So while the infants' internal physiological distress levels had not changed, their outward displays of that stress were extinguished by sleep training.

CNN: Should babies be allowed to 'cry it out'?

Half the babies were placed in a sleep-training group and the other half in a control group that did not use sleep training.

CNN: Should babies be allowed to 'cry it out'?

Observing 25 infants aged 4 to 10 months in a five-day inpatient sleep training program, researchers monitored levels of the stress hormone cortisol in the babies, who were left to cry themselves to sleep without being soothed.

CNN: Should babies be allowed to 'cry it out'?

On the pro sleep-training side, an Australian study published in September followed 326 children with parent-reported sleep problems at 7 months.

CNN: Should babies be allowed to 'cry it out'?

They had been planning to attend political training classes and participate in group activities during the day and sleep in tents at night, he said.

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She added this could specifically help medical and military training and further research could be carried out to see if sleep loss affected other brain and memory functions in a similar way.

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