behavior (crying or giving a sad face) will evoke a physiological response in you (Middlemiss et al., 2012).

Research has also shown that children who have insecure attachment patterns are linked to “increased cortisol levels in response to threat and challenge” (Middlemiss et al., 2012). Basically, their bodies have a stronger response to stressful situations.

Have I lost you yet? Essentially, the authors are laying down some trends for you, and the main takeaways are:

· Physiological synchrony between mothers and infants has not been well studied.

· If physiological synchrony has the same benefits as behavioral synchrony, then it may have a positive impact on a child’s development when Mom and Baby’s bodies are in tune.

· Something we do know from previous research is that children who display behaviors of insecure attachment also tend to have higher cortisol levels in response to a stressful situation. Children with secure attachments still have cortisol in their bodies when stressful situations occur, but their cortisol levels are lower.

The authors go on to mention that there are many other things that can disrupt physiological synchrony including the environment, maternal sensitivity, and a mother’s psychological health (i.e. maternal depression) (Middlemiss et al., 2012).


The Study

The study involved 25 mothers and babies between 4 and 10 months old. The researchers brought these mothers and babies to an inpatient facility to conduct a sleep training program. During a five-day period, the mothers would do a bedtime routine with their babies, then they would put their babies down in bed and go to a room nearby. The mother can still hear their babies, but they cannot intervene.  The nurses at the hospital would perform an extinction-based sleep training method and any interactions after bedtime were tended to by a nurse. The nurses would check on the babies after 5-10 minutes of crying to ensure that the baby was safe.

The Hypothesis

In case your science language is rusty from high school, a hypothesis is an educated guess that is going to drive the research experiment! In this case, the authors hypothesized that the mother’s and child’s cortisol levels would stay in sync until the third night when the infant stopped signaling for the mother. Since there was not a behavior to trigger the mother, the mother’s cortisol levels would decrease while the infant’s remained high. They also hypothesized that before bedtime each night the mother’s and baby’s cortisol levels would be synchronized again because they had spent the day bonding together (Middlemiss et al., 2012). 


The Findings

The authors report that this study highlights the fact that the behavioral cues that an infant gives (crying) are essential for the physiological synchrony of mothers and babies. If our babies do not cry, then we are less likely to experience a stress response with our babies. 

The researcher’s hypothesis was correct that prior to each bedtime routine the infant's and mother’s cortisol levels were in sync. After the bedtime routine, the cortisol levels were in sync for the first couple of days; but, after the third day, they were no longer in sync because the infant was no longer crying for longer than 5-10 minutes.

Now if you skim the information and stop there, the research doesn’t look good for sleep training, does it? Babies and moms are beginning to display different cortisol levels, so they are physiologically out of sync. And we want to be in sync because behavioral synchrony is a positive predictor for secure attachments, so we think physiological synchrony might matter too. Cue freak out!

That’s why we are not going to stop there, we are going to read the rest of the article and put some more context to this study…


If you Continue Reading…

The authors state that in this study it is clear that by the third night of sleep training infants were not “learning how to manage their experiences or stress and discomfort” (Middlemiss et al., 2012). The sleep training plan worked because infants were eventually able to self soothe to sleep, BUT it was not a habit yet. The behavior response to stress adjusted more quickly than the physiological response to stress (Middlemiss et al., 2012).

The authors go on to state that if the study lasted more nights, then it is possible that the infant’s physiological response (cortisol levels) would also adjust, meaning the mother’s and baby’s physiological states would be synchronized again (Middlemiss et al., 2012).


My Takeaways

If you have read this whole blog, then this is the theme I want you to keep in mind while we dissect it together:

Scare media rarely comes from lies, it comes from partial truths.

When you run across blogs and social media pages that use this study as a reason to reject sleep training, you will notice that they are using partial truths. They are correct in that the infant’s cortisol levels remained high after three days of sleep training.

Here’s what I think they are not telling you:

  1. Infants can’t be expected to have no stress after three days of sleep training. Think about the last time that you tried something new that was also challenging, did you feel stress-free by the third day of doing it? For my breastfeeding mommas, on the third day of breastfeeding, did you feel completely calm and stress-free about it? Probably not. It was still new to you! Now, some of your stress behaviors may have decreased – maybe you cried a little less out of frustration. Let’s compare that to a mother who has been breastfeeding for 4-6 weeks. They probably still don’t feel like a pro at it, but chances are their stress levels have decreased by now because they are feeling more confident in their new skills. This is also true for a baby who is learning to fall asleep independently. This is a challenging skill to learn, which is why the researchers chose it for their study! We can’t expect a baby to feel fully confident in a new skill after 3 days of practice. Even the authors said that it was possible for this to improve, given more time and consistency.  

2.  There were other things impacting cortisol levels than just sleep training. Not only did the babies have increased cortisol levels because they were learning a new skill, but they were also in a new environment and being tended to by a new caregiver. We know that unfamiliar environments and caregivers can increase cortisol levels as well. Likewise, the authors state that the mother’s cortisol levels may have decreased more quickly due to the knowledge that their child was in the care of a medical professional.

3. We’re missing some baseline information: we don’t know anything about infants’ cortisol levels to prior to the study. If our goal were to study how much an infant’s cortisol peaked during sleep training, then we would want to study their cortisol levels at bedtime prior to starting sleep training. The reason the authors didn’t record this information is because that wasn’t the purpose of their study. They were interested in seeing how a child’s behavioral cues impacted the parents’ physiological symptoms, and if they became out of sync.


This research is a starting point in looking at synchronized physiological responses between mothers and babies, but in no way does it conclude that babies remain under a constant state of raised cortisol levels at bedtime after they are sleep-trained. It also doesn’t conclude that a child will have an insecure attachment if a parent does sleep train.

Neuroscientist Dr. Ilanit Gordon pointed out in a Psychology Today article that synchrony accounts for the minority of interactions we have with each other. Most of the time we are not synchronized with each other. Even if we don’t sleep train, we are not going to be synchronized with our babies all the time, not even most of the time. This suggests that moments of being out of sync with each other is not a cause for alarm, nor an immediate predictor that your child will have an insecure attachment pattern (Gordon, 2020). Another case that proves that you are truly good enough for your baby, even if you are not connected with them all the time.


To Sum it Up

Sleep training is not the deciding factor as to whether your child will have a secure or insecure attachment style. Instead, look for those “synced up” moments during the day. Look for the moments when you look at your baby and you both smile or even the moments when you are both calm and snuggling on the bed. It’s the little moments in this case that truly lead to big benefits.

All being said, I know sleep training can still feel overwhelming and stressful. I would love to help guide you through the process. To set up a call, just click the link below:


The biggest thank you to my editor Emily Schafer,

she takes on the nearly impossible task of providing

clarity in my writings without losing my message or passion.

She nails it every time.



Bell, M. A. (2020). Biobehavioral Synchrony. Biobehavioral Synchrony - an overview | ScienceDirect Topics.,voice%2C%20affect%2C%20and%20touch.

Gordon, I. (2020, June 12). What is synchrony and why is it important? | psychology Today. Psychology Todayt.

Leclère, C., Viaux, S., Avril, M., Achard, C., Chetouani, M., Missonnier, S., & Cohen, D. (2014). Why synchrony matters during mother-child interactions: A systematic review. PLoS ONE, 9(12).

Middlemiss, W., Granger, D. A., Goldberg, W. A., & Nathans, L. (2012). Asynchrony of mother–infant hypothalamic–pituitary–adrenal axis activity following extinction of infant crying responses induced during the transition to sleep. Early Human Development, 88(4), 227–232.

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