Not too strong and not too weak. That’s what makes the perfect cuddle, according to a new study in Japan, which measured the calming effect a hug can have on infants at different pressures – also comparing the differences between receiving one from strangers and from parents.

Researchers at Japan’s Toho University in Tokyo, Japan, monitored heart rates of infants. They also placed pressure sensors on the hands of the adults. These two measurements allowed them to compare how babies reacted in different situations – from just being held by an adult to a tight hug. The results showed that a medium-pressure hug was the most soothing option.

The length of the hug was kept at 20 seconds as “it was almost impossible to avoid infant’s bad mood during a one-minute or longer hold or hug,” the researchers wrote in their paper. For infants older than 125 days, the calming effect was more significant when receiving a hug from parents rather than strangers.

But the infants aren’t the only ones who can benefit from a soothing hug. The study showed that parents also felt calmer while hugging their child. Normally, the oxytocin hormone is released during close physical contact, but researchers at Tohu University argued that the duration of the hug was too short for the hormone to play a role.

“The infants older than four months old showed a high increase ratio of heartbeat intervals during hugging by their parents than by female strangers,” lead author Sachine Yoshida said in a statement. “Parents also showed a high increase ratio of heartbeats intervals by hugging their infants. We found that both infants and parents come to relax by hugging.”

During the hug, parents and infants had an increase in what’s known as the R-R interval (RRI) on an electrocardiogram, which shows a slower heart rate. The R-R interval is the time between a particular waveform that measures electrical activity of the heart.

The younger babies, who were less than four months old, didn’t show the same RRI increase during a hug, according to the study Nevertheless, they had a slowed heart rate when a parent’s hand put pressure on their back while being held. Researchers claim that this suggests that babies don’t make the same distinction between X and Y as older infants do between being held and being hugged.

The researchers expected a hug would lead to visible and clear changes in an infant’s behavior, such as turning a bad mood into a good one. But this wasn’t the case all the time. The soothing effects of the hug were only visible in infants who were calm, and weren’t crying or fussy.

According to the researchers, this is the first time the physiological impact of hugging infants has been measured. The findings will help to know more about parent-child bonding and child psychology. At the same time, as the study focuses on the sensory inputs received during a hug, it could also have implications for the early detection of autism.

“Children with autism spectrum disorder (ASD) have difficulties in sensory integration and social recognition,” Hiromasa Funato, study author, said to AFP. “Therefore, our simple hug experiment might be utilized in the early screening of the autonomic function, sensory integration, and development of social recognition in infants with high familial risk for ASD.”

The study was published in the journal Cell