From : The Yoga Sutra's of Patanjali

1.28. The repetition of Om should be made with an understanding of its meaning
1.29. From that is gained introspection and also the disappearance of obstacles.
1.30. Disease, inertia, doubt, lack of enthusiasm, laziness,
sensuality, mind-wandering, missing the point, instability- these
distractions of the mind are the obstacles.
1.31. Pain, despair, nervousness, and disordered inspiration and
expiration are co-existent with these obstacles.
1.32. For the prevention of the obstacles, one truth should be
practiced constantly.
1.33. By cultivating friendliness towards happiness and compassion
towards misery, gladness towards virtue and indifference towards vice,
the mind becomes pure.
1.34. Optionally, mental equanimity may be gained by the even
expulsion and retention of energy.
1.35. Or activity of the higher senses causes mental steadiness.
1.36. Or the state of sorrowless Light.

From : The Yoga Sutra's of Patanjali