Three Grades
It should be known that there are three grades of fasting: ordinary, special and extra-special. Ordinary fasting means abstaining from food, drink and sexual satisfaction.
Special Fasting means keeping one's ears, eyes, tongue, hands and feet -- and all other organs -- free from sin.
Extra-special Fasting means fasting of the heart from unworthy concerns and worldly thoughts, in total disregard of everything but God, Great and Glorious is He. This kind of Fast is broken by thinking of worldly matters, except for those conducive to religious ends, since these constitute provision for the Hereafter and are not of this lower world.
Those versed in the spiritual life of the heart have even said that a sin is recorded against one who concerns himself all day with arrangements for breaking his Fast. Such anxiety stems from lack of trust in the bounty of God, Great and Glorious is He, and from lack of certain faith in His promised sustenance.
To this third degree belong the Prophets, the true awliya and the intimates of God. It does not lend itself to detailed examination in words, as its true nature is better revealed in action. It consists in utmost dedication to God, Great and Glorious is He, to the neglect of everything other than God, Exalted is He. It is bound up with the significance of His words: 'Say: "Allah (sent it down)": then leave them to play in their vain discussions.' [al-An'am, 6:91]
http://www.tasawwuf.org/basics/ghazali_fasting.htm
