Nightmares and forgetting
There is a lot of research to suggest that we have more bad dreams than we realize. Asked to state how many nightmares they have, most people say only 2 or 3 a year, but once they start keeping dream diaries, the number goes up to one or two a month. The frequency of nightmares varies with age, with about 25% of children ages 5-12 having them at least once a week, more as they climb thru adolescence to peak in young adulthood, and then dropping gradually into old age. Women have significantly more nightmares than men, which some believe is due to their higher rates of anxiety.
Interestingly, bad dreams may serve to create ‘fear extinction memories’, allowing us to process, demistify and forget dreadful memories, thereby allowing us to move on and make room in the brain for new things to be afraid of. Otherwise, we would fear the same things in adulthood as we did in childhood.
In the dreamscape of nightmares, why we dream at all, NYTimes.com