Why Does Excessive Thinking Lead to Fatigue?

Question: The physical body is composed of the four elements, and the four elements within the physical body are also constantly in flux. Humans absorb the four elements from outside the body through diet, breathing, etc., transforming them into nutrients composed of the four elements that the body requires. Simultaneously, the four elements within the human body are constantly depleting, being expelled through various excretions. And only the Tathagatagarbha possesses the function to operate all of this. When a person thinks, the Tathagatagarbha continuously absorbs the four elements at the subtle sense faculties, transforming them into dharma objects for the consciousness and the mental faculty to perceive. Therefore, excessive thinking consumes too much of the four elements' nutrients at the subtle sense faculties, causing the brain to feel fatigued. If the supply of the four elements' nutrients to the subtle sense faculties is insufficient, thinking capacity declines. When a person is excessively fatigued, they need to sleep, allowing the Tathagatagarbha to operate the four elements to replenish the nutrients of the four elements at the subtle sense faculties. Is this the case?

Answer: Dharma objects of the exclusive mental realm (独影境法尘) do not require consuming the four elemental substances of the subtle sense faculties. However, form included within the dharma-ayatana (法处所摄色) does require consuming the four elemental nutrients within the subtle sense faculties. But when the mental faculty (意根) and the mental consciousness (意识) operate, they must activate the subtle sense faculties and neurons, which will release bioelectricity and consume the nutrients of the four elements. Consequently, when thinking too much, one feels mental fatigue and needs to rest for a while to allow energy within the brain to accumulate again.

When the conscious mind moves, the brain nerves jump, releasing bioelectricity and expending energy. Even when entering samadhi where the six consciousnesses are still, the mental faculty is subtly perceiving, operating faintly, and still consumes a tiny amount of the four elemental nutrients. The lighter the activity of the mental faculty, the less it consumes. The deeper the samadhi, the lighter the activity of the mental faculty, and the less it consumes. Upon entering the samadhi of non-perception (无想定), one can remain in samadhi for several kalpas or even longer, without needing food or drink, but the body will still become emaciated.