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Sleep on It: Design in the Subconscious Brain

Evolution News
Photo credit: Kasper Rasmussen via Unsplash.

Why do we sleep? It’s something we spend about a third of our lives doing, yet to this day scientists have debated why. The period of torpor in which conscious awareness descends into a subconscious state is shared by all animals with a central nervous system, and is tied to the diurnal cycle. Yet it cannot be a matter of safety in the dark, because many animals are nocturnal, and most animals active in the daytime take naps. Nighttime, furthermore, is a function of latitude. Near the poles, the sun never sets for months at a time. Scientists have learned a lot about sleep; they can measure brain waves, classify patterns of sleep such as REM (rapid eye movement) and NREM (non-REM), and observe the effects of sleep deprivation. They have hypotheses, but the purpose of sleep has remained elusive.

Five scientists from five institutions across America and the UK examined “the most comprehensive published data on sleep throughout human development and across species” to answer the question. Publishing their results in the AAAS open-access journal Science Advances1, they believe they have found patterns that constrain the possible functions. 

Comparative, developmental, physiological, and human studies have all been fruitfully used to address questions about the nature of sleep. However, because data are seldom analyzed in a way that connects them with mathematical models or quantitative predictions, conclusions about the function of sleep have remained slow to evolve. In this context, we develop a general theory for the function of sleep that provides a framework for addressing several fundamental questions, such as what are the relative roles of repair and reorganization during sleep? And do these change during ontogenetic development? [Emphasis added.]

In the statement above, the word “evolve” refers to intelligently designed searches for answers by scientists. In the paper, the scientists clearly subscribe to another kind of evolution — Darwinism — believing that sleep, like everything else in biology, evolved by random mutation and natural selection. And yet their sixth sense knows that a function must exist for sleep. This is a common fallacy among evolutionists; they think they can get purposeful ends from purposeless processes. They follow a distorted Cartesian maxim: “it exists, therefore it evolved.”

Sleep is such an engrained and necessary part of our lives that we often take its functions and origins for granted. Presuming that sleep evolved to serve some primary function, it is almost certain that multiple physiological functions have piggybacked onto this pervasive and time-consuming feature of animal life.

Dying to Rest

Experiments show that animals will die without sleep. Tests of prolonged sleep deprivation show fruit flies, rats and dogs never waking up after a point. Even humans, whether testing their limits in a dance marathon for a Guinness world record, escaping danger or being tortured, will collapse at a point, as if nature enforces the requirement to sleep. Firefighters and other emergency first responders can be trained to wake up suddenly out of a deep sleep, but they eventually must get long enough breaks to get the benefits of unbroken sleep. From an evolutionary point of view, though, sleep is risky for fitness: a predator or enemy can attack when the prey is helplessly unaware. Surely natural selection would weed out the unfit who fall prey in sleep and would favor those able to stay awake, wouldn’t it? Yet that is not the case in insects, reptiles, birds, mammals or humans.

The pervasiveness of sleep during development and throughout the animal kingdom suggests that it is a biological process that is necessary for survival. Although we spend approximately a third of our life asleep, its explicit physiological and evolutionary function remains unclear with myriad hypotheses being postulated. Two of the leading hypotheses are that sleep enables (i) the repair and clearance needed to correct and prevent neuronal damage and (ii) the neural reorganization necessary for learning and synaptic homeostasis. These hypotheses are compelling because neither of these processes can be easily achieved in waking states, and there is supporting empirical evidence that they occur during sleep.

Hypothesis #1 about repair and clearance was discussed in our recent article about circadian clocks. This team found additional support for that function, but also for hypothesis #2 about neural reorganization. One major finding was that the function switches from #2 to #1 at a predictable stage in life that is quite sudden. Combing through the data from animals and humans, they made these primary findings:

We collate and integrate data for total sleep time, REM sleep time, brain weight, body weight, cerebral metabolic rate, and synaptic density based on a systematic review of the literature. The resulting dataset spans from 0 to 15 years of age and cumulatively represents about 400 data points. We then use the empirical data to find patterns of sleep during ontogeny, compare them with phylogenetic patterns, and test predictions from our theoretical framework. In so doing, we:

1) develop distinct quantitative theories for both neural repair/clearance and neural reorganization;

2) use extensive human sleep and brain data from birth to adult to cleanly test and discriminate between these theories;

3) provide strong evidence of a remarkably sharp transition at about 2.4 years of age in the primary purpose of sleep from being for neural reorganization that occurs during the active sleep/REM cycle in earlydevelopment to neural repair and metabolite clearance in late development.

In short, infants need sleep to organize their neurons. After two years of age, when neural reorganization has completed sufficiently, the need for repair and waste removal predominates. Those seem to be the main functions of sleep. “Neural signaling and computation in the brain are extremely costly, accounting for 80 to 90% of its metabolic expenditure,” the team says.

When looking at functional brain development in humans, Johnson found that the first 2 years of life is the period that most of the pronounced advances in brain structure and behavior occur. Brains develop extremely dynamically in the first 2 years, and most brain structures have the overall appearance of adults by the age of around 2. One notable exception is the delayed development of the prefrontal cortex, the onset of which perhaps corresponds with a surge in REM sleep around later puberty, which would be predicted by our theory. Overall brain size increases markedly during the first 2 years of life and reaches 80 to 90% of adult size by the age of 2.

Does Ontogeny Recapitulate Phylogeny in Sleep?

The team halfway expected to see hints of Haeckel’s recapitulation theory in the data:

The major focus of this paper is to address the intriguing question as to whether these general relationships for sleep times remain valid during growth, implying that ontogeny recapitulates phylogeny, or whether new patterns emerge during development, reflecting a different dynamical origin for sleep. More pointedly, during both development and across species, sleep times systematically decrease with brain and body size. But do they do so at similar rates? And are they attributable to the same underlying dynamics? If new patterns emerge, what do those patterns reveal about neurological development and the growth of the brain? To answer these questions, we derive a quantitative ontogenetic sleep model across species that combines both ontogeny and phylogeny in a single framework and use this model to guide the analysis of human sleep data from birth to adult. We compare our new findings with previous empirical and theoretical results for how sleep changes across phylogeny to ask whether explanations for why a mouse sleeps roughly five times more than a whale can also be used to explain why babies sleep roughly twice as long as adults.

Alas, they did not find support for Haeckel’s “law” (which was, for many years, considered strong evidence for Darwinian evolution in his long-debunked embryo drawings).

However, during early ontogeny, the brain is undergoing rapid changes in size and synaptogenesis that require a relaxation of the power optimization and other constraints that determine how metabolic rate scales with size for mature organisms. Therefore, the canonical theory for the scaling of metabolic rate needs to be reformulated for the brain to recognize that ontogeny may not recapitulate phylogeny.

In the final Discussion section, they repeat this negative result more forcefully:

The large change in percent REM sleep across development is thus a key indicator that the function of sleep, and particularly of REM sleep, is very different during development than in adults. It shows that ontogeny does not recapitulate phylogeny because ontogeny does not show qualitatively similar patterns to phylogeny for REM sleep. Rather, it differs from it in the most fundamental of ways (e.g., invariance versus rapid change) and exhibits a phase transition between early and late development….

Our ontogenetic findings differ markedly from previous phylogenetic findings both in terms of the magnitudeand sometimes, the direction of changes and the corresponding scaling exponents. These results are quite unexpected, yet by transitioning from our model for neural reorganization to one for repair and clearance, we are able to simultaneously explain the scaling of sleep time across species and across growth.

In Search of Haeckel

They looked in earnest but did not find Ernst (as in Haeckel). Actually, they didn’t find much of Charles, either (as in Darwin). They did find a lot about function, however — a word mentioned 61 times in the paper. The word “purpose” was used as well. Those are design words. ID science looks for evidence of functional information in biology. Sleep has at least two important functions, as shown here. ID science also looks for evidence of foresight in biology. Considering the needs of infant brains to continue organizing for a time outside the womb, and the lifetime needs of repair and waste clearance, finding a brain state that performs both functions shows evidence that sleep was intended. 

Knowing that Haeckel and Darwin did not contribute anything to the “explanatory and predictive power” of their model, they want to continue investigating if the same two functions of sleep, along with the sudden transition of function with age, occur as well in rats, zebra finches, fruit flies, roundworms, and many other species. Design science can take the lead here.


  1. Poe et al., “Unraveling why we sleep: Quantitative analysis reveals abrupt transition from neural reorganization to repair in early development.” Science Advances 18 Sep 2020: Vol. 6, no. 38, eaba0398. DOI: 10.1126/sciadv.aba0398