Does using the manas to recollect the Buddha and to ponder questions feel tiring?

When recollecting the Buddha (buddhānusmṛti), just recite and that is all; there is no need to rack your brain pondering the Buddha's various merits, or questions such as how to recite in order to achieve an effect. This consumes relatively little mental effort, energy, and vitality. If the time spent on reciting the Buddha's name is not too long, and one eats normally, one will not feel very tired. However, when one has just begun to learn to recite the Buddha's name with the manas, due to unfamiliarity, some mental effort is required, and one will feel somewhat tired. When the recollection of the Buddha gradually enters the state of meditative absorption (samādhi), the meditative concentration becomes relatively deep, the circulation of qi and blood functions well, and the nourishment and energy are sufficient, so one no longer feels tired; on the contrary, the physical body becomes tranquil and at ease, and the mind feels joyful. In the state of samādhi, the energy consumed is very little.

Using the manas to ponder problems is different from reciting the Buddha's name. When pondering problems, one needs to rack one's brains, and the mental effort, energy, and vitality consumed are much greater. If there is not an ample supply of energy, one will feel tired. When investigating Chan, it is mainly the manas that carries out the investigation and deliberation (manana), which requires mobilizing the nervous system; the energy consumed is great, so one will feel very tired. Therefore, those who investigate Chan will eat a bit more, those who use their brains a lot will eat more, and they need to rest at intervals; otherwise, they will feel very tired. But when meditative concentration is very deep, because the qi and blood are abundant, the energy supply can keep up, and one does not easily feel tired, whereas with shallow meditative concentration one easily feels tired. If the investigation of Chan enters a very deep state of samādhi, at that time the qi and blood circulate well, and body and mind are tranquil, at ease, and joyful, so one will no longer feel tired.

Deliberation with the manas consumes a great deal of energy, while thinking about problems with the mind consciousness consumes less. Because deliberation with the manas requires engaging the brain's nervous system, it consumes energy, and so one feels tired. The nervous system of the entire body is mobilized by the manas and cannot be mobilized by the mind consciousness, so thinking with the mind consciousness is relatively easy. Even if one is very scattered and distracted, thinking about everything, one will not feel very tired. For this reason, many people like to use the mind consciousness and are unwilling to think deeply and investigate.