Everyone fasts for at least some part of the day, generally the eight or so hours that one spends sleeping every night. Physiologically, fasting can refer to a person's metabolic status after not eating overnight, or even the metabolic state after the complete digestion of a meal. Once you've gone eight to 12 hours without eating, the body enters a state of "fasting."
The practice of fasting can lead to a number of metabolic changes within the body. These changes typically begin approximately three to five hours after eating, when the body enters a "post-absorptive" state – rather than the state on ongoing digestion, where eating frequent meals means the body is always involved in some sort of digestive activity.