By Hannah Richardson
BBC News education and family reporter
Many parents want to know how to get a baby to sleep through the night
Leaving young babies to cry themselves to sleep can harm their developing brains, a parenting expert claims.
Dr Penelope Leach says recent scientific tests show high levels of the stress hormone cortisol develop in babies when no one answers their cries.
If this happens over long periods and repeatedly, it can be "toxic" to their brains, she says in a new book.
Dr Leach suggested unattended extreme crying bouts of 30 minutes or more could be damaging to babies.
The claims come in a new book called The Essential First Year - What Babies Need Parents to Know.
Dr Leach told the BBC News website: "We are talking about the release of stress chemicals. The best known of them is cortisol, which is produced under extreme stress."
"One is not talking about a wakeful baby lying there gurgling, one is talking about a baby that is crying hard and nobody is responding.
"When that happens, and particularly if it happens over a long period, the brain chemical system releases cortisol and that is very bad for brain development.
"Some neuroscientists describe it as toxic."
The psychologist and parenting expert, who first found fame in the 1970s with her book, Your Baby and Child, said the important thing was for a parent to respond.
The mother-of-two said it was not necessary for the parent to discover why the baby was crying.
"If you do not respond and if you refuse to respond, the baby knows no response is coming," she added.
"The reason that a baby gives up after half an hour, three-quarters of an hour or an hour is that it has given up and that its expectations have been altered.
"I've heard it said that babies stop crying because they have learned that mummy wants them to go back to sleep.
"Babies are not capable of that sort of learning."
There was no scientific evidence that suggested allowing a baby to "cry-it-out" taught them how to go sleep, she added.
It was possible for a parent to make it clear to a baby that they always come, but that they would not always do what the baby wanted, she added.
"You can show you are quite different at night - that you don't pick him up and play with him, try to stimulate him or get his Lego out.
"We are trying to teach the baby to become diurnal - to know the difference between day and night."
She said it was much harder work than closing a door on a shrieking baby but made for a better loving relationship.
She said she was not "getting at parents" in her new book, just trying to provide them with good advice grounded in science.