Human babies adapted to sleep less but better

The sleep intensity hypothesis in human evolution

What is different about the way humans sleep, compared to other mammals and primates?

A recent study from Duke University, published in the journal Evolutionary Anthropology in 2015, reveals a lot about sleep evolution for mankind. They studied sleep environments and analyzed research on sleep patterns among many mammal and primate species. They found that (adult) humans sleep on average 7-8 hours in a day, whilst for example the chimpanzee spends on average 11-12 hours in sleep mode and other species even more. In fact, they found that humans are among the mammals that sleep the least. However, the quantity of human sleep is less, but the quality is best. Our sleep is deeper and more productive for our brains, compared to other species.  We spend most of sleep time in REM sleep – active eye movement, sleep with dreams – in contrast to other mammals. REM sleep comprises about 25% of human adult sleep, when in all other primates the percentage of REM sleep doesn’t surpass 5%.

Humans are not distinctive from other primates only for their developed thumbs and upright posture. Our sleep is significantly and uniquely different compared to sleep of our evolutionary cousins. But why has our sleep evolved in such a way? Research cannot explain these differences in sleep only by the use of artificial light in humans. Human tribes that have no access to night lights and electricity sleep less as well. The evolutionary reason in humans developing into less sleepers is probably the following; as we evolved into using sleeping environments from trees to ground, and sleeping together in family groups, we could thus be warmer during the night and less worried about predators and dangers. This made it possible for us to gradually spend less time sleeping and more time interacting with each other, alert and manipulating our environments. This gave humans more freedom and time for intense activity throughout the day, to develop learning abilities and skills, to enhance cognition, to produce civilization. And the consolidation of these daytime experiences and new endeavors crucially occurs in our more prolific and intense REM sleep. Our cognitive capacity was augmented by two factors, by less hours in sleep – thus more hours in activity and interaction – and by more hours in productive REM sleep. This is the plausibility of the so-called sleep intensity hypothesis in human evolution. Sleep rich in quality, interaction and touch, little in quantity, and here we are. Sleeping less but better has made it possible for us humans to dominate the planet.


Sleep intensity and normal sleep in human infants

How do we transfer these findings in the way our babies sleep? Do they have any significance in how we feed our babies? Is there any connection between these anthropological findings and infant nutrition and sleep?

Human evolution, a process that has taken thousands of years to unfold, actually is repeated and recreated in every single human  being, from conception through embryonic life through birth and first years of life. Early life of every human, her development and maturation from womb to adulthood, in fact represents significantly, comprises a remonstration of the evolution of the human species. In other words, the way human brains mature from embryo to early and later childhood takes the exact steps, something like a miniature model, of the development of human brains throughout history. The fish-like embryo of first days after conception develops to a mature human being depicting in steps memories of thousands of years of human evolution.

Human babies are born too immature, compared with other species. Their brains mature significantly under the environmental influences during the crucial first 3 years of life. The size of our brains triples during the first year of our lives alone, neuronal connectivity is hugely enhanced by early cognition and experiences, myelinization – maturation of neuronal transfer signals and circuits – occurs after birth. It’s the reciprocal social interaction with humans, the responsive appropriate and prompt care of mothers which represents a major factor for our brains and cognition to develop.

Human babies spend even more percentage of their sleep time in active REM sleep, compared to adults. The all-new stimuli and experiences during daytime activity need consolidation in sleep dreams to be processed and scanned. They tend to awaken frequently, in small sleep circles of 2-3 hours and go back to short, light dream sleep. Normal nutrition and care of human infants includes breastfeeding on cues, with no imposed time restrictions. Breastfed babies tend to nurse frequently, day and night. Night breastfeeding is important for the continuation of breastfeeding and for mothers having adequate milk supply. But it is also important for nurturing rich interaction, touch, cuddle, small baby talk, and reciprocal communication in babies, all of which cannot stop at nighttime. Babies on formula feeding frequently show different patterns of sleep, spending more hours in sleep, sleeping more easily “through” the night, spending more time in deep non-REM sleep, self – soothing, and getting accustomed to not demanding contact with parents in awakenings.

As it happened with our ancestors, who discarded needless hours of sleep and consolidated their sleeps in productive dreaming stage so that they become more social and smarter, the biological imperative for human infants is to keep social interaction throughout the night, awaken frequently, get prompt and appropriate responses of care and feed day and night. Tactile reciprocal skin to skin contact has been proven so critical for further development and maturation of the born immature human brain and for enhancing cognition in a human infant. The same with reciprocal mother to infant baby talk even during the night, with acoustic stimuli, scents and other senses communicated regularly from mother to baby – and vice versa, promoting this way strong mother-infant bonding.


The cultural violation of our babies’ biological imperatives

Every single breastfeeding family I come across in my work as a paediatrician and a lactation consultant has major, intense and repeated worries about their babies spending “too little” time sleeping and waking up “too often”. This is by far the number one concern for modern mothers intending to follow WHO guidelines and breastfeed their babies on cues and exclusively for 6 months, followed by introduction of solid food and continuation of breastfeeding for at least 2 years.  It must also be the number one reason for discontinueing breastfeeding after 6 months of age. Most mothers will tell you that even if their babies were sleeping well between 2 and 6 months of life, their sleep pattern “worsens” after six months awakening more often at night and crying for cuddle and breastfeeding. Many at this stage strive to get baby back to sleep not with breastfeeding and cuddles but with substitutes – giving dummies, separating baby in other room so that he “learns” solitary sleep, getting father involved, rocking baby for hours, giving baby to the grandmother or imposing strict practices such as “crying it out” and not responding.

At the same time, this is the developmental stage when human infants learn object and people permanence, search for mother when she leaves, develop separation anxiety and strangers’ anxiety, begin to have aches from teething and open up to the world resulting in the development of fears and vivid dreams in their sleep. That is, this is the time, 6-18 months of life, when babies need their parents most, thrive from even more responsive, appropriate and prompt care.

And yet mothers commonly try to impose cultural substitutes to initiating prompt contact and breastfeeding attuning to regular awakenings of their infants – cultural alternatives such as striving to put baby back to sleep without breastfeeding, by rocking or swaddling or using a dummy or letting the father strive to put baby back to sleep, or even imposing sleep training practices, not responding to their baby or crying it out with the goal that the baby finally resigns, despairs and “auto-regulates”, putting himself back to deep sleep by self-soothing, many times by finding his finger to suck and soothe for years to come.

During the last 200 hundred years of human civilization – minimal time in evolutionary respect – our babies have experienced a radical shift in the way they sleep, in an opposite direction that contrasts the one of thousands of years of human sleep evolution; We have predominantly given our babies milk from another species instead of their mothers’, imposed solitary sleep in separate rooms, enhanced substitutes to nighttime human touch and breastfeeding, answered their needs with not responding, reiterated cultural views of babies sleeping through the night, self-soothing through the night, and prevailed paucity of nighttime interaction with babies. All these in order for adults to separate early from their kids, work more, produce more. Early independence of babies from their parents so that parents carry on with their busy lives without the “bothers” of infant nurturing.

We have experienced a cultural detrimental reverse of the sleep intensity hypothesis for mankind; the adverse cultural force of infantile solitary sleep and paucity of interaction and breastfeeding overnight that contradicts human evolution of thousands of years. Modern parents expect that their babies do not disturb their precious sleep soon after birth. Modern stresses of life for parents demand for their babies to be separated early, sleep a lot and deeply, so that adults get on with their busy working lives and productivity remains high. It seems that current west civilization prefers our babies not to have the expected human biological imperatives, those of reciprocal interaction and affection day and night, but rather the biological imperatives of veals, who tend to sleep long hours, pass much time in deep nonREM sleep and display little interaction with parents. That’s just not likely to pay any service to humanity on the long term.

The whole business – commonly in economical, industrial and company terms –  of systematically driving women away from breastfeeding and sleeping babies through the night stands in sharp contrast with our evolutionary trends and needs. Human infants are not evolutionarily designed to sleep alone or to sleep through the night. Modern cultural impositions distance human babies from their needs, their biological and psychosocial imperatives being systematically violated.


Adapting to babies’ nighttime developmental needs in a modern world

How can we respect and promote our evolutionary trends whilst simultaneously promote family harmony and adaptation in the modern world?

Families need to invest more time and energy in attending and meeting the needs of their infants responsively, because what distinguishes human beings from other species is the gross immaturity of newborns and the intense needs they have for touch, affection, responsive care and social interaction for their brains to mature. Societies need to acknowledge the need for extra input mothers and fathers need to invest in children below three years of age, and acknowledge the importance of giving more to small kids for the development of healthy human beings and healthy societies. Breastfeeding on demand is a key part of this crucial investment. We need to change our cultural views on normal infant sleep – solitary sleep, in separate rooms, sleeping through the night – with a view that respects babies’ biological rhythms.

The way we treat our babies at night has important consequences not only on an individual basis, not only on the current social basis but even on evolutionary basis for mankind. The adverse effect of recent cultural changes in infant feeding and sleep on human evolution needs to be reversed. Breast milk and breastfeeding have been evolved in human history so that they promote brain maturation and cognitive development at night, thus need to be actively protected and promoted. Society needs to acknowledge the extra burden parents have in nurturing healthy – in somatic, emotional and cognitive terms –  kids. They should not be left alone and be expected to carry the burden on their own against all odds. Maternal leave, family provisions and fostering proper environments for optimal early life experiences is of paramount importance for future generations being competent and happy, for, even on the simplistic and restrictive fiscal terms, a more healthily productive adulthood.

Sleep rich in quality, interaction and touch, little in quantity, bear in mind that this is actually the evolved normal for our babies.


Stelios Papaventsis MBBS MRCPCH DCH IBCLC 2016



Sampson DR et al. Sleep intensity and the evolution of human cognition. Evolutionary Anthropology, Dec 2015; Vol 24, Issue 6: 225-237

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2 Responses to “Human babies adapted to sleep less but better”

  1. Πόπη says:

    Τα άρθρα σας με βοηθάνε να μη χάνω το στόχο. Συμφωνώ. Συμπαράσταση είναι αυτό που λείπει, σε εμένα τουλάχιστον, για να ξεπεραστούν οι δυσκολίες που προκύπτουν από το συνταίριασμα όλων των ρόλων που πρέπει να παίξουν οι σύγχρονοι γονείς.

  2. Mat181 says:

    Συγχαρητήρια. Και ως μητέρα και ως συνάδελφος ψυχίατρος με ιδιαίτερη ευαισθησία σε θέματα σύνδεσης μητέρας βρέφους!

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