A nightmare is a dream that occurs during rapid eye movement (REM) sleep that results in feelings of strong terror, fear, distress and extreme anxiety. Nightmares can happen to anyone at any given time of their lives. Generally your typical nightmare will evoke negative emotional responses from the mind, typically the anxious, fear or dread feeling.
Nightmares and symbols in them can be unconscious messages to the conscious alerting the dreamer to pay close attention to the meaning. Constant nightmares might be classified under a type of sleep disorder.
If nightmares persists on a regular basis that stop you from getting a healthy sleep, then it might be classified under a sleep disorder. Parasomnia falls under a nightmare or sleep disorder that involves a series of abnormal jerks, emotions, thoughts and behaviours while falling asleep, or during sleep stages.
Most parasomnias are dissociated sleep states which are partial arousal’s during the transitions between wakefulness and NREM non rapid eye movement sleep. It can occur during wakefulness and REM sleep as well.