Is Sleep Training Bad for Babies? What the Research Actually Says

If you've spent any time searching for answers about your baby's sleep at 2am, you've probably stumbled across very strong opinions on both sides. Sleep training is "cruel and damaging." Sleep training is "the only thing that saved us." The internet is not short of certainty.

But what does the research actually show? Not the blog posts, not the parenting forums — the peer-reviewed studies that looked at real babies and measured real outcomes. This article walks through what scientists have found, where the evidence is clear, where it's still debated, and what all of it means for your family.

Why Parents Worry About Sleep Training

The concern is understandable and worth taking seriously. It usually centers on a few key fears:

• Attachment damage: Will leaving my baby to cry undermine the trust we've built?
• Stress hormones: Does prolonged crying flood a baby's brain with cortisol and cause lasting harm?
• Emotional security: Will my child grow up feeling less safe, less loved?

These aren't irrational worries. They come from a genuine place of love and from real (if sometimes misapplied) developmental research. Attachment theory tells us that consistent, responsive caregiving builds secure children — and the leap to "any crying alone = attachment damage" is an easy one to make.

So let's look at what the studies actually measured.

What the Research Shows

The Cortisol Question

One of the most-cited studies raising concern about sleep training is Middlemiss et al. (2012), published in Early Human Development. The researchers measured cortisol levels in infants and mothers during an extinction-based sleep program and found something striking: by the second and third nights, babies had stopped crying — but their cortisol levels remained elevated even when they were quiet.

This finding alarmed many parents and commentators. If babies are still stressed even when they've stopped crying, doesn't that mean they've simply given up?

It's worth reading what the researchers themselves concluded. Middlemiss and colleagues called for more research and noted that the dissociation between outward behavior and cortisol was the key concern — not that sleep training was definitively harmful. Critics of the study also point out that elevated cortisol alone does not equal psychological damage; the body uses cortisol in many normal contexts, and a single snapshot of stress hormones cannot tell us about long-term outcomes.

The Large-Scale Randomized Trial

One of the most robust studies in this space is Price et al. (2012), a randomized controlled trial published in Pediatrics following 326 families. Parents in the intervention group received advice on behavioral sleep techniques — including controlled crying — starting at seven months. The researchers followed these families until the children were six years old.

At every follow-up point, there were no significant differences between children who had been sleep trained and those who hadn't on measures of emotional and behavioral development, parent-child attachment, or child sleep and behavior. The children who received the intervention slept better in the short term, and that improvement did not come at the cost of long-term wellbeing.

This is one of the few studies with a long enough follow-up period to actually answer the attachment and emotional outcome questions that parents care about most.

Graduated Extinction vs. Bedtime Fading

Gradisar et al. (2016) compared three approaches in a randomized trial: graduated extinction (the "controlled crying" approach where parents check in at increasing intervals), bedtime fading (delaying bedtime until the child is drowsy enough to fall asleep quickly), and a control group with no intervention.

Both sleep training methods reduced infant sleep problems and parental stress. Critically, the study also measured infant cortisol, stress, and emotional and behavioral outcomes — and found no significant differences between the groups. Infants in all conditions, including the sleep training groups, showed similar patterns of attachment behavior toward their parents.

The authors concluded that both methods were effective and safe, and that parents could choose based on their own comfort and preference.

The Melbourne Study

Hiscock et al. (2007), published in BMJ, followed 738 families and found that a behavioral sleep intervention at eight months significantly reduced maternal depression at ten months. Sleep deprivation in parents is not a trivial concern — it is strongly linked to postnatal depression, relationship breakdown, and reduced parental sensitivity. When we talk about what is best for the baby, the wellbeing of the caregiving parent has to be part of that conversation.

What "Cry It Out" Actually Means — and What It Doesn't

Much of the debate around sleep training is muddied by imprecise language. "Cry it out" (CIO) has become a catch-all term that parents and commentators use to mean very different things.

Full extinction means placing the baby down awake and not returning until morning. This is the strictest version and the one most parents find hardest to implement.

Graduated extinction (the Ferber method and its variations) involves responding to your baby at set intervals — checking in, offering brief reassurance, then leaving again. The intervals increase over time. The baby is never left entirely without contact.

Chair method / Sleep Lady Shuffle involves the parent sitting beside the crib and gradually moving further away over one to two weeks — no leaving the room at all in the early stages.

Bedtime fading involves no crying by design. You push the bedtime later until the child falls asleep quickly, then gradually move it earlier.

None of these are the same thing. Studies that look at "sleep training" outcomes often lump them together, and critics of sleep training often describe the strictest version as if it represented the entire category. Your pediatrician, your friends, and your mother-in-law may all mean something completely different when they say "sleep training."

What the Research Does Not Show

The research does not show that sleep training is harmless under all circumstances, for all babies, at all ages. The studies above were conducted on healthy, full-term infants, typically from six to eight months of age, with responsive and attentive parents. Extrapolating their findings beyond those conditions requires caution.

There is also very little high-quality long-term research on sleep training in the first six months of life. Most experts and pediatric organizations do not recommend behavioral sleep interventions before four to six months, partly because frequent night waking in newborns serves a biological function — including supporting feeding — and partly because the evidence base simply doesn't exist yet for younger infants.

When Sleep Training Is Not Recommended

Regardless of your views on the evidence, there are circumstances where behavioral sleep approaches should be avoided or approached with particular care:

• Under four months of age: Infants need frequent night feeds and are neurologically too immature for most sleep training approaches to be effective or appropriate.
• Preterm infants: Developmental age, not chronological age, matters here.
• Medical concerns: Reflux, airway issues, or any condition causing pain at night means sleep problems may have a medical cause that needs addressing first.
• Attachment concerns: Infants who have experienced significant early neglect, trauma, or disruption to caregiving relationships may need specialist support before behavioral approaches are appropriate.
• Recent major life disruptions: Starting sleep training during a house move, parental separation, illness, or another significant stressor is unlikely to go smoothly and may add unnecessary stress to an already difficult period.

If any of these apply to your family, talking to your pediatrician before starting any sleep training approach is the right first step.

Putting It Together

The honest summary of the research is this: for healthy babies from around six months of age, behavioral sleep training methods — when implemented thoughtfully — have not been shown to cause harm to attachment, emotional development, or long-term wellbeing in the studies that have looked for these outcomes.

The cortisol finding from Middlemiss et al. is real and worth knowing about. But it exists alongside multiple larger, longer studies that followed children to school age and found no differences in the outcomes parents care most about.

That does not mean sleep training is the right choice for every family. Some parents will try a method, find it feels wrong for them, and stop — and that is a completely legitimate decision. The research gives you permission to sleep train if you want to. It does not tell you that you have to.

What is clear is that severe sleep deprivation — in babies and in parents — has its own costs. Exhausted parents are less able to be responsive during waking hours. That matters too.

The question is rarely "is sleep training bad?" in the abstract. The more useful question is: what approach fits my baby, my family, and our current circumstances — and am I doing it in a way that is consistent, appropriate for my child's age, and supported by people who know what they're doing?

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If you're trying to figure out the right approach for your family, a certified sleep consultant can help you choose a method that fits your baby's age and temperament, your parenting style, and your goals — and guide you through it with the kind of individualized support that no article can replace. Browse our directory to find a qualified consultant near you.