The manas constantly deliberates—why is sleep needed?

This is because the manas regards the physical body as its own possession, regards the six consciousnesses as its own possessions, and regards the five aggregates and eighteen realms all as its own possessions; it also relies on the eighth consciousness to know the condition of the physical body in every instant. When the physical body becomes fatigued and needs sleep to restore energy, the manas will feel like sleeping and no longer wants the six consciousnesses to keep operating. The manas itself wants to shut out the six sense objects and cease knowing them, and thus the five sense consciousnesses and the mind consciousness gradually cease. Therefore, it is not that the manas needs to sleep, but that the physical body needs rest to replenish energy and restore vitality. This is precisely the result of the manas's constant, attentive deliberation (manasikāra).

The activity of the six consciousnesses consumes energy. In order to reduce energy consumption, the manas decides not to let the six consciousnesses continue operating, actively shutting down the six consciousnesses' discernment of the six sense objects, no longer wanting to know the six sense objects, relying only on the manas's own knowing. In this way, energy continuously accumulates, and once vitality has recovered to a certain degree, the manas will decide to wake up, make contact with sense objects, and continue the activities of the five aggregates.