London, April 23 (IANS) Distressed babies who are left to cry are at greater risk of developing problems later in life, a childcare expert said.

Long periods of crying can damage developing brains, leading to learning difficulties later in life, said Dr Penelope Leach, author of the 1977 book "Your Baby And Child: From Birth To Age Five".

"It is not an opinion but a fact that it's potentially damaging to leave babies to cry," she said.

Leach's theory is contrary to popular child-rearing beliefs that a baby should be allowed to cry for up to 20 minutes.

"A baby who is left crying for long enough will eventually stop, but not because he has learned to go to sleep happily alone, but because he is exhausted and has despaired of getting help," she said.

She added continued crying led to the increased production of the "stress hormone" cortisol. Long periods of crying produced so much cortisol that it could damage a baby's brain, reports telegraph.co.uk.

"That doesn't mean that a baby should never cry, or that parents should worry when she does. All babies cry, some more than others. It's not the crying that is bad for babies - but crying that gets no response," Leach said.