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Scientists find the best way to soothe a crying baby

The sound of a crying infant can be very distressing to parents. They seek to soothe the baby and may resort to numerous ploys in order to restore the baby to a state of peace. None of these methods has been scientifically proven, however, relying rather on trial and error or on the advice of experienced caregivers. In a new study by Japanese researchers, four different soothing methods were tested and the efficacy of each was assessed based on changes in the baby’s heart rate and whether the baby stopped crying. 

“Many parents suffer from babies’ nighttime crying,” says corresponding author Kumi Kuroda of the RIKEN Center for Brain Science in Japan. “That’s such a big issue, especially for inexperienced parents, that it can lead to parental stress and even to infant maltreatment in a small number of cases,” she says.

The study details how crying babies are physiologically affected by being held, carried, and laid down. The researchers made use of ECG data on heart rate, and hand-held video cameras, to track the physiological and behavioral state of the baby as it was soothed. Mothers were given four methods for calming the baby: hold the crying infant, hold and walk with the infant, place the infant in a cot or crib, and place the infant in a mobile crib or stroller that can be moved back and forth. At each heartbeat, behavior was assessed as asleep, alert, or crying, and scored accordingly. This way the researchers could track changes in both behavior and physiology with sub-second precision.

In previous research, Kuroda and her colleagues found that mother mice pick up their babies (pups) by mouth and carry them when they are distressed. The mouse pups show a specific response, termed the “Transport Response,” when picked up by their mothers. This involves a complex series of parallel biological processes that results in reduced crying and lower heart rates, which helps the parents to transport the infants. Kruoda and the team wondered whether human infants would show similar physiological responses to being carried around, and whether this would have a calming impact when the infant was distressed.

The Transport Response is found in many mammals that have altricial young (born underdeveloped and helpless) that are not able to walk around on their own. Such mammals include dogs, cats and monkeys. We are more familiar with this response when picking up young puppies or kittens by the scruff of the neck. They tend to hang motionless and become more docile, but their bodies do not go entirely limp; they maintain a certain compactness of posture, with legs pulled in slightly, perhaps making it easier for their parents to carry them to a new location.

The researchers compared the responses of 21 human infants to soothing using the four different methods. They found that when the mother walked while carrying the baby, the crying infants calmed down and their heart rates slowed within 30 seconds. In fact, when mothers walked in this way for a period of five minutes, the infants became so calm that around 50 percent of them fell asleep. A similar calming effect occurred when the infants were placed in a rocking cot, but not when the mother held the baby while sitting, or placed the baby in a motionless cot.

As Kuroda explains, “walking for five minutes promoted sleep, but only for crying infants. Surprisingly, this effect was absent when babies were already calm beforehand.” Among the babies studied, all had stopped crying by the end of the five-minute walk and had reduced heart rates. However, sitting and holding a crying baby was not calming; the heart rate tended to increase and crying persisted. Heart rates also increased when walking mothers turned, or when they stopped walking, showing that babies are very sensitive to their mother’s movements.

Unfortunately, the study found that when the mothers tried to put down their sleeping babies after calming them, more than one-third of the infants became alert again within 20 seconds. Analysis of the data showed that all the babies produced physiological responses, including changes in heart rate, the moment they became detached from the mother’s body. However, if the infants were asleep for a longer period before being laid down, they were less likely to awaken during the process, the team found.

“Even as a mother of four, I was very surprised to see the result. I thought whether a baby awoke during a laydown was related to how they’re put on the bed, such as their posture, or the gentleness of the movement,” Kuroda says.  “But our experiment did not support these general assumptions. Although we did not predict it, the key parameter for successful laydown of sleeping infants was the latency from sleep onset.” Babies often woke up if they were put down before they got about 8 minutes of sleep.

Based on their findings, the researchers propose a method for soothing and promoting sleep in crying infants. They recommend that parents hold crying infants and walk with them for five minutes, followed by sitting and holding infants for another five to eight minutes before putting them to bed. The protocol, unlike other popular sleep training approaches such as letting infants cry until they fall asleep themselves, aims to provide an immediate solution for infant crying. Whether it can improve infant sleep in the long-term requires further research, Kuroda says.

Although the research involved only mothers and their infants, Kuroda expects the effects are likely to be similar for fathers and any other caregivers. She emphasizes that this study only included a small samples of infants and will need to be repeated with bigger sample sizes in order to verify the results. Additionally, this procedure does not address why some babies cry excessively and cannot sleep, but it does offer an immediate solution that can help parents of newborns. 

The researchers recognize the usefulness of heartrate data in this approach to soothing a baby and hope to make it accessible to parents. “We are developing a ‘baby-tech’ wearable device with which parents can see the physiological states of their babies on their smartphones in real-time,” says Kuroda. “Like science-based fitness training, we can do science-based parenting with these advances, and hopefully help babies to sleep and reduce parental stress caused by excessive infant crying.”

The research is published in the journal Current Biology.

By Alison Bosman, Staff Writer

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