Guest post by Professor Helen Ball

To understand how babies sleep, it can be helpful to think about the biology of other aspects of infant care—such as feeding and contact—and how they might influence the ways in which babies and their parents experience sleep.

After all, new parents still receive lots of advice about dropping night feeding and resisting the urge to cuddle their baby “too much.”

But, human babies are particularly helpless precocial mammal babies, totally dependent on their caregivers for warmth, safety, comfort and food— none of which they can obtain for themselves during their first year of life, given their lack of strength and poor neuromuscular coordination.

Babies’ biology is intimately intertwined with, and responsive to, that of their caregivers during this period—and for babies who are breastfed the biological interconnections between a baby and their mother are extensive and can influence sleep behavior in various ways.

In my new book, How Babies Sleep: A Science-Based Guide to the First 365 Days and Nights, we discuss in more detail how feed type and sleep patterns are related, but for now we will focus on how the biology of human milk and lactation affects the biology of human infant sleep.

The milk produced by mothers of precocial mammalian babies generally has a low-fat, high-sugar composition. For most species with precocial babies, the sugar (lactose) supplies the energy needed to cling to or run after their mothers all day long.

For baby humans, who have insufficient strength and coordination to cling or run, the high-energy sugar fuels rapid brain growth. Because there is little fat in the milk of precocial species, however, it does not satiate babies for long periods, and so they feed often, day and night.

The Tiny Stomachs of Newborn Babies

When they are born, human babies have tiny stomachs, the size of a cherry or a grape. It takes very little milk to fill them up, and they digest it quickly.

As your baby grows, their stomach enlarges, but they still cannot consume much milk at each feeding in the first few months. When babies regurgitate milk following a feeding (often known as posseting), this can be a sign that they do not have room in their stomachs for all the milk they have swallowed—this may happen more frequently when babies are given milk from a bottle, as it is more difficult for them to control the flow of milk, how much they consume and when they can stop.

When feeding directly at the breast, babies must actively work to extract the milk and take pauses during feeding, so it is easier to stop when they are full. This means that babies fed using bottles can consume more milk than they need—and if this is not human milk, it can take longer for a baby to digest it—leading to the popular practice of giving babies a big bottle of formula before bedtime to “help” them sleep.

Do Formula-Fed Babies Get More Sleep Than Breastfed Babies?

It is a common belief that breastfed babies wake more than formula-fed babies, and some parents therefore consider feeding their breastfed baby with formula during the night so that the breastfeeding mother can get more sleep.

A growing number of studies have looked for differences in the sleep patterns of breastfed and formula-fed babies and found that there is no difference in the total amount of sleep obtained by feeding type, so long as babies are fed an appropriate amount for their age and size.

And while studies confirm that the frequency of awakenings differs—with breastfed babies waking more frequently during the night— research finds that breastfeeding dyads (mother–baby pairs) get the same amount of sleep, or more sleep overall, compared to formula-feeding dyads. 

This is because, while breastfed babies might wake frequently, neither breastfeeding babies nor their mothers have to wake fully to feed, and they return to sleep more quickly, under the relaxing influence of breastfeeding hormones oxytocin and prolactin, and due to the presence of melatonin— the hormone that promotes sleep—in breast milk.

Melatonin is secreted during the night by adults but not by infants. It has a calming effect that promotes sleep, as well as relaxing the smooth muscle of the gastrointestinal tract, allowing for better digestion. Newborn babies who are breastfed during the night therefore benefit from the calming and sleep-inducing effect of melatonin, helping them return to sleep quickly.

Why Your Baby Craves Your Touch for Sleep

But there is another factor at play here. The difference in sleep fragmentation between babies may also be due to differences in mother–baby proximity. Where babies are sleeping—near to their mothers or in a room alone—influences sleeping and feeding patterns. As we know, most newborn babies are intrin- sically contact seeking, feeling most relaxed and secure in the arms, or on the body, of a familiar caregiver, and touch is the first of a baby’s senses to develop. 

Skin-to-skin contact after birth is now a staple in many delivery suites (including after C-sections, and with fathers if mothers had general anesthesia), as it is well known that immediate physical contact between mother and baby helps calm babies, helps them thermoregulate, and encourages them to feed early, which simultaneously helps kick-start maternal milk-production.

Although the research on the biological mechanisms for how contact, comfort and touch affect and promote infant sleep is still in its own infancy, the importance of comfort and physical contact for babies’ social and emotional development has been recognized for decades.

The work of John Bowlby and Harry Harlow on infant attachment emphasized the importance for babies of physical contact and comfort, which has relevance for how babies sleep, but they did not examine the biology of touch and how it calms babies. A growing body of evidence on the importance of touch is beginning to explain why babies find contact and touch pleasurable, calming, relaxing and sleep promoting.

The bottom line is that physical contact and touch are extremely important in helping babies to relax, and very useful tools for helping them dial down sufficiently to fall asleep.

“Some Nights Will Be Difficult, Some Less So”

What is important to know is that needing to feed frequently night and day is normal for human babies, not just in the first days and weeks but for several months.

Parents develop different ways of dealing with the interruption to their own sleep, which we will explore in later chapters, as this is consistently the aspect of early parenthood that new parents find most difficult.

I often hear from parents who are desperate to know when sleep will “improve,” or whose search for a quick fix has left them even more frustrated. I reassure them that if they can go with the flow, anticipate that nighttime will be fragmented for at least a few months and possibly much longer, and—where possible—ask for and accept help from family and friends when they need it, they will manage.

Some nights will be difficult and some less so. When they are anticipating their baby will sleep, wake, seek contact, and feed frequently night and day for the first few months, they find this period in their baby’s life much less difficult.

Useful Sleep Tips for Parents

• In the early days remember that your baby will sleep in many short bouts with no day–night rhythm. Feeding and contact will help them fall asleep. Wait until they are floppy before you try to put them down.
• As they reach the end of the Fourth Trimester (three months), watch for the buildup of sleep pressure and help them relax so sleep pressure can do its thing. No one can fall asleep on command, and babies are no exception.
• Help your baby’s circadian clock develop by taking them outside in the morning and keeping them in the daylight with the noise of normal activities during the day. While you can’t force your baby’s sleep to fit around your schedule, you can let them sleep on the go, as and when they need to, so you aren’t tied to the house and their sleep–wake schedule.

Key things to remember about baby sleep biology

Sleep biology may seem complex because it involves multiple biological systems, but when you grasp how these work, sleep is straightforward and predictable. With babies, an added com- plication is that the systems mature at different rates in different babies, because biological variation exists.

So it is important to have realistic and flexible expectations for how baby-sleep biology unfolds, and how variable it can be. It is helpful to be aware of general developmental trends over time, but resist the urge to benchmark them against specific weeks or months—every baby develops at their own rate, and this variation is completely normal.

As we will discover in the next chapter, when it comes to baby-sleep development, biological variability is the norm, and both cultural and statistical normalcy can be misleading.

About the Author

Professor Helen Ball is a pioneering infant sleep researcher whose groundbreaking work has transformed our understanding of how babies naturally sleep. As Director of the Durham Infancy & Sleep Centre and founder of the Baby Sleep Information Source, Helen has dedicated her career to bridging the gap between biological reality and cultural expectations around infant sleep.

Her research influences national and international policy guidelines, and she serves as Chair of the Scientific Committee for the Lullaby Trust and as a Board Member of ISPID. In 2018, Durham University received the Queen’s Anniversary Prize for Helen’s research and outreach work—a testament to her impact on families worldwide.

Helen’s new book, How Babies Sleep: A Science-Based Guide to the First 365 Days and Nights (released today, May 27th!), offers parents a research-backed alternative to mainstream sleep advice. Rather than trying to “fix” baby sleep, Helen empowers parents to understand and work with their infant’s natural biology, helping families find harmony in those precious—and often challenging—early months.

Through her work, Helen validates what many parents instinctively know: that responsive, connection-based approaches to infant sleep aren’t just kinder—they’re scientifically sound.