Why is thinking clearer at night than during the day?

Answer: If the problem you are contemplating is rather deep and keeps hanging in your mind, it means the manas is also participating in the deliberation (manana). If no conclusion has been reached and the thinking has not yet become clear, the mind consciousness will grow fatigued, shift its attention elsewhere, or cease altogether, while the manas will continue deliberating. At some unknown moment, the manas will have thought the problem through clearly.

Many problems considered at night become clearer than those considered 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, once the six consciousnesses quiet down, the manas can concentrate its energy on deliberating a single problem on its own. 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. In this way, it is possible that in the middle of the night the meaning of the Dharma will suddenly become clear.