The biology behind your baby's sleep – from Durham University's Infancy and Sleep Centre

The team at Durham University's Infancy and Sleep Centre look at babies' sleep patterns from an anthropological point of view. They consider the importance of mammalian biology and evolution to understand how (and where) human babies feed and sleep. Their Basis report raises some interesting concepts about how our biological ‘make up’ expects something quite different from what 21st century life provides. Here’s an extract from that report – we think it’s fascinating!

Human milk and infant formula: what our biology ‘expects’

One thing that is clear from considering the anthropology of infant care is that how we feed babies is intimately linked to how babies sleep, and what is biologically normal for human babies. For this reason, the information presented in Basis discusses differences in sleep behaviour between babies who are fed human milk, and those who are fed infant formula, as formula (which is based on cows’ milk or soya protein) is a recent invention, and therefore is a type of food that babies have not biologically evolved to expect.

Humans as mammals – and primates

In the animal kingdom, humans are both mammals and primates. All mammals are warm-blooded, infants are fed with their mother’s milk and require care following birth. Mammal babies fall into two types: altricial and precocial.

Altricial mammals

Altricial species, such as mice, rabbits, cats and dogs give birth to many offspring – litters – which (in the wild) are kept safe in a nest or den. They are very helpless at birth, often blind and hairless, unable to cling to their parent. They don’t feed often, as their mothers produce high-fat milk that takes them a long time to digest.

Altricial babies therefore sleep for most of the time, safe and warm in nests with their litter-mates, digesting milk, and growing!

Precocial mammals

Precocial species, such as deer, horses, sheep and monkeys, on the other hand, give birth to a small number of babies – usually just one or two – that are well developed at birth. They can usually see well, and are able to walk or cling shortly after being born, and so are able to make sure they remain close to their mother.

They feed frequently and at will on relatively low-fat but high-calorie milk (high calories come from sugar in the form of lactose). This milk is easily and quickly digested and provides these active and alert babies with energy.

Our close cousins: the primates

Primate species, including our close cousins the great apes (chimpanzees, bonobos, gorillas and orangutans), fall into the precocial category. They are born relatively well developed, infants are able to cling onto, and travel about with, their mothers, and they feed frequently (on demand) on low-fat, high-calorie milk.

Humans, however, are somewhat unusual primates and appear to have a mixture of altricial and precocial traits.

Brains, biology and sleep: vulnerable human babies and their large brains

Humans fit into the mammalian precocial pattern in most ways — we are usually born singly, have good sensory development (sight, hearing, et cetera) at birth and are fed with milk that is similar in composition to other precocial mammals – high-calorie, low-fat, quickly and easily digested.

However, we also have a smaller proportion of our adult brain development completed by the time of birth than do other primates, which means human babies have some relatively undeveloped characteristics.

While the milk of other precocial species is high in sugar to provide infants with the energy they need to follow or cling to their mothers, the high sugar content of human milk is used to fuel rapid brain growth over the first few months of life.

We are unable to cling or walk for months following birth, and are less able than other primates to regulate some of our biological systems – including temperature control, heart rate and breathing.

This means that human infants are totally dependent on their caregiver to keep them close, to provide warmth and protection, to ensure frequent access to maternal milk, and to help regulate aspects of their physiology. (Trevathan and Rosenberg’s book Costly and Cute: helpless infants and human evolution (2016) discusses the research into how we understand human infancy if you wish to know more!)

What this means for ‘normal’ infant sleep

As unusually helpless precocial primates, human babies have a very particular set of characteristics, some of which have evolved over many millions of years and are deeply engrained in our biology and behaviour:

We have a long period of development after birth, which takes place at a very fast rate, and requires a lot of energy.
We have the need to feed often and on-demand on high-calorie, low-fat milk which is digested quickly.
We are born with some well-developed senses – especially sight, hearing and touch.
We are unable to cling to a caregiver and so rely on being carried to stay close.
We have some underdeveloped physiological systems – body temperature, breathing and heart rate control.

Throughout our evolutionary history, human babies would not have survived without the constant presence of a caregiver – in most cases, the mother. Together with the need to feed frequently, this means human babies are designed to be close to their mother, both day and night.

Human infants are therefore biologically evolved to sleep near to (and probably in contact with) their mother’s body during the first months or years of life. In the past, we could not have survived without doing so.

Today, we live and sleep in very different environments than those we evolved in, and so understanding infant sleep involves drawing together information about what is normal for infant sleep based on our evolved biology, and the ways in which our history and culture have shaped what we consider to be normal today.

Infant sleep in history and around the world

Present-day UK sleeping arrangements – sleeping alone or in pairs in warm, quiet, private rooms with spacious beds – is a relatively recent historical development. Less than 200 years ago it was the norm, in UK and US households, for mothers and babies, and indeed for whole families, to sleep in close contact with each other.

Around the world in non-Western cultures, sleep is a social activity that people do in groups. Beds are basic, often on the floor, and pillows are thin or hard, or non-existent! Special rooms for bedrooms, special clothes for sleeping, and separate sleep locations for parents and children became popular as working people began to earn disposable incomes.

As houses and living arrangements changed, so did our sleep behaviour, and our attitudes about infant sleep.

Nowadays, parents can feel under pressure to help their babies become independent from an early age. Popular myths suggest ‘good’ babies sleep through the night, sleep alone and do not require attention in the night.

As a result, parents may try to ‘help’ their baby ‘sleep through’ as early as possible. But expecting a human baby to sleep alone, and for prolonged periods, is unrealistic and can be harmful.

The mismatch in what today’s parents might expect or desire regarding infant sleep, and their baby’s biological abilities regarding sleep, can lead to some unnecessary conflicts.

Around the world, many babies and mothers are in continuous contact, regardless of time of day or night. Babies sleep on the mother’s body or close to her, often in some form of carrying device during the day, and in her sleep space at night. Babies fall asleep while their mother works and/or completes her daily activities.

These babies do not require silence or darkness to sleep, and they are not expected or ‘trained’ to conform to a schedule. Within Western societies before the late nineteenth century, mother-infant sleep contact was also the norm, as demonstrated by comments found in physicians’ child-rearing guides for mothers.

A message from MFFY!

If you feel like your baby never sleeps, hopefully understanding the biology behind newborn rest might be reassuring through these sleepless nights. You can find the full Basis report here.

You might also enjoy our article on Sleepy (and not-so-sleepy) babies!