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Why Babies Sleep As They Do

To understand why our babies sleep the way they do, we need to understand something about what has happened to human babies during our evolutionary history. An 'evolutionary perspective' can tell us how and why the biology and behaviour of human babies has come, over millions of years, to be the way it is. Here we will explore how humans compare to animals of other species, their different characteristics, and what it means for the way babies behave and sleep. We also look at how our present-day Western approach to baby-care has developed, and where we may have mismatches between what we do in the 21st century, and what newborn babies have evolved to expect, with respect to infant care.

As discussed under 'normal sleep development' feeding method is intimately related both to how babies sleep, and what we must consider as biologically normal in an evolutionary sense. For that reasonthe information presented in ISIS will often 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 cow's 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

Length of gestation period and developmental state at birth varies among mammals with infants generally categorised into two well-known types. Altricial newborns are poorly developed, sequestered in nests, and fed infrequently with high fat milk. Precocial newborns are well-developed, able to follow or cling to their mothers, suckling frequently and at will, on milk that is relatively low in fat but high in calories (lactose) providing energy in a quickly digested form (Small, 1998). Among the primates, monkey and ape infants are precocial; human infants conform, by consequence of evolutionary relatedness, to this precocial primate pattern.

Yet human infants also display 'secondarily altricial' characteristics-primarily lack of neuromuscular control-a consequence of the limits imposed on gestational brain development by the evolution of the human pelvis. Human newborn brains are 25% of their adult volume (compared to 50% for infant chimpanzees and gorillas) due to the constraints of a birth canal that was modified for bipedal walking. Although displaying many precocial traits, therefore, human infants are unable to independently locomote or cling, and therefore maintain proximity with their mothers, or to effectively regulate temperature and breathing during the first few months of rapid brain growth and development (Small, 1998; Hrdy, 1999).

Human milk has a similar composition to that of other precocial primates, being relatively low in fat and protein, but high in sugar (Jelliffe & Jelliffe, 1978). It is milk produced for infants who suckle frequently and of their own volition day and night, and the high sugar content in the case of humans provides the energy needed for rapid brain growth. Due to poor neuromuscular control, however, human neonates require their mothers to ensure that proximity is maintained, frequent feeding facilitated, and physiology regulated.

Ethnographic data from societies around the world confirm that mothers in traditional human societies are in almost constant contact with their infants, carrying them strapped to their bodies by day, sleeping beside them at night (Ball, 2006a), and breastfeeding at will. From an evolutionary perspective, this behavior increases infants’ life chances as they remaining safe whilst achieving optimal nutritional requirements. However, the benefits do not end there. It is argued that frequent suckling, particularly at night, is an evolutionary adaption of the infant to extend their mothers’ lactational amenorrhea. This serves to delay the birth of a younger sibling and further enhances the infant’s chances of survival (Haig 2014). Consideration of the human neonate from an evolutionary perspective therefore highlights the fact that many aspects of what is considered to be 'normal infant care' in contemporary western societies are historically recent culturally adopted practices (Crawford, 1994) that do not support evolved infant biology. This means we need to consider what is normal for early infant care. 

Last modified: 16th December 2016