Parenting expert Dr Penelope Leach has sparked a heated debate by claiming that leaving young babies to cry themselves to sleep can damage their brains.
She says babies develop high levels of the stress hormone cortisol if they are left to cry.
In her new book, "The Essential First Year - What Babies Need Parents to Know" she claims that this can be toxic to their brains if it happens repeatedly over long periods.
Dr Leach says babies should not be left to cry for more than 30 minutes.
She 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."
Dr Leach, who has been writing parenting books since the 1970s, told the BBC that babies are not capable of learning that they should go back to sleep if they are left to cry.
Dr Leach told the BBC that there are ways to help babies sleep at night without leaving them to howl.
"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," she said.
"We are trying to teach the baby to become diurnal - to know the difference between day and night."
Dr Leach says she is not "getting at parents" but trying to give them good advice.
Many parents swear by controlled crying and I'm not sure how helpful it is to suggest that they may have damaged their babies' brains.
I'm not sure there is any right answer - I think some babies naturally settle themselves easily while others are more difficult. I've never tried controlled crying because I never got the sense that it would work - and I didn't have the guts for it.
What do you think? Did controlled crying work for you?