Be warned, I'll write a lot, but I'll try to make the text as readable as possible. If you spun a lot of explanations on the buoy, it is better to immediately scroll to the finale. From a biochemical and physiological point of view, it looks like this:
The fact is that glucose in our body, for the most part, is needed for the brain. (Glucose ---> the chain of biochemical reactions -----> Energy (ATP) -----> the work of the brain, as well as other organs and tissues). Nutrition of the brain is its main function. Of course, there are others, the importance of which cannot be diminished, but the question is not about them. The brain plays the main role in the body, works most of all, accordingly has the greatest need for energy and, accordingly, has the most intense blood supply of all organs. In addition, the nervous tissue is one of the so-called glucose-dependent tissues, together with erythrocytes and the medulla of the kidneys. That is, the brain can burn only glucose in order to obtain energy on which it can function. And not to use the organ for the sake of saving what is intended and necessary for its work is simply pointless.
Of course, with increased mental activity, the blood supply to the brain increases, and its consumption of glucose and oxygen increases. But this is a _natural_ physiological process.
Glucose is contained in our blood at a certain concentration (~ 3.3 -5.5 mmol / l), and besides, it is stored in the form of glycogen in the brain itself (in a very small the amount of which is enough for 5-6 minutes after the cessation of the supply of glucose in the blood), in the muscles and, most of all, in the liver.
When the concentration of glucose in the blood decreases or increases, a number of chain reactions are triggered, leading to its increase or decrease in the blood, respectively. That is, everything is kept at a certain level, not only glucose. And such a self-regulating balance is called homeostasis.
Carbohydrates cover no more than 60% of all energy expenditures. In many tissues of the body, it is possible to use for decomposition for energy and other intermediate metabolic products of proteins, fats and other carbohydrates (that is, in addition to glucose). And also glucose can be synthesized from all this (proteins, fats, carbohydrates) through the Krebs cycle (google if interested). Somewhere these or those of them are used to a greater extent, somewhere to a lesser extent, with different nuances, different additional conditions and different energy output.
(For example: the brain works mainly by 90% , on oxygen-dependent breakdown of glucose, that is, on aerobic glycolysis, with an energy release of 38 ATP molecules from one glucose molecule, and 10% on anaerobic - only 2 ATP molecules from the same glucose molecule. For example, muscles are capable of anaerobic glycolysis , as a result, lactic acid is formed due to which the muscles ache after intense physical activity. As it was correctly written by Denis Tsaregorodtsev, muscles spend a lot of energy at the time of work, especially intense work. In general, yes, they spend more energy, but with glucose a little drugaya story. Muscles are capable of receiving energy in several other ways than the way it is received in the brain. Athletes and workers of heavy physical labor, for energy, burn mainly fats. And glucose continues to feed their brains. The generated energy (ATP) is used further in chemical reactions, depending on the tissue in which it was formed. In muscle - ATP is used to contract this muscle.)
Why does the brain need aerobic oxidation? This oxidation has an important advantage - the brain does not swell, the person does not fall into a coma and does not die. This is a separate topic that will require the same canvas of text.
"Not thinking" will help, due to a decrease in the intensity of blood supply to the brain, imperceptibly for you, to slow down glucose consumption by microscopic values .
The brain cannot function without glucose. Not thinking to save glucose = not breathing to save air. In principle, it is possible, but why? (if you allow a complete shutdown of consciousness, as in the situation described by Dmitry, with hibernation, then you will benefit).
All metabolic pathways in the body intersect. They leave in a spiral and intersect again. Therefore, the body will find where to get glucose from, no matter how hard you try to spend it with persistent thoughts or physical exertion. And it makes no sense to "protect" it in the way you proposed.
Not thinking is ADVANCED! Survival in nature is provided primarily by the power of the mind, and not at all by physical capabilities. Both the strong and the weak can escape from the enemy, but a stupid one who misjudges his capabilities and chooses the wrong line of behavior will not be able to survive.
A "intracranial muscle" needs constant training, like any other. Therefore, using energy for the thought process is completely normal. That is why it, energy, exists.
By the way, according to physiologists, 80% of the energy absorbed from food is spent by a living creature on mental activity!
The brain, of course, consumes a lot of energy and, accordingly, glucose for its activity. But muscles (and especially muscles involved in physical work or sports) consume much more of it when they work. From this, to conclude that it is beneficial to lie down and not move, and athletes are just crazy squandering of valuable energy - this is some kind of wrong conclusion. It seems to me that there are no benefits from not thinking. Yes, and there are no problems with food and carbohydrates either.
Well, except to imagine a fantastic situation that the spaceship crashed, and the crew is taking it to no one knows where, and food is limited, but there is a chance that rescuers will arrive - well then, yes. Better to hibernate - the chances of survival will increase.