Behavior and physiological responses are motivated to maintain a stable internal physiological environment (homeostasis). Negative feedback systems operate to maintain homeostasis for body temperature, water, salt, and an energy balance (energy input minus energy output). Alliesthesia is a change in a person's internal environment that makes behaviors restoring homeostasis pleasurable and behaviors that disrupt it painful. Hunger motivates eating but stops when a person is satiated and food no longer tastes as good. The palatability of food also motivates eating. Humans prefer sweets and fats; familiar foods, and food that restores energy. Dieting refers to cognitively restraining the start and stop of eating. Stress releases diet restraint, which can lead to overeating. Anorexia nervosa develops from restricted energy intake, a fear of getting fat, and leads to an emaciated appearance.