A nightmare is an unpleasant dream that can cause a strong emotional response from the mind, typically fear but also despair, anxiety and great sadness. The dream may contain situations of discomfort, psychological or physical terror. Sufferers often awaken in a state of distress and may be unable to return to sleep for a small period.

Nightmares can have physical causes such as sleeping in an uncomfortable or awkward position, having a fever, or psychological causes such as stress, anxiety, and as a side effect of various drugs. Eating before going to sleep, which triggers an increase in the body's metabolism and brain activity, is a potential stimulus for nightmares. Recurrent nightmares may require medical help, as they can interfere with sleeping patterns and cause insomnia.