A nightmare is a frightening or disturbing dream that can wake you up feeling scared, anxious, or upset. You might dream that you're being chased, that you're falling, or that something terrible is happening to someone you love. When you wake up from a nightmare, your heart might be pounding and you might feel relieved to realize it wasn't real.

Scientists believe nightmares happen during a stage of sleep called REM (rapid eye movement) sleep, when our brains are very active and we dream most vividly. Young children often have nightmares as their imaginations develop and they process new experiences and fears. Even adults have nightmares occasionally, especially during times of stress or after watching something scary.

The word also describes any extremely unpleasant or difficult experience in real life. If you forgot your homework, lost your lunch, and ripped your pants all in one day, you might call it a total nightmare. When someone says organizing the school talent show turned into a nightmare, they mean it became much harder and more stressful than expected. A nightmarish situation is one that feels almost as bad as a scary dream: chaotic and overwhelming in ways that are hard to control.