Bhagavad Gita 14.10रजस्तमश्चाभिभूय सत्त्वं भवति भारत।रजः सत्त्वं तमश्चैव तमः सत्त्वं रजस्तथा ॥

Just as winter has its sway when summer and autumn have gone, just as sleep has its sway when a man is neither dreaming nor waking, so also Sattva has its sway when Rajas and Tamas are suppressed and makes people say that they are happy. The Sadhana for increasing Sattva is given in the 17th and 18th chapters.Each quality acts in its own turn at different times. All the three alities cannot operate at one and the same time. When one quality asserts itself or prdominates by overpowering or suppressing the other two, it produces its own effect. Sattva produces knowledge and happiness Rajas action Tamas veiling of knowledge, inertia, error, indolence, sloth and sleep. When Sattva is in the ascendant in a man, he is endowed with discrimination. Sublime thoughts roll in his mind. He has pure understanding. His mind turns away from sensual pleasures and moves inward towards the Self.What is the characteristic mark by which you can know that a particular quality is predominant or is in the ascendant The answer is given in the following three verses.