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, etc.) 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 care-giver 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.