Dreams are still a huge mystery to us - there are many theories about why we have them, and why we have scary ones.
According to Best Health magazine in the US, 70% of Americans report having nightmares, and 30 % percent of us have them while they sleep at least once a month.
Causes can include stressful goings ons in your life, certain medications you may be taking, genetics, neurological diseases like Alzheimer's and even what you've been eating can cause them.
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One of the main causes according to one expert is that your mind is overloaded with emotion, and it's a way to process those stressful events as your brain begins to integrate your feelings.
Nightmares can just be dreams with scary plots - but sometimes they can become an issue.
There are certain disorders where nightmares will disrupt a person's ability to sleep as well as some people who begin to act out violent actions while sleepwalking.
In those cases it's recommended to see a health professional for help.