A study suggests that if we actively try to suppress a subject, we're more likely to dream about it than if we were to focus our thoughts on it.
The 300 college students who participated in the study were asked to think of a personal acquaintance before going to sleep. Then a third of the students were asked to focus their thoughts on the person for five minutes, another third were asked to actively suppress their thoughts of that person for five minutes, and the last group was asked to think freely about whatever came to mind.
The acquaintances showed up more frequently in the dreams of those students who'd been asked to put the person out of their minds. The psychologists who conducted the experiment hypothesize that suppressed thoughts resurface in dreams because the prefrontal cortex, the area responsible for mental control, is less active during sleep.
While you doze, the floodgates are opened and suppressed thoughts float to the surface.
"Banished Thougts Resurface In Dreams" (Scientific American)