The science of healthy baby sleep

Mention you've had a baby, and almost everyone will ask one thing: how is she sleeping?

After all, many exhausted parents look forward to the time when their baby finally sleeps through the night. Particularly in the West, an industry of sleep coaches, books and articles has sprung up, promising to help families achieve what many see as the holy grail: a baby who sleeps in a crib, alone, all night, and has several long naps during the day. Even some paediatricians warn parents that, if these goals aren't reached, children are less likely to get the sleep they need to grow and thrive.

But not only is this idea of independent, uninterrupted baby sleep far from universal, it is also very different to how human infants have slept through most of our species' history. Taken too far, it can cause a great deal of anxiety and stress for parents – and even be unsafe for the babies themselves.

"The way in which we sleep now in the 21st Century is kind of odd, in an evolutionary sense, because we weren't evolved to sleep like we're dead for an eight-hour period, and not wake up, in total silence and total darkness," says Helen Ball, professor of anthropology at Durham University and the director of the Durham Infancy and Sleep Centre. "But that's what people in Western societies have become accustomed to.

"And that affects the way in which we think about what babies should be able to do, and how babies should be treated." (Read more about how sleeping through the night is a relatively new phenomenon, even for adults.)

Sleeping enough?

Worrying about whether babies are getting enough sleep isn't new. The first "scientific" guidelines date as early as 1897, when, in a book on sleep for the London-based Contemporary Science Series, a Russian physician recommended that newborns should sleep 22 hours a day. Throughout the following century, although these suggested amounts declined, recommended sleep consistently ran around 37 minutes more than the actual sleep babies were getting, paving the way for decades of concerned parents.

Experts agree that sleep is crucial for babies and young children (and, for that matter, for adults). A lack of sleep has been associated with cardiometabolic risk factors, an increased risk of ADHD and low cognitive performance, and with poorer emotional regulation, academic achievement and quality of life.

Many of these longer-term findings, however, involve school-aged children, not babies. They are also correlations, not causations. The only way to know if a certain amount (or lack) of sleep "causes" a specific condition such as ADHD, as might seem to be suggested by research showing a correlation between children who consistently slept less overnight and ADHD, would be to set up a randomised controlled study. This would involve sleep-depriving one group of children over years. That's obviously unethical. So, it is difficult to unravel how much of the association may be the reverse: children with ADHD may simply sleep less.

Of course, it's likely that the relationship between sleep and development goes both ways. Short-term randomised controlled trials have found that babies given a memory task did better when they napped and, in findings that will surprise exactly zero parents, that fatigued infants had a harder time dealing with a stressful episode than alert infants.

But while that might mean we shouldn't do anything (such as deliberately forcing a child to stay awake) to inhibit sleep, it doesn't mean that every baby requires 12 hours of unbroken sleep a night and several two-hour naps per day, either.

"Just as adults differ in terms of their sleep, so do babies," says Alice Gregory, a psychology professor specialising in sleep at Goldsmiths University of London and the author of the book Nodding Off: The Science of Sleep.

She points out that it has been recommended by the US's National Sleep Foundation that babies up to three months old should obtain 14-17 hours of sleep in a 24 hour period, but that as few as 11 or as many as 19 hours might be appropriate. Meanwhile, sleep length recommendations from the American Academy of Sleep Medicine make no recommendations at all for infants under four months old. Neither body makes specific recommendations for nap versus nighttime sleep amounts.

"These slightly different guidelines highlight the fact that even leading experts disagree about infant sleep," Gregory says.

The amount of variation is also clear if you look at how babies actually sleep. In one Australian study, the average amount of sleep over a 24-hour period among 554 four- to six-month-olds was 14 hours. But look closer at the data and it becomes clear that there were more than eight hours' difference between those getting the most and the least sleep. "There are huge differences in sleep duration at the 98th percentile vs the 2nd percentile," says Harriet Hiscock, a paediatrician at Royal Children's Hospital Melbourne and one of the study's authors.

Staying on schedule

What about following a preset routine that schedules naps (and feeds) throughout the day? Or the nighttime schedule known as the seven-to-seven (where the baby sleeps through from 7pm to 7am), considered the gold standard by countless baby sleep books and trainers?

In the earliest days, this kind of regular schedule can be especially difficult to follow. That's because the physiological functions that tell adults that nighttime is for sleeping, such as melatonin excretion and a body temperature rhythm, don't start emerging until at least eight to eleven weeks of age in healthy, full-term babies. Exposing newborns to light during the day and to darkness at night can help get these systems going. (And despite some sleep coaches' claims, babies don't produce melatonin during the day – and it would confuse their circadian rhythms if they did – so it's not necessary to have pitch-black naps for the purpose of melatonin production.)

"The main theory of sleep regulation proposes that there are two processes controlling sleep and wake," says Gregory. "First is the homeostatic process (the idea that the longer we have been awake the sleepier we become), and the second is the circadian process (a clock-like process, which results in us being more likely to be sleepy or alert at certain times of the day and night).