Dreams are basically stories and images that our mind creates while we sleep.  Dreams can happen at any time during sleep and most of the time you don't even remember them. The dreams you do remember are when you have your most vivid dreams which are during a process phase called REM (rapid eye movement) sleep. The random or rapid movement of the eyes, and by low muscle tone throughout the body, will cause the sleeper to dream vividly. There can be a number of ways that cause nightmares, usually depends on the person and the problems they might have, for example, a person could have anxiety or depression and it could cause nightmares.