Japa nourishes and purifies the subtle body just as food and medicine nourish the gross body. Nama japa includes chanting OM, and requires no prerequisite.
Dhyana pacifies the causes of suffering, purifies chitta, & normalizes a disorganized mind. Sages used meditation & dhyana to attain jnana (spiritual knowledge).
Manas is an object, the visible “tip of the iceberg of chitta.” Mental vrttis refer to external objects. Mano vrtti nirodha is the gateway to chitta vrtti nirodha.
Japa is a form of dhyana that everyone can do to nourish the subtle body. Manas is the “tip of the iceberg of chitta.” Dreamless sleep restrains manas (mind) but not the chitta vrttis.
Mental clarity requires sensitivity in asana & pranayama. Adhyatmika sadhana (practice pertaining to Self) ascertains the known, knowing, & knower as objects of knowledge.