https://youtu.be/gbMB-uEXSBA?si=KsVQJyQhpKH7sPsA
Yes — and the interesting part is that “not focusing” does not mean the brain becomes metabolically quiet.
The correction I’d make is this: the resting brain as a whole is enormously energy-consuming, but it is probably too strong to say that the DMN itself consumes most of the brain’s energy. The adult brain is only about 2% of body mass, yet it uses roughly 20% of resting oxygen consumption, corresponding to about 20 watts continuously. When you switch from quiet wakefulness to a demanding cognitive task, the increase in total brain energy use is surprisingly small—often only a few percent.
So when you stop concentrating on an external task, your brain doesn’t enter anything like a computer’s sleep mode. Instead, activity is redistributed. The default mode network—medial prefrontal cortex, posterior cingulate/precuneus, inferior parietal regions, medial temporal systems—tends to become more active during autobiographical memory, mind-wandering, imagining the future, self-related thought, social inference, and other internally generated cognition. Many of these regions become relatively less active during attention-demanding external tasks.
In that sense, 「無所事事」在腦的層次，真的不是無所事事。
You may be sitting there doing apparently nothing:
no reading → no calculation → no conversation → looking out the window
yet the brain may be doing something like:
remembering → recombining → simulating → anticipating → narrating → wandering.
That is one of the important implications of DMN research.
There is one numerical claim I’d handle cautiously. You’ll sometimes see statements that the DMN accounts for 60–80% of the brain’s energy consumption. Some reviews repeat numbers in that range, but the safer interpretation is that intrinsic/resting neural activity and signalling account for a very large portion of the brain’s metabolic budget—not that the anatomically defined DMN alone literally burns 60–80% of all cerebral ATP.
And this is perhaps the more interesting answer to your question:
Focusing doesn’t necessarily save or spend dramatically more energy; it reorganizes where the energy is being used.
Very schematically:
A. 坐著發呆
DMN activity relatively ↑
internally generated cognition ↑
B. 專心解題
parts of DMN relatively ↓
dorsal attention/frontoparietal systems ↑
The second state can feel much more effortful subjectively, yet the total metabolic difference may be modest. A 2025 review summarized this nicely: brains are expensive, but cognition is often cheap. The costly thing is maintaining an awake, functioning brain in the first place.
So I’d put it this way:
The DMN is not simply an energy-hungry program that turns on when you stop working. The brain is already running a very expensive intrinsic process all the time; when external task demands loosen, more of that intrinsic activity appears as DMN-dominated cognition.
Which also leads to a thought close to what you’re asking: focus may be less an addition of mental activity than a narrowing of possibilities.
萬念 → 一念
and when the externally imposed task loosens,
一念 → 三千
—not because “三千” suddenly costs three thousand times more energy, but because the costly machinery was already running.
That is also why mind-wandering, reverie, free association, and staring out the window shouldn’t automatically be equated with cognitive inactivity.
