Why Babies Sleep As They Do

As anthropologists we think about human babies differently to most other people — in trying to understand why our babies sleep the way they do, we look at humans as mammals and consider the importance of mammalian biology and evolution in understanding how we are and what we do. Western baby-care practices are recent developments, and the biology of human babies expects something rather different than 21st century families might provide.

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 may discuss 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

IIn 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 (above photo on the left) and precocial (above photo on the right).

Primate species, including our close cousins the great apes (chimpanzees, bonobos, gorillas and orang-utans), 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. We look at the reason for this on the following pages, as it affects both human infant development and sleep behaviour.

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 feed do not 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 species such as deer, horses, sheep, 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 are not left in a nest while their mother hunts or forages but stay with their mother wherever she goes. 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 compared to the high fat milk consumed less frequently by altricial species. It provides these active and alert babies with energy.