For most healthy babies over 4 to 6 months old, letting them cry for short periods, particularly during sleep training, does not cause lasting harm. The research on this is reassuring: studies measuring stress hormones and tracking children’s emotional development over time consistently find no negative long-term effects. But the answer depends heavily on your baby’s age, the context of the crying, and how you approach it.
What the Research Says About Stress
The biggest fear parents have is that letting a baby cry floods their system with stress hormones and causes some kind of damage. The evidence points in the opposite direction. A study published in the journal Pediatrics evaluated 43 sets of parents and babies in Australia, measuring cortisol (the body’s main stress hormone) in the babies’ saliva. Babies in the sleep training groups actually showed slightly lower cortisol levels than babies who had no sleep training at all. After one year of follow-up, there was no difference among any of the groups in children’s emotional and behavioral health or in the strength of the parent-child bond.
This pattern holds across multiple studies. Babies who go through sleep training that involves some crying don’t end up more anxious, less securely attached, or emotionally different from babies whose parents responded to every cry immediately. The relationship you build with your baby across hundreds of daily interactions matters far more than a few nights of crying at bedtime.
Age Makes a Major Difference
Newborns and very young babies are a different story. In the first three to four months of life, babies have almost no capacity to calm themselves down. Their self-soothing ability develops gradually across the entire first year, and it develops precisely because a caregiver shows up to comfort them. Knowing a loving adult will be there when the world gets overwhelming is, according to child development experts at ZERO TO THREE, a baby’s first experience with learning self-control.
For babies under 4 months, responding promptly and sensitively to crying fosters secure attachment, improves sleep over time, and actually reduces overall crying. This is the period sometimes called the “fourth trimester,” when babies still need near-constant regulation from a caregiver. Letting a very young infant cry it out is not recommended, not because a few minutes of crying is dangerous, but because these babies genuinely cannot settle themselves yet, so the crying serves no developmental purpose.
By around 4 to 6 months, most healthy, full-term babies have developed enough neurological maturity to begin learning to fall asleep on their own. This is when structured sleep training approaches become appropriate.
Different Approaches to Crying and Sleep
There’s no single “cry it out” method. The approaches exist on a spectrum, and they perform similarly in studies.
- Unmodified extinction: You put the baby down awake and don’t return until morning (or the next scheduled feeding). This is what most people picture when they hear “cry it out.”
- Graduated extinction (the Ferber method): You check on the baby at increasing intervals, offering brief reassurance without picking them up. Intervals typically start at a few minutes and stretch longer over successive nights.
- Camping out: You stay in the room with the baby but gradually reduce your involvement over days or weeks, eventually moving your chair out the door.
When researchers have compared these methods head-to-head, the outcomes for babies are largely the same. Where differences do emerge, they tend to favor gentler approaches. Camping out has been found to be less stressful for parents and more helpful for babies experiencing high levels of separation anxiety. For parents who can’t stomach the idea of walking away entirely, graduated methods or staying in the room work just as well.
When Crying Is Telling You Something Else
Sleep training is one thing. A baby crying during the day is communicating a need, and learning to read those signals matters. Crying is actually a late sign of hunger. Before a baby cries from hunger, you’ll typically see earlier cues: hands going to the mouth, head turning toward your breast or a bottle, lip smacking or licking, and clenched fists. If you can catch these signals before the crying starts, feeding goes more smoothly and your baby spends less time distressed overall.
Context is everything. A baby crying at 2 a.m. after being fed, changed, and put down in a safe sleep space is in a very different situation from a baby crying inconsolably during the day with no obvious cause. Persistent, intense daytime crying in a young infant, especially if it seems painful, always warrants a closer look to rule out illness, reflux, or other medical issues.
Premature Babies Need Extra Caution
One group that deserves special mention is premature babies. Research on blood oxygen levels shows that while all babies experience a temporary dip in oxygen saturation during crying, preterm infants recover much more slowly than full-term babies. Their systems are less mature when it comes to regulating the physical stress that crying creates. If your baby was born early, talk with your pediatrician before starting any approach that involves sustained crying.
The Effect on Parents
The conversation about crying usually centers on the baby, but parental wellbeing is part of this equation. Sleep deprivation in new parents isn’t just uncomfortable. It’s a risk factor for postpartum depression and anxiety. A study published in BMJ Open found that after a sleep intervention (which involved allowing some infant crying), maternal depression scores dropped by 66% on average. The proportion of mothers meeting criteria for any degree of depression fell from 32.5% to just 5%. Anxiety decreased by 44% and overall stress by 42%.
A parent who is rested and emotionally regulated is better equipped to be responsive and attuned during all the waking hours. Sleep training isn’t selfish. It’s one of those situations where taking care of yourself directly benefits your baby.
Practical Guidelines
If you decide to let your baby cry as part of a sleep training approach, a few principles make it safer and more effective. Wait until your baby is at least 4 months old (adjusted age for preemies). Make sure the crying isn’t from hunger, a dirty diaper, illness, or pain before you begin a training session. Keep the bedtime routine consistent: same time, same sequence of events, same sleep environment.
If full extinction feels too intense, start with graduated check-ins. Go in at short intervals at first, briefly reassure your baby with your voice or a gentle touch, and leave again. Lengthen the intervals over successive nights. Most babies show significant improvement within three to five nights, though some take longer. The key is consistency. Starting and stopping repeatedly tends to make the process harder, not easier, because the baby learns that enough crying eventually brings you back for good.
Some babies take to it quickly. Others protest more intensely. Neither response says anything about your baby’s temperament long-term or about your relationship. It says something about how entrenched the current sleep habits are and how your individual baby handles change.