The human brain is, by every available measure, an unusual organ in terms of its energy demands. The brain accounts for approximately two percent of total body weight in an adult human. The brain consumes approximately twenty percent of the body’s total energy. The ratio of these two numbers is, on close examination, one of the more striking facts about how human metabolism is allocated. The brain, in adult humans, is consuming energy at roughly ten times the rate that its weight alone would predict.
The standard cultural framing of this fact tends to attribute the high energy consumption to thinking. The brain is, in the standard framing, the organ of cognition, and cognition is presumed to be expensive. The framing makes intuitive sense. The framing also turns out, on close examination, to be considerably less accurate than the wider register has been treating it as. Most of the brain’s energy is not, on the available evidence, being spent on thinking in any conscious sense. Most of the brain’s energy is being spent on what neuroscientists now call the resting state, which is the ongoing background activity the brain conducts continuously, regardless of what the conscious mind is currently doing.
What Marcus Raichle actually noticed
The structural recognition of this came, in some real way, from the work of the neurologist Marcus Raichle at Washington University in St. Louis in the late 1990s. Raichle was conducting routine brain imaging experiments, using PET scans and later fMRI to measure brain activity while participants performed cognitive tasks. The standard procedure involved comparing the brain activity during the task to the brain activity during a baseline period of rest, with the difference being attributed to the cognitive work the task was eliciting.
What Raichle noticed, that previous researchers had not particularly attended to, was the baseline. The baseline was supposed to be the inactive condition, the period during which the brain was, in some sense, idling. The baseline turned out, on careful examination of the data, not to be inactive at all. The brain during the rest condition was operating at roughly ninety-five percent of the energy consumption it operated at during demanding cognitive tasks. The difference between hard thinking and apparent doing-nothing was, in energy terms, approximately five percent.
The five percent figure has been confirmed repeatedly in subsequent research. A 2024 review in the wider neuroscience literature reports the same finding. The energy consumption of the brain in the task state is only about five percent higher than in the resting state. The wider implication, on close examination, is that approximately ninety-five percent of the brain’s energy budget is being spent on something other than the conscious effortful thinking the wider culture associates with using the brain.
What the brain is doing during the resting state
It is worth being precise about what the resting-state activity actually consists of, because the wider cultural register has not, on the available evidence, developed particularly good vocabulary for it.
The resting state is not, on close examination, the brain doing nothing. The resting state is, more accurately, the brain doing a particular kind of ongoing background processing that does not, in most cases, surface to conscious awareness. The processing includes the consolidation of recent memories. The processing includes the integration of new information with existing knowledge structures. The processing includes the maintenance of the various neural systems that monitor the body’s internal state. The processing includes the continuous low-grade preparation of the brain for whatever cognitive demands might arrive in the next several seconds. The processing includes, on the available evidence, considerable amounts of what the wider register tends to call mind-wandering, daydreaming, and spontaneous internal thought.
The specific network of brain regions most active during the resting state has been identified and is now called the default mode network. The default mode network includes the medial prefrontal cortex, the posterior cingulate cortex, the precuneus, and several other regions that activate together when the brain is not engaged in any specific external task. The network is, in some real way, what the brain defaults to doing when nothing else is demanding its attention. The network consumes considerable energy. The network is, on the available evidence, conducting most of the cognitive work that the conscious mind is not aware of conducting.
Why this matters for what cognition actually is
The structural implication of all this, on close examination, is that the wider cultural understanding of cognition has been calibrated to the wrong target. The wider understanding treats cognition as the visible work the conscious mind does during effortful thinking. The cognition, in this framing, is the writing of the email, the solving of the problem, the deliberate planning of the afternoon. The framing treats these activities as the bulk of what the brain is doing.
The energy accounting tells a different story. The energy accounting suggests that the visible effortful cognition is, in terms of brain resource allocation, a small surface activity sitting on top of a much larger ongoing background process. The background process is what the brain is actually doing most of the time. The visible cognition is, in some real way, a small modulation of the background process, requiring only a few percent additional energy to produce.
This means, structurally, that whatever cognition actually is, it is emerging from the background process rather than being primarily constituted by the visible effortful activity. The conscious mind has access to the visible activity. The conscious mind does not have direct access to the background process. The conscious mind, accordingly, has been overestimating the role of the visible activity in producing the cognition that the conscious mind experiences itself as performing.
The actual production of cognition, on the available evidence, is happening largely below the threshold of conscious awareness. The integration of memories. The accessing of relevant prior knowledge. The construction of the various contextual frames that allow incoming information to be interpreted at all. These operations are not, in most cases, conscious. These operations are, on the available evidence, what most of the brain’s energy is being spent on. The conscious effortful thinking is the thin top layer of a much deeper ongoing process.
What the wider implication is
The wider implication of this, for how one understands one’s own cognitive life, is on close examination considerable. The conscious mind has tended, in the standard cultural framing, to identify itself with the visible effortful thinking. The thinking is the part the conscious mind has access to. The thinking is the part the conscious mind feels itself to be doing.
The neuroscience suggests that this identification is, in some real way, misplaced. The thinking is the small visible portion of a much larger process the conscious mind is not directly participating in. The actual cognitive work is, by every available energy measure, being done elsewhere, in systems the conscious mind does not have access to, by mechanisms the conscious mind does not direct. The conscious mind is, more accurately, the recipient of the outputs of this larger process, presented to itself as if they were its own deliberate productions.
This is, on close examination, a significant revision of the standard cultural framing of what it is to be a thinking creature. The revision does not, in any obvious sense, diminish the experience of thinking. The experience continues to feel like one is doing the thinking. The revision does, however, suggest that the doing is more complicated than the experience suggests. The doing involves an enormous amount of background work that the experience does not have access to. The work is what most of the brain’s energy is being spent on. The work is, in some real way, what cognition mostly is.
The acknowledgment this article wants to leave
The human brain consumes approximately twenty percent of the body’s energy while constituting approximately two percent of its weight. The ratio is striking. The standard cultural framing has tended to attribute the high consumption to thinking. The available neuroscience suggests, more specifically, that approximately ninety-five percent of the consumption is being spent on the ongoing background work of maintaining the resting state, with only the remaining five percent attributable to the difference between effortful thinking and apparent rest.
The resting state is not, on the available evidence, inactive. The resting state is, more accurately, where most of the cognitive work the brain is doing is actually being conducted. The conscious experience of thinking is the small visible portion of this work. The unconscious background processing is the much larger part. The wider cultural register has, on the available evidence, been calibrated to the visible portion and has accordingly been underestimating the scale of the unconscious portion.
The recalibration is, in some real way, available to anyone willing to take the energy accounting seriously. The energy accounting is unambiguous. The brain is mostly spending its energy on what the conscious mind is not directly aware of. The conscious mind is, accordingly, the small visible feature of a much larger cognitive process that is occurring continuously, regardless of what the conscious mind happens to be doing. The conscious mind, on close examination, is not the cognitive process. The conscious mind is, more accurately, what the cognitive process produces when it has the resources to spare for the additional activity of presenting some of itself to itself.
