Researchers at the University of North Texas found that youngsters who are deliberately left to cry themselves to sleep eventually appeared to settle themselves without being comforted.
But the levels of the stress hormone, cortisol, remain high in them even after they have apparently learnt to settle themselves. In other words, the child is still unhappy but just keeping quiet about it, the researchers said.
The study, published in Journal of Early Development, is likely reignite the debate about the popular parenting method called "controlled crying" which many believe trains babies to settle into a routine, the Daily Mail reported.
In the study, the researchers put a group of four to ten months old babies -- who had trouble getting into a routine or settling without being comforted -- to bed and left to soothe themselves to sleep. The mothers stayed in a room near enough to hear any cries but were not allowed to go to their babies.
Levels of cortisol were measured in the women and babies on the first night of the study and on the third day, by which the infants cried little before dropping off. However, their cortisol levels remained high, the researchers found.
In contrast, the amount of cortisol in the mothers had dropped, suggesting that they had relaxed due to the lack of crying from their baby.
"Although the infants exhibited no behavioural cue that they were experiencing distress at the transition to sleep, they continued to experience high levels of physiological distress, as reflected in their cortisol scores," said study author Wendy Middlemiss.
"Overall, outward displays of internal stress were extinguished by sleep training. However, given the continued presence of distress, infants were not learning how to internally manage their experiences of stress and discomfort."
The brevity of the study means it's not clear if cortisol produced by the babies does eventually drop. The researchers are now doing a longer study to see if the hormone level falls with time, as babies learn to cope with going to sleep alone.
