CIO stands for “cry it out,” a sleep training method where you put your baby down awake at bedtime and allow them to cry until they fall asleep on their own. It’s one of the most talked-about (and debated) approaches to teaching babies independent sleep, and it has a more formal name in pediatric research: extinction, or unmodified extinction. The core idea is simple: instead of rushing in at the first cry, you give your baby the chance to self-soothe and drift off without your help.
How CIO Actually Works
The method itself is straightforward. You follow your normal bedtime routine, place your baby in the crib while they’re drowsy but still awake, say goodnight, and leave the room. If your baby cries, you don’t go back in during that initial falling-asleep period. That’s the part that makes it “extinction,” the idea that the association between crying and being picked up gradually fades.
This doesn’t mean ignoring your baby all night. You still go in for regular night feedings and check on them if something seems genuinely wrong. Marc Weissbluth, one of the pediatricians most associated with this approach, is explicit about that. The “no returning” rule applies only to the falling-asleep window, not the entire night.
CIO vs. the Ferber Method
You’ll often see CIO and the Ferber method mentioned together, but they’re different. Full extinction (CIO) means you don’t return to the room at all while your baby is falling asleep. The Ferber method, sometimes called graduated extinction, has you come back at timed intervals to briefly reassure your baby, then leave again. Those intervals get longer each time, maybe three minutes the first check, then five, then ten.
Graduated extinction appeals to parents who find full CIO too intense. Both methods aim for the same outcome: a baby who can fall asleep independently. The trade-off is that graduated extinction typically takes a bit longer, while full extinction tends to produce faster results with more intense crying upfront.
When Babies Are Ready for Sleep Training
The American Academy of Pediatrics recommends building consistent sleep habits starting around 2 months of age, but that doesn’t mean sleep training at 2 months. At that stage, the focus is on routines like placing your baby in the crib drowsy but awake, so they start learning the difference between being held to sleep and falling asleep in their own space. Formal sleep training, including CIO, is generally considered appropriate starting around 4 to 6 months, when babies are developmentally capable of longer stretches of sleep and no longer need as many overnight feedings.
How Long the Crying Lasts
This is the question every parent dreads, and the answer is more reassuring than most people expect. A 2018 study tracking real-world sleep training found that crying peaked on the very first night and resolved within a week. Most pediatric sleep specialists say you should see clear improvement by the end of the first week, with crying largely finished by two weeks. If there’s no improvement in that window, it’s worth pausing and reassessing with your pediatrician rather than pushing through.
The first night is almost always the hardest. Babies may cry for 30 minutes to over an hour. By night three or four, many babies are settling in under 10 minutes.
Does It Actually Work?
A randomized trial of 235 infants (average age 7 months, all waking at least twice a night on most nights) found that after six weeks, only 31% of sleep-trained babies were still waking twice or more per night, compared to 60% in the group that received no intervention. In practical terms, that means for every four families who try it, one family gets a baby sleeping through the night who otherwise wouldn’t have been.
Safety and the Stress Question
The biggest concern parents have is whether letting a baby cry causes lasting harm. The research here is more reassuring than the internet debate suggests. A study published in the journal Attachment & Human Development found no adverse effects on attachment or behavioral development at 18 months in babies whose parents used cry it out.
The cortisol question, whether CIO floods babies with stress hormones, gets a lot of attention. One often-cited 2012 study by Middlemiss found elevated cortisol in infants during an extinction protocol. But subsequent research has painted a more nuanced picture. A pilot study comparing responsive sleep interventions to extinction methods found no differences in infant cortisol between groups across time points. Babies whose mothers were more emotionally responsive during daytime interactions and at sleep time tended to have lower cortisol overall, regardless of sleep training method. In other words, what happens during the rest of your 24-hour relationship with your baby matters at least as much as what happens at bedtime.
One interesting finding from the cortisol research: maternal stress was positively correlated with infant cortisol. When mothers were more distressed, their babies showed higher stress hormones. This creates a difficult loop, because the sleep deprivation that prompts parents to try CIO in the first place contributes to the very stress that can affect their baby.
Effects on Parental Mental Health
Sleep training doesn’t just change the baby’s sleep. A large cluster-randomized trial found that mothers who received a sleep intervention reported significantly fewer depression symptoms at both 10 and 12 months postpartum. Their mental health scores on a standardized scale improved by nearly 4 points compared to mothers in the control group, a meaningful difference. By 12 months, 52% of mothers in the sleep training group reported poor sleep quality, compared to 63% of control mothers. The effect was strongest for mothers who already had elevated depression scores at the start of the study.
This matters because postpartum mental health directly affects parenting, bonding, and family functioning. For some families, the short-term stress of sleep training produces a real and measurable improvement in the home environment.
What CIO Looks Like in Practice
If you’re considering CIO, the process typically follows a few basic steps. First, make sure your baby is at an appropriate age (at least 4 months, ideally closer to 6) and is healthy, gaining weight well, and cleared by your pediatrician to go longer stretches without feeding. Establish a consistent bedtime routine: bath, feeding, book, song, whatever sequence works for your family. The routine signals to your baby that sleep is coming.
Put your baby down awake. This is the key piece. If your baby falls asleep while feeding or being rocked and then wakes up in the crib, they’ll cry because their environment changed. The goal is for the crib to be the last thing they see before sleep, so waking up there in the middle of the night feels normal rather than alarming.
Leave the room. With full CIO, you don’t return until your baby is asleep or until a scheduled night feeding. With graduated extinction, you return at increasing intervals but keep visits brief (under a minute) and boring. No picking up, no feeding, just a quiet reassurance and then out again. Consistency matters more than which version you choose. Responding sometimes but not others tends to make crying worse and longer, because your baby learns that enough crying eventually brings you back.