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Recurring dreams are a surprisingly common phenomenon: research shows that up to 75 percent of adults experience at least one during their lifetime. These dreams exist on a spectrum: sometimes they’re nearly identical each time they occur, but they may also have recurring themes, locations or characters set against different backdrops. This fluctuation sets recurring dreams apart from bad dreams triggered by post-traumatic stress disorder, a psychological condition in which people relive specific memories from their waking life with far less variation while asleep.