47% of people have a nightmare at least once a month.

Before you curl up in bed tonight and doze off peacefully, here's some eye-opening news to ponder: Nightmares, at least among young adults, apparently occur more than twice as often as scientists previously thought. Furthermore, emotionally stable people have about the same number of nightmares as highly anxious individuals.

The researchers defined a nightmare as "a dream that frightens the dreamer," thus excluding some upsetting or sad dreams some view as nightmares, Wood says. Nearly 47 percent of the students reported at least one nightmare in their dream logs. Based on the logs, the estimated average annual number of nightmares for each student totaled 24 - about 2-1/2 times the annual frequency estimated from the retrospective reports.