1. The sighing and groaning of a man of Israel is very precious, for it is the making whole of all that lacks. However, it is impossible to make a lack whole through sighing except through drawing closer to the [particular] tzadik who is the true Rabbi of in the generation. For from him the spirit of life is received to make whole any lack via sighing. (From Likutei Moharan I, 8; see sec. Tzadik, 11)
2. When a person prays properly, with devekut, or one who does hitbodedut properly, but in the middle [of it] he falls from his level - that is a result of damaged faith. Thus, he needs to break his heart within him and become ashamed of himself for having fallen and been thrown down from Heaven to earth; and he should have mercy on himself until he sighs. Through the sigh he will return to his level. (From Likutei Moharan I, 108)
3. Sighing of holiness is very precious because through one sighing over being far from holiness one is released from the cord of impurity and is hooked up to the cord of purity. Also, when one sighs over [as a result of] some lust that he yearns for - the opposite [happens], God forbid. (From Likutei Moharan I, 109)
4. The sighing that a person sighs over his signs or over his lack of attainment [of holiness, Torah, etc] - this is better than several mortifications and fasts. (there^)
See more about the idea of sighing in sec. Brazenness, 2 and 3 that through the sighing of holiness, one breaks the brazenness of the body and then the soul can draw closer to the body and make known to it the attainments [of Heavenly wisdom] that it reaches, and more; see there.
(Continue to next section, Eating)