Why is thinking clearer at night than during the day?

Question: At night while sitting in meditation contemplating the meaning of the Dharma, even after rising from the seat I only half-understood it. I fell asleep with the unresolved question still hanging in my mind, and in the middle of the night I suddenly understood it. At the moment of understanding, my mind was especially clear and bright, and I was so excited that I could not fall back asleep. In the middle of the night, who was it that understood?

Answer: If the problem you are deliberating on is rather profound and keeps hanging in your mind, it means the manas has also participated in the deliberation (manasikāra). If no conclusion has been reached and it has not been thought through clearly, the mind consciousness will become fatigued, shift its attention, or cease, while the manas will continue deliberating. At some unknown point, the manas will have deliberated the problem through clearly.

Many problems are considered more clearly at night than during the day. Why? Because there is less interference, the manas is relatively focused, and it participates more in the deliberation. As for grasping (upādāna)—is it the mind consciousness that grasps, or the manas? It is the manas that grasps, but because there is no cooperation from the six consciousnesses, the manas's grasping is reduced. Therefore, after the six consciousnesses quiet down, the manas can concentrate its energy on deliberating a single problem by itself. At night, without the six consciousnesses, the manas grasps less; the six consciousnesses no longer disturb the manas with the information they discriminate, so the manas begins to ruminate, single-mindedly deliberating the meaning of the Dharma that the mind consciousness could not think through clearly. Then it is possible that in the middle of the night the meaning of the Dharma is suddenly understood.