It's a debate that has divided the parenting community for generations: should you leave a baby to cry until it settles itself or pick it up and comfort it?
Now a new study seems to have the answer.
Researchers found that once babies have stopped crying they still have high levels of the stress hormone cortisol and are still unhappy – they're just keeping quiet about it.
The study involved tracking hormone levels in babies and their mothers. Many of the children, who were aged between four months and ten months, had trouble getting into a routine or settling without being comforted.
During the study they were put to bed and left to soothe themselves to sleep, and the length of time that they cried was recorded. Their mothers were nearby but weren't allowed to comfort their children.
Levels of cortisol were measured in their babies on the first night of the study and on the third. By the third night, the infants cried little before dropping off. However, their levels of cortisol remained high.
The research has been published in the respected journal Early Human Development.
One of the researchers, Dr Wendy Middlemiss, said: "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.
"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."
Researchers are now undertaking a longer study to test how the hormone level is affected as sleep patterns settle over more time.
Tamsin Kelly, editor of Parentdish.co.uk, said: "I think that trying to establish a routine with an element of controlled crying of a short time can help to settle your baby in the long term. Older babies and toddlers who have regular, comforting sleep routines are happy children."
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