To put this whole vague matter into a nutshell:
If something is in use, is working, it consumes energy. Point :)
So to answer your question i looked for changes of brain activity (sleep, resting, cognitive tasks...) and overall energy consumption of the brain. 2 very good articels putting your question into a bigger picture, quite academically written, but focus on the statements concerning energy consumption. The articels also discuss, what actually can be derived from brain imaging data (fMRI, PET), how higher conscious brain functions relate to physiological changes measured by these techniques.
The basic conclusion is, that the brain doesnt variate much its energy consumption, whether resting, tasked,...Contrary it needs a average high activity (high metabolism, energy consumption) to make specific functionality possible at all. So its not like a computer, where you start a program (analog higher conscious brain function, e.g. playin chess) and then processor and memory consume raise, the energy consumption is already and constantly on a high average level, otherwise the operating system (brain) couldnt run at all a distinct software (function).
quoted the to me most important parts, but both articles give pretty good overview and draw bigger picture around ur question.
brain represents about 2% of the body
weight. Remarkably, despite its
relatively small size, the brain
accounts for about 20% of the oxygen
and, hence, calories consumed by the
body (1). This high rate of metabolism
is remarkably constant despite widely
varying mental and motoric activity
This should clearly indicate, that we use and need the brain pretty much from a evolutionary point of view
showing that the maximum values of
oxygen consumption and spike frequency
achieved during stimulation were
approximately the same from both
baselines (i.e., both levels of
anesthesia). The authors assert that
an overall level of ongoing activity
must be achieved for a particular
function to occur
This high metabolic activity is
present when we are completely passive
and resting as well as when we are
observably doing something. Two lines
of investigation have recently
converged in their analysis on how
this energy is being used. Both have
focused on the metabolic requirements
associated with glutamate signaling in
the brain. This focus would seem
reasonable, considering that greater
than 80% of neurons are excitatory and
greater than 90% of synapses release
glutamate (6, 7). Attwell and Laughlin
(8) have taken a bottom up modeling
approach using extant data on the
blowfly retina and the mammalian
cerebral cortex. Estimates from their
approach indicate that most of the
energy used in the brain is required
for the propagation of action
potentials and for restoring
postsynaptic ion fluxes after
receptors have been stimulated by the
neurotransmitter. In contrast,
maintenance of the resting potential
in neurons and glial cells accounts
for less than 15% of the total energy
consumption. Shulman and his
colleagues (9, 10) in a very different
approach using MRS in anesthetized
rats have shown remarkably converging
evidence that a very large fraction
(≈80%) of the energy use in the brain
is correlated with glutamate cycling
and, hence, active signaling processes
An intriguing hypothesis has emerged
that the responsiveness of neurons to
changes in their input depends on a
continuous, high-level but balanced
input of both excitatory and
inhibitory activity (for review, see
ref. 29). Importantly, it is the
balance between this continuous
excitatory and inhibitory input that
determines the gain or responsiveness
of the neurons to correlations in
their input. In this formulation,
spontaneous ongoing activity becomes a
critical enabling factor in the
creation of functional connections
within circuits responsible for
specific behaviors. Furthermore, this
correlation-induced functional
connectivity can be modified without
causing variations in the mean firing
rates of the involved cells. As
Salinas and Sejnowski have pointed out
in their review (29), balanced neurons
have rich dynamics and can react to
external stimuli on effective
timescales that are much smaller than
the membrane time constant of a single
neuron.
So, how might this relate to our
analysis of the energy budget of the
brain? It should be noted that most of
the neurophysiology discussed above
concerns synaptic activity at the
input to neurons. Because the highest
energy-demanding processes in the
brain are centered at these sites (27,
28), it suggests that much of the
ongoing or baseline metabolism is
devoted to processes occurring there.
We might therefore posit that, in the
brain, a large majority of its
metabolic activity is devoted to
ongoing synaptic processes associated
with maintaining a proper balance
between excitatory and inhibitory
activity. Maintenance of this balance
allows neurons to respond
appropriately to correlational changes
in their input and establish the
functional connectivity as required
for a particular task.
Thus, we may entertain the possibility
that the very high baseline or ongoing
metabolic activity of the brain not
only supports processes necessary for
the maintenance of the proper
responsiveness of neurons for the
transient and ever changing functions
of the brain but also instantiates a
sustained functionality.
source
Indeed, relative to the high rate of
ongoing or “basal” brain metabolism,6
the amount dedicated to task-evoked
regional imaging signals is remarkably
small (estimated to be less than 5%).
The brain continuously expends a
considerable amount of energy, even in
the absence of a particular task
(i.e., when a subject is awake and at
rest). A significant fraction of the
energy consumed by the brain (quite
possibly the majority) has been shown
to be a result of functionally
significant spontaneous neuronal
activity.7 From this cost-based
analysis of brain functional activity,
it seems reasonable to conclude that
intrinsic activity may be as
significant, if not more so, than
evoked activity in terms of overall
brain function.
source
So overall high average energy consumption of brain in conjunction with share of the total body energy consumption should make ur point very clear. Saying "but how often we actually use our conscious brain functions" is no counter argument, as the whole "software package" is needed, the brain doesnt behave as a multi-core cpu, its one big decentral core and software is consistently rewritten (e.g. dreaming, the brain doesnt deeply rest, when you sleep. Also some savants show ability to memorize immense amount of data when drawing conscious e.g. a detailed landscape they saw, while the picture saving with their eyes was a highly short & unconscious process, they dont see more than a average joe, they manage somehow unconscious to recall or save simply more information out of/in the brain)
