A nightmare, also called a bad dream, is an unpleasant dream that can cause a strong emotional response from the mind, typically fear but also despair, anxiety and great sadness. However, psychological nomenclature differentiates between nightmares and bad dreams, specifically, people remain asleep during bad dreams whereas nightmares awaken individuals. Further, the process of psychological homeostasis employs bad dreams to protect an individual's Homeostatically Protected Mood (HPMood) from the impact of elevated anxiety levels. During sleep, nightmares indicate the failure of the homeostatic system employing bad dreams to extinguish anxiety accumulated throughout the day. The dream may contain situations of discomfort, psychological or physical terror or panic. After a nightmare, a person will often awaken in a state of distress and may be unable to return to sleep for a short period of time.