What could be a cause of constant nightmares?

Nightmares: Several possibilities -- among them, being on medications that are associated with nightmares, such as narcotics, antidepressants, and some antihypertensives. Drug and alcohol withdrawal can cause it. Also nightmares can certainly come with ptsd, anxiety, and depression. Sleep apnea and restless leg syndrome can lead to nightmares as well. See your doctor for help with this problem.

No More Nightmares: Dreams that wake you up may be a symptom of unresolved trauma, or ongoing chronic stress. Think of it as a scratch on a vinyl record--it keeps skipping and repeating disturbing material that wakes you up. With the skill of Planned Dream Intervention you can learn how to sleep through your dreams. More from this provider.

Lots of things: If its a child, could be discord in the family, or allowing your child to view stupid movies like "chucky" or medications. Does the person have recall of the dreams? Some worry themselves into problems that they replay when they sleep. No recall of a dream (s) could be a night terror.