Anxiety is an expectation of a future threat. It serves to activate the body and prepare it to respond to a perceived stressor. While some presence of anxiety is normative in day to day functioning, high levels of persistent anxiety is extremely unpleasant and typically includes sensations such as tension in muscles, difficulty concentrating, hyperawareness, and fatigue. Individuals may withdraw from situations where they have experienced distressing situations in the past, experience negative thoughts about outcomes that have not yet occurred, and feel intense physiological responses such as rapid heartbeat, stomach pain, and shortness of breath. High levels of anxiety can lead to fear of new situations or people, difficulty with changes in routine, avoidance of challenging situations, and reduced performance on tests and other known activities.