Nightmares Offer Clues to Why We Dream

Nightmares offer clues to why we dream, how our dreaming and waking lives may intersect, as well as how we are able to create a virtual reality within our skull. Studies have shown that about 75% of our dreams are associated with negative emotions and that about 25% of children between the ages of 5 and 12 are awakened from bad dreams at least once a week. The frequencey of nightmares varies by sex and age, with young adults, particularly women, having the most nightmares.

Scientists have begun determining which areas of the brain are active and which are inactive during sleep. Most dreaming occurs during REM, or rapid eye movement, sleep and many changes occur in the brain when one enters this sleep stage. The limbic system, which is involved in emotions, is much more active during sleep, while the prefrontal cortex, functioning in rational thought and critical reasoning, is largely offline. The primary visual cortex is relatively inactive, which is the area of the brain that receives visual signals, but the secondary visual cortex, involved in intrepretation of the signals, remains active. A small region of the brainstem also paralyzes most of the body to prevent physical activity during dreaming. All of these changes in the brain contribute to the nature of our dreams.

It was proposed by Dr. Neilson and Dr. Levin that dreaming creates "fear extinction memories," helping people clear away fearful memories during bad dreams, but that this "fear extinction" doesn't occur in nightmares.

http://www.nytimes.com/2007/10/23/science/23angi.html?pagewanted=2&_r=1&ref=health