The study, which appeared in the journal Pediatrics, followed on a 2007 study by the same researchers that found babies and their parents benefited when the infants were taught to settle themselves to sleep with behavioral techniques.

London - It is a blissful moment for any parent, when a once fractious baby finally learns to fall asleep without a murmur.

But mothers and fathers should not be lulled into a false sense of security, because their child may actually still be upset.

A study found that levels of the stress hormone cortisol remain high in “cry babies” even in the days after they have apparently learnt to settle themselves.

In other words, the child is still unhappy but just keeping quiet about it.

The research will reignite the debate about the pros and cons of controlled crying – letting unsettled babies sob themselves to sleep.

Sticklers for routine, such as childcare guru Gina Ford, say that if babies cry during designated sleeping hours they should not be picked up.

But others, including fellow author and childcare expert Sheila Kitzinger, claim mothers should be guided by their instincts and not by prescriptive routines.

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 logged.

Their mothers stayed in a room near enough to hear any cries but were not allowed to go to their children. Levels of cortisol were measured in the women and 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 journal Early Human Development reports.

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.

Wendy Middlemiss, a researcher at University of North Texas, 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.”

The brevity of the study means it is not clear if cortisol produced by the babies does eventually drop. The researchers are now doing a longer study, to see if the hormone’s level falls with time, as babies learn to cope with going to sleep alone.

Siobhan Freegard, of the parenting advice website Netmums, said: “I don’t think anybody would ever say that you shouldn’t use controlled crying – it is about getting the balance right.

“If you are on maternity leave with your first child and can have a nice lie-in and breastfeed the baby in bed, that is very different to being a single mom who needs to go out to work or no one will eat.

“I have been advised many times to try controlled crying, but it caused me much more stress than picking up the baby and doing what comes naturally.

“But I know other moms who have found controlled crying short, sharp and successful.” - Daily Mail