By Daniel J. McLaughlin
What should a parent do when they hear their crying baby? According to an author of a new parenting book, they should be left to "cry it out".
It may be good for both the baby's sleep and the parent's satisfaction to allow "controlled crying".
However, responding to a crying baby could be important for their development.
An economics professor has advised parents to leave their babies to ‘cry it out’ at night, the Daily Mail reports.
In her new parenting book Cribsheet, Emily Oster says babies sleep better and were happier after sleep training - or 'controlled crying' - than they were before.
She told MailOnline: "In studies where parents were encouraged to use this technique and others were not, they found - on average - after the sleep training, babies sleep better.
"Many studies found parents reported their babies are happier after the sleep training than before.
"In addition, there seems to be some benefits to parents, including less maternal depression and better marital satisfaction."
The academic said that she went through "the thousands of papers of academic literature" for her research.
Professor Oster adds: "Having parents who are exhausted and depressed may also have consequences for children even if we do not want to put value on the parents at all."
In an article for La Leche League, an educational and advocacy organisation for breastfeeding, Teresa Pitman explains why responding to a crying baby is important for their development.
She writes: "For one thing, it reduces the amount of crying as the baby gets older. Silvia Bell and Mary Ainsworth from John Hopkins University found that mothers who responded quickly to crying in the first few months had babies who cried less often and for shorter periods of time at one year, compared with the mothers who delayed responding or sometimes ignored the crying.
"And now more recent research is showing that responding to your crying baby protects him or her against future mental health problems and negative reactions to stress."
A baby's stress hormone system is also "very reactive" in the first three or four months after birth. Pitman explains: "The baby is easily stressed by many things that seem minor upsets to the rest of us: being away from his or her mother, being bathed, having diapers changed and being hungry.
"These everyday events can send the baby’s cortisol levels shooting up."
As soon as the parent responds to the baby's cries, these cortisol levels go back down. The levels are important because it "affects the way the baby’s brain develops, the way he responds to stress in the future, his immune system, his risk of obesity and other areas of development".
According to Stanford Children's Health, sleep needs for babies vary depending on their age. Newborn babies generally sleep about eight to nine hours in the daytime and about eight hours at night.
Most babies do not start to sleep through the night without waking until about three months of age. By the age of six months, about two-thirds of babies are able to sleep through the night on a regular basis.
For the first six months, the NHS says, babies should be in the same room as their parents when they are asleep, both day and night.
They also recommend that it is a good idea to teach a baby that night-time is different from daytime from the start.
The NHS explains: "During the day, open curtains, play games and don't worry too much about everyday noises when they sleep."
At nighttime, the lights should be kept down low, voices should be quiet, and the baby should be put down as soon as they have been fed and changed.