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How Babies Sleep

A Factful Guide to the First 365 Days and Nights

Helen L. Ball Professor, Infancy & Sleep Centre, Durham University

How Babies Sleep

A Factful Guide to the First 365 Days and Nights

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To Megan and Rhianna, who taught me how, and Den who learned with me.

Introduction

Imagine the unseen night-time world of mothers and babies. How does it look? Are they sleeping? If so, how, what on, in what space? Where are the baby and the mother you imagine? Are they together or apart? Are they alone or are other people with them? If they are not sleeping, what are they doing? Is your mental scene peaceful and calm, or are your imagined mother and baby stressed and unhappy? And where in the world are they? This last question is crucially important for our mental image of what mother–baby sleep entails, colouring our perceptions of what is and is not possible, acceptable and normal.

Let’s consider three new mothers: Kristi (US ), Lizzie (UK ) and Yuki (Japan). Their babies are three months old.

In the US , Kristi is anxious. She has a decent job and has been able to take 12 weeks off so far, but soon she will be returning to work. She feels fortunate she has a good employer – many US mothers must return to work much sooner. She has been following US guidelines to keep her baby in a cot at night, near to her bed, and has been getting up several times a night to feed her baby in a nursing chair she bought for this purpose. US sleep safety guidance says she must not bed-share. Her husband takes turns to feed the baby with milk Kristi has expressed during the day, but she is worried this will not be sustainable when she is back at work. She is thinking that in the next month she must wean her baby to formula and sleep-train so that she can get a good night’s sleep. She must drive 40 miles to and from work every day and needs to be alert. Sleep training will involve having her baby

how babies sleep x sleep alone in another room and not responding even if they cry. She doesn’t want to do this to her baby – it feels wrong not to respond – and US guidance recommends keeping your baby in your room for the first year. But the sleep training books, even the ‘gentle’ ones, aim for you to ultimately leave the baby in their own room and close the door, so she has to make a choice. She decides she’ll switch to formula next week so her milk will be gone before she has to resume commuting, and once her baby is happy taking formula the sleep training process will begin. She thinks the next few weeks are likely to be rough, but she and her husband resolve to do their best to ‘get it over with as quickly as possible’.

In the UK , Lizzie is feeling despondent. She has been diligently following the guidance of a popular baby app that recommends getting babies into a routine to support their sleep development. She times the duration of her baby’s daytime naps and wake periods, trying to anticipate when her baby needs to sleep. Every evening, she follows a bedtime routine and tries to put her baby down drowsy-but-awake by 7pm, but she is finding this difficult. Her baby often falls asleep while feeding, so Lizzie finds she can’t easily stick to the schedule or teach her baby to self-settle. She is following the guidance to keep her baby’s cot at her bedside at night but her baby doesn’t want to sleep there. At night when they breastfeed lying side-by-side in bed, Lizzie is so exhausted she sometimes falls asleep. On these occasions she finds both she and her baby sleep better than when she persists in returning her to the cot. But Lizzie is worried that by not teaching her baby to sleep independently she will develop bad habits, and that this may have a negative effect on her development in the future. Her partner says she is obsessing too much but her mum insists babies need to be in a routine, and Lizzie feels guilty that she hasn’t managed this yet, even though her entire life revolves around her baby’s sleep schedule and she rarely manages to get out of the

house. She doesn’t have to return to work until her baby is a year old, but she is dreading that the next nine months will be more of the same; she isn’t enjoying being a mum.

In Japan, Yuki is feeling daunted by the responsibilities of motherhood; she is expected to integrate her baby into the family and foster strong bonds of kinship. Although Euro-American infant care ideas and sleep products have been adopted by some of her friends, like many contemporary mothers Yuki values the traditional family sleeping arrangements; she feels uncomfortable with the idea of putting her baby in a crib all alone. She remembers sleeping between her mother and father (a practice known as kawa ) on a futon on the floor of her parents’ tatami room until she was well into childhood. In their modern apartment Yuki and her husband favour Western-style furniture and have bought themselves a raised bed with a mattress rather than a futon. In the furniture store the sizes of the beds and mattresses were illustrated by how many adults and children they would accommodate sleeping side-by-side. They bought a bed that would accommodate themselves and two future children. Yuki keeps her baby with her in the evening and takes him to bed with her when she is tired; her husband joins them when he is ready to sleep, but night-time care is her responsibility, and her husband never wakes. She thinks her baby sleeps well because he is touching her all night and feels secure. Yuki would like her husband to take part in more baby care, but is happy that her mother who lives in a nearby apartment building likes to help with her baby during the day, which allows Yuki the time to run errands. She is enjoying motherhood, despite the responsibility of prioritising her son’s care, planning for his future education and managing the family’s finances. Unlike her own mother who gave up her work life in order to have children, Yuki plans to return to work, albeit part-time, when her son is a year old.

How babies sleep is both exceedingly simple and excruciatingly complex. It is simple because it is based on a few straightforward biological principles that affect all babies. It is complex because we have made it so.

Over the past century and a half, in modifying the world to better suit our needs in industrialising, industrialised and now digital-era societies, we have tried to manipulate baby sleep to fit with the rapidly changing nature of adult lives. Our ideas about sleep, and particularly baby sleep, have been influenced and shaped by the socio-economic pressures, political philosophies, religious values, academic theories and cultural ideologies that have waxed and waned during this time. The mismatch we have created between our babies’ biology and our contemporary lives has caused conflict between parents’ and babies’ needs – now framed as ‘baby sleep problems’ – for which babies are often ‘treated’ using behavioural and clinical interventions. In the cases of both Kristi and Lizzie, above, we can see the tension created by societal pressures, causing mothers stress as they try to conform to cultural ideals; Yuki does not express direct conflict with her baby but feels the weight of her responsibility as the primary care-giver, so for her infant care plays out in a different way.

I am both a biologist and social scientist: my background is in human biology combined with biological and cultural anthropology. This training path gave me a holistic perspective on human life and encouraged me to think outside academic silos, combining biological, behavioural and cultural perspectives. In my three decades as an academic, I have applied my knowledge and insights to parent–infant sleep. This means I view baby sleep differently than most academics who specialise in infant sleep research, most of whom have a psychology or medical background, and therefore focus on sleep primarily as a function of the brain, prioritising cognitive development; or sleep issues as a symptom of

ill-health, prioritising pathology or avoidance of death and disease. In contrast, my research studies reflect my academic training as a biologist, primatologist (observer of primate behaviour) and anthropologist. As a biologist and primatologist, I was taught to observe closely and systematically, and to ask questions and formulate hypotheses about the things I was observing. As an anthropologist, I was taught to question assumptions, understand the philosophy of science and how scientific knowledge progresses, think holistically and on a grand scale (considering evolutionary time), and to critique dogma (particularly medical dogma). I emerged from my graduate and undergraduate studies with a unique set of perspectives that inform the way I understand how babies sleep, which I will share with you in this book. But first a disclaimer.

All of biology is ultimately about evolution,1 but some proponents of applying an evolutionary perspective to aspects of human biology and behaviour (such as infant development) take an approach that I consider to be ill-informed.2 They invoke what is known as the ‘environment of evolutionary adaptedness’, which refers to a time period (usually unspecified, but often vaguely suggestive of a Pleistocene-epoch savannah-type huntergatherer lifestyle or a Palaeolithic-era cave-dweller lifestyle)3 when key human traits such as tool use or language are thought to have arisen in our behavioural repertoire. In these visions of our human evolutionary past, such environments of evolutionary adaptedness are invoked as reference points to which we might look for comparisons when seeking to benchmark contemporary human activities such as diet, family composition, parenting, infant care and more; the ‘Palaeolithic Diet’ is an example of this. I would like to be clear that this book does not take this approach. There is no single moment or era in our evolutionary history that has defined how human babies sleep. Indeed, there

are multiple evolutionary transitions that have shaped both how babies sleep and how parents respond to how babies sleep. It is also important when thinking about evolutionary influences on contemporary living organisms such as humans, to be aware that the process of evolution does not ‘design’ organisms with a purpose in mind; evolutionary change proceeds by tinkering with the bodies and behaviours of existing organisms whose characteristics diverge and converge in response to their ability to survive and reproduce in different environments. Most of the species that have been produced in this way have already failed to survive (become extinct). Those that are alive today are the products of a long series of evolutionary changes that happened to result in a combination of successful characteristics that were able to overcome particular environmental challenges – not because that combination was designed to overcome them. As Nobel prizewinner François Jacob famously said, ‘evolution is a tinkerer and not an engineer’.4 Contemporary living organisms, including humans, are not elegantly designed; we have been cobbled together along a path of twists and turns that enabled the ancestral species in our lineage to survive – and as a consequence our biology comes with a lot of evolutionary baggage. We need to understand that baggage and how it affects us and our babies if we want to understand how babies sleep. It is once we understand how the key features of human parents and babies evolved – including milk production, prolonged gestation, large brains and extreme dependency – that we can understand and resolve the challenges they pose in the first year of an infant’s life. In addition to the foundations of our evolutionary biology, we have layered many cultural beliefs and practices onto our understanding of baby sleep, which vary hugely from location to location, and over time. Parents’ perceptions and popular discourse in western societies have been influenced by philosophical

and socio-cultural trends over the past two centuries in more ways than we might realise in response to rapid social and political changes. For clarity I should explain that I am using the term ‘western society’ as more of an ideological descriptor than a geographic one to refer to those societies that exhibit a shared set of beliefs, values, political systems and cultural norms, regardless of whether they are geographically located in the West. These concepts often encompass democracy, capitalism, individualism and human rights. In these societies industrialisation, urbanisation and medicalisation transformed family life and baby care, setting in motion the development of numerous ideas, traditions and expectations about babies, sleep, and the role of parents that many of us are still tethered to. Our recent historical past helps to explain where all the myths you’ve ever heard about baby sleep have come from – and the myths you’ve heard will depend on your cultural origins.

It is not only the public who have been influenced by specific historical and cultural perspectives on babies and baby sleep. The emerging science of baby sleep and clinical guidelines about how babies ‘should’ sleep was influenced by the same biases and expectations. Many infant sleep research studies reflect western cultural values and lifestyles, and are what might be called ‘aspirational’, in that they aspire to find ways to ‘help’ babies to sleep longer, wake less often, settle more quickly, and disturb their parents less. They offer families the promise of a less sleep-deprived future. Throughout this book I hope to encourage you to consider whether studies that aim to find out how to help babies sleep more are asking the right question. The culture and history of guidance issued by ‘experts’ for parents in western societies have shaped expectations of how ‘good’ babies should behave and how they are created by ‘effective’ parents, while the media (both traditional and social) reinforces simplistic and outdated

rules and rigid ideas. None of this is helpful for new parents in the twenty-first century, and it serves only to build anxiety and disharmony between parents and babies.

How Babies Sleep brings together the science, the anthropological context and various practical insights to explain what you need to know about your babies’ sleep, your own expectations, and how to navigate the first 365 night-times with your baby. Often this is information no-one ever tells you, or that you find out too late. Although (most) babies spend a lot of time sleeping, their sleep patterns rarely fit with adult schedules, and their sleep habits change dramatically throughout their first year – sometimes from month to month, week to week, or even day to day. The cultural expectations we have about babies’ sleep habits are often unrealistic, and this makes caring for a new baby unnecessarily challenging. Understanding what babies’ evolved biology has prepared them to expect, and what our cultural history and social environment lead us to think babies need, can help to make sense of this paradox, and remove some of the anxiety and frustration of new parenthood.

All parenting choices are personal and different, so please accept my assurance that this book is not intended to judge any individual parents who did what they needed to do to get through the night-times, save their own sanity and meet their own sleep needs. It is intended to reveal how biology shapes infant sleep behaviour and to challenge the cultural norms about night-time parenting that many of us have grown up with.

Applying anthropological and evolutionary thinking to infant sleep and night-time parenting does not produce a quick fix to make a baby sleep ‘better’ or resolve new-parent exhaustion. But it does help to explain why we might feel exhausted and want a quick way to fix our baby’s sleep. It also helps in understanding when and why the search for a quick fix could be an inappropriate

solution, or good for parents but not for babies. Some parents who have gone ahead with quick fix solutions (which, in reality, are often neither quick, nor solutions) may wish they had heard about all of this sooner and made different choices. Wherever you are in your parenting journey, this book will offer you a new understanding of ways to harmonise your own needs with those of your baby, to experiment to find strategies that work for you and your family, and will empower you with the confidence to reject approaches that make you uncomfortable.

1. The helplessness of baby humans

The first days with a newly born baby can be both blissful and terrifying. They are small and cute, yet fragile and utterly helpless, adorable yet unpredictable, and absolutely, completely, dependent on you. You may feel confident and well prepared, or you may have no idea what you are doing or how you will keep this floppy, squirmy, squally creature alive for the next 18 days, never mind 18 years. Either way, to understand why human babies are the way they are, including getting to grips with how they sleep, we need to recognise the importance of two crucial human evolutionary traits: making milk and growing big brains.

Producing milk to feed our babies is an ancient trait. Arising 310 million years ago,1 it is what makes us mammals, along with all other hair-wearing, milk-producing, temperature-regulating animals for whom this is a defining characteristic. Only female mammals have functioning mammary glands, and fascinatingly all of us produce milk that is unique to our species and particularly tailored to the nutritional, growth and activity needs of the babies we produce.2 The milk that is made by human mothers has evolved to fuel the rapid and prolonged brain growth that results in the enormous and complex adult brains that make us uniquely human. Our big brain is a relatively recent trait, arising only in the past 1 million years, and is an extremely energetically expensive organ to grow and maintain.

It is usually the brain that gets all the attention, but, as milk came first in our evolutionary past, let’s begin by looking at what being mammals and making milk entails, and how the kinds of

milk mammals make affects the way they must care for their babies.

Three kinds of mammal babies

I first saw a platypus in real life during a visit to Australia in 2007 to speak about my research at an infant feeding conference (aptly named ‘Hot Milk!’). My family had joined me, with the intention of exploring the east coast of Australia after the conference. On arriving in Sydney we headed out into the sunshine to convince our brains it was daytime and to check out the neighbourhood near our hotel. Here we happened across Sydney Zoo on Darling Harbour, which educates visitors about the unique wildlife that inhabits Australia. In a dark room housing a simulated creek behind a glass wall, visitors were able to observe the rare and endangered platypuses: these unusual hair-covered duck-billed creatures were swimming in the rapidly flowing water and burrowing in the creekbed for crustaceans to eat. They were bizarre-looking, smaller than I had expected, about half the size of a domestic cat, and with a flat paddle-like tail, webbed-feet, a furry body and a beak like a duck. From the information displays I learned that when Western scientists encountered the (taxidermically preserved) platypus in the eighteenth century they initially considered them a hoax, given their strange combination of features.3

Mammals are organised into three groups – monotremes, marsupials and eutheria (or placental mammals) – but all descended from one common ancestor, who lived some 300 million years ago. Examining the traits these three groups have in common, and how they differ, gives us a clue as to how the features that define our own group – the placental mammals – and our own species have evolved over time.

The platypus belongs to the rarest of the three kinds of mammal, the monotremes, whose babies are unusual because they hatch from eggs. Their mothers produce milk with which to feed them, even though they have no nipples, and the babies, which look like small pink beans with tiny limbs, squirm up their mother’s body to lap the milk secreted from glands in her armpits. The platypus’s unique characteristics provide evolutionary biologists with clues that help to explain how lactation evolved.4

Lactation (milk production) is the defining characteristic of all types of mammal and a trait that all three groups inherited from our common ancestor. The nipple-less milk-feeding of egg-laying monotremes like the platypus suggests that lactation developed before live birth (viviparity) in mammalian evolutionary history, and that mammary glands evolved from modified sweat glands (hence platypuses lactating from their armpits). The origin of lactation is important because the ability to feed babies with milk has been central to the evolutionary success of mammals (including humans), and the composition of human milk plays a crucial role in understanding how human babies sleep.

Marsupials, such as the more well-known Australian animals – koalas, kangaroos, wombats and wallabies – are the second type of mammal. Unlike monotremes they do not lay eggs but produce extremely undeveloped babies who, after a very short pregnancy, crawl from the birth canal to a pouch on their mother’s belly. Although marsupials have nipples inside their pouch, these undeveloped, foetus-like marsupial babies (very similar in appearance to the bean-like monotreme babies that hatch from an egg) are incapable of sucking, and so permanently attach on to a nipple that supplies them with milk for the first few months of life. Marsupials fill most of the terrestrial niches in Australia but differ from ‘true mammals’ found outside Australia in generally having much shorter gestation periods and smaller and less

complex brains. The development of extended internal gestation (pregnancy) supported by an organ known as a placenta, which delivers oxygen and nutrients to the developing foetus, is what allowed for the evolution of the group of mammals that we belong to.

In my late teens I had a weekend and holiday job working at the local veterinary surgery, where I became familiar with the reproductive characteristics of this third group of mammals, which we call the eutheria, or placental mammals. The veterinarian I worked for operated a one-man practice covering all domestic animals (pets and farm animals). It was hugely varied and interesting, despite being messy and sometimes disgusting. I scrubbed the kennels and small animal pens, sterilised and organised surgical implements, held and soothed animals, answered the phone, assisted with surgical operations, and took samples during post-mortems.

One aspect of animal life that often needs the input of a vet is reproduction, and so by the age of 18 there were few aspects of mammalian reproductive biology I hadn’t observed first-hand (spaying cats was particularly fascinating). An issue I became curious about early on was why the small pet animals typically produced many more babies per pregnancy than most of the much larger farm animals. It was not until five or six years later when I was in graduate school studying comparative reproductive biology that I discovered the answer.

Professor Robert Martin, an evolutionary anthropologist and reproductive biologist, spent his forty-year career studying the sexual behaviours and reproductive strategies that are unique to placental mammals, including the types of uteri and placentas that are used to support mammal embryos and foetuses throughout gestation.5 He describes how some mammals have a placenta that lines the inside of the whole uterus and can support multiple

embryos, while others have a localised disc-shaped placenta that is attached to one part of the uterus wall and typically supports a single embryo. As a consequence of the nature of the placental organ, placental mammal babies do not all develop in the same way, being born after different periods of gestation, singly or in litters, and with hugely different characteristics at birth.

The variation in types of babies produced by placental mammals means that mammal females (who are the primary care-givers in most mammalian species) need to provide different amounts and types of care to their new-borns. Small-bodied placental mammals typically produce large litters of babies after a relatively short gestation (Figure 1.1). Well-known examples of these are the cats, dogs, rabbits, hamsters and guinea pigs that I encountered when working for the local vet. Individual babies of these species are almost completely helpless for several weeks after birth. Known as altricial, these babies are blind, deaf and hairless, with weak limbs and little ability to move. Altricial babies, like puppies and kittens, spend the first few weeks of their lives in a nest prepared by their mother, which provides warmth and safety for them and their littermates, while they get on with the business of sleeping and growing.

Larger-bodied mammals tend to produce single babies after a longer gestation period supported by the thick disc-shaped placenta. These babies are well-developed at birth (Figure 1.1) and are described as precocial; examples include many large farm animals as well as horses, antelopes, elephants and monkeys. Precocial new-borns can see, hear, call, and have sufficient strength and coordination that they are able to stand or cling soon after birth. The mother is the source of these babies’ safety and warmth, and they stay close to her for several weeks or months.

While the females of placental mammal species who produce altricial babies have a bicornuate – or two-chambered – uterus

lined by the placenta in which multiple small foetuses develop simultaneously, females of those species that produce precocial infants have a single-chambered or simplex uterus in which the two chambers have fused to create a larger space, with a thick disc-shaped placenta in one location. By producing one (and usually no more than two) babies at a time, the simplex uterus and thick disc-shaped placenta allows the mother to produce a much larger and more well-developed foetus during a prolonged pregnancy. Monkeys, apes and humans all have this simplex uterus and specialised placenta, and produce large babies (relative to their mothers’ body size) that are well developed at birth. Counterintuitively, however, human babies are not as well developed at birth as we might expect based on our mammalian history, so let’s

Birth Uterus

Figure 1.1 Characteristics of altricial and precocial mammal infants (from Martin, 1992)6

The basic distinction between altricial o spring (a) and precocial o spring (b). Altricial o spring are born after a relatively short gestation period (stippling) and their eyes and ears are still sealed at birth. They typically develop for some time in a nest before emerging into the outside world. Precocial o spring are born after a relatively long gestation period, and their eyes and ears are open at birth or soon afterwards; the ‘nest phase’ of altricial neonates is thus part of uterine development in precocial mammals.

The basic distinction between altricial offspring (a) and precocial offspring (b). Altricial offspring are born after a relatively short gestation period (shading), and their eyes and ears are still sealed at birth. They typically develop for some time in a nest before emerging into the outside world. Precocial offspring are born after a relatively long gestation period, and their eyes and ears are open at birth or soon afterwards; the ‘nest phase’ of altricial neonates is thus part of uterine development in precocial mammals.

now think about how humans’ evolutionary trajectory has taken a different turn.

The importance of brains and milk

Once mammal babies are born and the placenta ceases to provide the baby with nutrition, lactation is the key adaptation that allows the mother’s body to serve as a buffer between the infant and fluctuations in the availability of foods that infants might consume.7 Non-mammal babies (such as reptiles) must fend for themselves and compete for food against stronger and more competent adults, meaning many fail to survive. Mammals, therefore, have an advantage in being able to produce food for their babies during this vulnerable phase. The ways in which placental mammal mothers care for (or in evolutionary terms ‘invest in’) their babies vary according to the type of infant they produce.

Mothers of altricial infants (rabbits, mice, cats, dogs, etc.) supply little direct care during early life other than feeding their babies once or twice per day. The nest they have constructed prior to giving birth provides their babies with safety and warmth; after birth, mothers spend most of their time foraging to both feed themselves and produce the high-fat, nutrient-dense milk that will fuel their babies as they sleep and grow. They are often said to ‘cache’ their babies.8 This type of mammal baby therefore finishes their gestation in the nest, and to avoid attracting the attention of predators they are silent for the long periods when their mother is absent.

Mothers of precocial infants typically do not construct nests – their bodies provide safety and warmth for their babies, with whom they stay in close contact day and night. Born singly or in small multiples, these babies enter the world in a more

neurologically advanced state.9 Not only are their eyes and ears functional at the point of birth, but they are capable of coordinated neuromuscular control a short time after birth. Precocial infants take to their feet and follow their mothers, or cling to her body to be carried, alerting her with cries should they be separated. Precocial babies suckle often and at will when hungry. In these species, milk is often watery and low in fat but high in sugar (lactose), providing rapidly digested calories to fuel energy expenditure and growth. As babies stay with their mother as she forages, the mothers of precocial infants are often said to use a ‘carry/follow’ infant care strategy.10

Humans are primates, and like other primates we produce precocial infants.11 We have many precocial infant characteristics: our babies are typically born singly or in pairs, with their senses well developed. They cry if separated from their mother. Human mothers12 also produce milk that provides energy (sugar) rather than fat, and to obtain sufficient calories human babies feed frequently throughout the day and night.

But human babies are also the most neurologically undeveloped of all primates at birth,13 lacking the muscle tone and neuromuscular coordination to either cling to their mother or run after her for months, never mind within hours of birth. So, our babies are precocial in most respects, but are unusually helpless due to their lack of strength and coordination, unable to maintain close contact with their mother without help – a situation that makes human babies both unique and uniquely vulnerable.14

Evolutionary anthropologist Karen Rosenberg observed that the helplessness of human babies in comparison with other primate species was not recognised by zoologists and anthropologists until the middle of the twentieth century, and this has since informed a gradual understanding that our slow and prolonged childhood

development is a key aspect of our biology that differentiates us from other species.15

The explanation for why human babies are born so helpless can be found in the key biological changes that happened during the course of human evolution, which resulted in humans having exceptionally large adult brains. Human babies’ postnatal brain growth and neurological development takes much longer than for other primates,16 and so human babies are born prior to developing sufficient coordination to control their limbs and effectively cling to or follow their mothers.

As I was taught during my first undergraduate anthropology course, the two specific and defining evolutionary features of the human species are bipedalism (walking on two legs) and encephalisation (massively large brains). These two features arose millions of years apart in human evolutionary history, but for several decades it was thought that bipedalism, or more specifically the way the human pelvis had evolved as a consequence of bipedalism, was what limited the duration of human pregnancy and resulted in human babies being born in a neurologically undeveloped state.17 It was argued that getting the head of a largebrained baby through a bipedal pelvis was the key limitation on foetal brain development before birth (named the ‘Obstetric Dilemma’ by Sherwood Washburn in 1960).18

Today it is debated whether the narrow birth canal of a bipedal human was indeed the key evolutionary limitation, or whether it was in fact the ability of pregnant females to sustain the energetic demands of a large foetus for longer than nine months.19 Holly Dunsworth, an American biological anthropologist, has made a very effective case against the key premise of the Obstetric Dilemma, which is that humans would be inefficient walkers if their hips were slightly wider (allowing a little more space inside

the birth canal). Instead, she argues that it is the overall body size and basal metabolic rate of human females that limits gestation length, because we are not big enough to support the expensive metabolic demands of a rapidly growing foetus for a period that exceeds the current duration of gestation.

Whichever of these mechanisms was the limiting factor, human babies are born with relatively smaller brains (about a quarter of their adult brain size) than all other primates (whose brains are about half their adult size at birth).20 Because of this, once they are outside their mother’s body, the brain of a human baby grows more rapidly and for longer than that of any other primate, only slowing down after 12 months of age.21 This explains the human infant’s curious mix of well-developed and poorly developed traits. Sight and hearing are well developed, as these are in place early in foetal development; however, control of muscular function and coordination does not develop for human babies until several months following birth, causing their prolonged period of helplessness. American anthropologist Ashley Montagu described human neonates as exterogestates: foetuses completing their gestation outside of the uterus, while anthropologist-midwife Sheila Kitzinger termed this phenomenon the Fourth Trimester, indicating that human babies continued foetal-like patterns of development for several months after birth.

Although the human baby’s brain grows rapidly after birth, it is not until nine or more months later that human babies are able to control their muscles, coordinate their limbs, and eventually begin to move themselves effectively and independently – first by shuffling and crawling – and eventually around the end of the first year, by standing and walking. So, by 12 months of age, when human babies are gaining control of their balance and beginning to take their first tentative steps under their own steam, they achieve the developmental stage at which other precocial mammals are born.

Now is the time when the rate of human infant brain growth finally begins to slow down, an inflection point that occurs a full year later than in other primate species (Figure 1.2).

What all of this evolutionary biology means for the sleep of babies and their parents is a central question at the heart of this book. With a firm grasp on the evolutionary history of human infancy we are able to explain many of the specific characteristics of human babies that new parents encounter on a daily basis, such as their changing sleep patterns, their need to be held and to feed often, and their distress at being left alone. The key implications are:

1. Growing a huge brain is an energetically expensive business, and a lot of babies’ brain growth (such as producing new cells and creating neural connections) happens during sleep. So, we should expect human babies to sleep a lot – both to conserve energy that can be devoted to brain growth and to provide opportunity for the brain to grow. This explains why human newborns spend much of their time asleep.

2. Despite sleeping for up to twenty hours a day, newborns do not have consolidated sleep – that is, they do not sleep in very long bouts. Typically, young babies will sleep in two-to-three-hour bursts at most, waking frequently throughout the day and night. This is in part because new-borns have tiny stomachs and, as we have seen, because humans are precocial mammals: our milk is low in fat and easily digested, so our babies need to feed frequently and wake regularly to do so. Many breastfed babies continue to wake and feed during the night throughout the first year, and night-time feeding, as we shall see, is an important driver of their mothers’

Non-human primate infants

Body

Figure 1.2 The growth trajectory of non-human primate and human infant brains22

The first graph shows that non-human primate brains grow rapidly relative to body size until birth, and then brain growth slows down. The second graph shows that human brains also grow rapidly relative to body weight before birth and continue at the foetal rate after birth. The inflection point where brain growth slows down happens at birth for other primates, but at the end of the first year for humans. Birth

milk supply. Understanding lactation is an important component of understanding how babies sleep.

3. Related to the need to feed frequently, as per other precocial mammals, human babies are designed to be in close contact with their mothers for warmth, safety and food, day and night. While these key aspects of infant care can be outsourced to other carers, and even to technological innovations (blankets, cradles and feeding devices), babies are hard-wired to seek out their mother’s arms and breasts. Consequently, the responsibility for early infant care disproportionately falls to mothers, and in response humans have culturally adapted to prolonged and intensive infant care patterns.

Meeting human babies’ needs

In spring 2015 I was invited to speak at a breastfeeding research conference in Japan. The audience comprised lactation consultants, midwives, paediatricians, and other clinicians involved in the care of mothers and babies in the post-birth period. In one of my talks I shared the results of a recently completed research study,23 which had been novel for two reasons: firstly, we had used infrared video cameras mounted at the end of hospital beds in a UK postnatal ward to observe the night-time interactions of mothers and their babies during their first night following birth, and secondly we had randomly assigned the mother and baby pairs to three different sleeping conditions. These three conditions involved: baby in a normal hospital bassinet by the bed; baby in a three-sided bassinet that attached to the mother’s bed like a sidecar; and baby in bed with the mother (using a mesh side-rail to ensure the baby would not fall).

We were interested in how mothers responded to their babies’ cues in these three locations and how this affected feed frequency. Typically in UK postnatal wards, babies are kept at their mothers’ bedside – an arrangement known as ‘rooming-in’ – but sometimes when babies are fussy and mothers are exhausted, staff will offer to take the baby away for a few hours so the mother can get some rest.24 This practice of separating babies from their mothers following birth was common practice in UK hospitals throughout most of the twentieth century, and in many US hospitals it is still the norm, continuing when babies are transferred home; the separation of babies from their mothers that began in the hospital is often perpetuated by placing babies to sleep in cribs in separate rooms from their parents. In Japan, however, mothers and babies traditionally sleep together side by side, and the idea of separating mothers and babies during the new-born period horrified the audience I was speaking to. Even though it was only one small aspect of my talk, it clearly struck a chord. At the end of my talk the podium was besieged by Japanese health professionals, women and men, imploring me to ensure that UK babies were not taken away from their mothers at night, because babies and mothers need to be together.

Indeed, multiple studies have found that many more Japanese than American babies are likely to sleep with their parents,25 and that in Korean and Chinese families likewise parent–infant sleep contact is normal and common.26, 27 Ethnographic accounts of Japanese sleeping habits clearly document the importance of parent–child co-sleeping for generating anshinkan (contentment, relief and security) for all participants.28

This experience highlights that, while we might reasonably expect all babies’ biological needs to be similar at birth, the interpretation of their needs in different cultures, and therefore the care they receive, varies widely. In most societies with a