Why do we have Nightmares?

I was up late last night watching movie called " Happy Gilmore". It was starred by Adam Sandler.. Was really nice and funny. And then go to bed at half past twelve. I didnt know that my sleep will turn up into a bad nightmare. But why do we really have nightmares?

According to the Real Meaning of Dreams. Nightmares are bad dreams.They can make you feel extremely scared and frightened. But fear not….these dreams are not real and are usually quite normal.
Characterized by vivid imagery and extreme feelings such as anxiety, fear, sadness or guilt they are powerful enough to wake you up.

Children experience frightening dreams more often than adults, especially between the ages of 3-4 and 7-8, as this is when they are dealing with separation anxiety.Bad dreams at this age show a normal struggle to cope with fears and issues and they usually involve being chased by wild animals. This is a normal stage of development for children and should be viewed as such.
In adults, about 50% report experiencing nightmares occasionally and of these, they occur more often in women than men.

There are certain factors that contribute to frightening dreams occurring in adults:1. Certain Medications and illnesses, especially fever, can cause bad dreams.2. Stress and anxiety in adults is the main reason that causes bad dreams. If there are some stressful events or huge changes in your life this can definitely lead to frightening dreams.3. Traumatic events such as deaths, serious injuries or being attacked can bring on a severe form called Post Traumatic Nightmares and these are more severe than regular bad dreams and are usually recurring dreams. Treatment can be sought for these forms of dreams as they are usually more disturbing and persistent in nature.