Answering questions on brain

Q: Why does our brain consume so much energy?
Krishna: Our brain consumes about 20% of the oxygen your body consumes which is incredible considering it is only ~2% of a person’s body weight on average.
This involves our conscious thoughts, memories, analyzing, using language and deciphering meaning, abstract thoughts, memorizing, identifying, and the list goes on and on.
This high energy consumption is primarily to maintain and transmit signals across 100 billion neurons. This immense energy demand is driven by constant ion transport, synaptic communication, and maintaining cellular "housekeeping" to support learning, memory, and cognitive functions.

It has predictive responsibilities for anticipating danger. And then, on top of all this, it has a role in running the rest of the organs in your body. Calorically, it is an incredibly demanding organ.
Key reasons for the brain's high energy consumption include:
 Active Signal Transmission: Approximately 66% to 80% of the brain’s energy is used to restore ion balance (\(Na^{+}\) influx) following electrical signaling (action potentials) and synaptic transmission.
Constant Maintenance ("Housekeeping"): The brain requires energy to sustain cell health, including packaging neurotransmitters into vesicles. This process is inherently "leaky," requiring constant energy to maintain readiness even during rest.
High Metabolic Cost of Mitochondria: To meet this demand, neurons rely on a massive number of mitochondria to convert fuel into ATP, which is vital for maintaining cellular vitality.
Evolutionary Trade-off: The brain lacks energy storage (like muscle glycogen) to minimize space and maximize speed, meaning it must be constantly supplied with oxygen and glucose. While the brain is highly energetic, it is also efficient at maximizing the amount of information transmitted per molecule of ATP. 

Q: Does oxygen and energy consumption increase when brain is in intense working as compared to sleep time?

Krishna: The brain consumes more energy while working intensely than while sleeping, but the difference is surprisingly small.
The brain is a "metabolic powerhouse" that constantly consumes about 20% of the body's total energy, even when at rest.
Here is a breakdown of the energy usage:
While Sleeping: The brain remains highly active, particularly during REM sleep, when it consumes nearly as much energy as it does during the day.
While Working Intensely: Intense, complex mental tasks (like solving hard math problems) increase energy consumption by only a small amount, estimated at 8% or less compared to a resting state.
Energy Shift: During sleep, the brain shifts from processing external information to internal maintenance (cleaning waste), whereas, during intense work, it uses more glucose for rapid, specific neural signalling.
While you might feel "mentally drained" after intense work, this is more likely due to stress or the brain's preference to avoid sustained high-intensity, "non-pleasurable" focus, rather than a massive, body-sapping increase in total energy consumption.