The three Gunas - Satwa, Rajas and Tamas - are the three essential energies of the mind. Genetically determined, the psychological characteristics of an individual are dependent on the relative dominance of the three Gunas.
While individuals have certain amounts of the three gunas, the predominant guna determines an individual's mental composition (call Manasa Prakriti). In equilibrium, the three Gunas maintain a healthy mind (and indirectly a healthy body). Any disturbance in this equilibrium results in various types of mental disorders.
Satwa is characterised by lightness, consciousness, pleasure and clarity. It is pure, free from disease and cannot be disturbed in any way. It activates the senses and is responsible for the perception of knowledge.
Rajas is the most active of the Gunas and is characterised by motion and stimulation. All desires, wishes, ambitions and fickle-mindedness are governed by Rajas.
Tamas is characterised by heaviness and resistance. It causes disturbances in perception and other activities of the mind. Delusion, false knowledge, laziness, apathy, sleep and drowsiness are a result of Tamas.
Rajas and Tamas, the gunas that can be disturbed, can be unbalanced by stress and negative desires as lust, malice, delusion, greed, anxiety, fear and krodha anger.