(CNN) - What's your most frequent nightmare? Is it dreaming that you're dying, or that one of your loved ones is suffering but you can't do anything about it? Or maybe you're waking up with confusion and a racing heart, simply glad that the dream ended.
Nightmares are classified as dream sequences that seem realistic and often awaken the person. They are a complex experience. Though fear is the dominant emotion felt during nightmares, a 2014 study reported that sadness, anger, confusion, disgust, frustration or guilt were also common.
The study, published in the journal Sleep, looked at the content of 351 adults' nightmares and found that the most frequently reported theme was physical aggression, followed by being chased and the presence of an evil force.
But if bad dreams start frequenting your nights, it could be a sign of health problems. An estimated 2% to 8% of adults can't get rest because terrifying dreams wreak havoc on their sleeping patterns.
In particular, nightmares can be an indicator of mental health problems, such as anxiety, post-traumatic stress disorder and depression.
A phenomenon called REM sleep behavior disorder, in which a person acts out aggressive dreams by screaming, moving around or jumping out of bed, could also predict neurological diseases like Parkinson's, according to research.
But some theories posit that nightmares can be a way for our brains to cope with and process unpleasant memories.
"We don't know an awful lot about dreams or nightmares beyond what Freud and some of the other psychoanalytic and psychodynamic tutors told us," explained professor Jason Ellis, director of Northumbria University's Centre for Sleep Research. Freud is famous for his "interpretation of dreams," in which he suggests that dreams are a representation of our wishes, some of which play out in a bizarre way.
One theory, Ellis said, is that dreams are problem-solving exercises. "Under that framework, we would generally see nightmares as part of a process of trying to deal with emotional material," Ellis said. During a nightmare, the emotions and problems we encounter during the day are turned into characters and scenarios to help us understand and manage them better, for psychological health.
Generally, "nightmares will occur if someone is having a longstanding problem with sleep," Ellis said, for such reasons as pain or insomnia.
In Jonny Benjamin's case, it was anxiety that caused him to struggle with sleep, especially during his 20s.
When his anxiety and stress levels were very high, Benjamin, now 31, of London, would go through periods of insomnia. Sleep "became a real issue. Going to sleep, waking up constantly in the night, with feelings of panic -- it was really horrible."
Struggling to fall asleep again would create a vicious cycle of worry. Fretting about his job performance the next day or whether he would be able to sleep normally the following night -- on top of not feeling like his normal self or wanting to socialize because of his fatigue -- added further stress.
Benjamin said that a lack of sleep, along with things like alcohol and stress, became the main trigger for his mental health problems; he has been diagnosed with schizoaffective disorder. "My mind really struggles when I don't get enough sleep. If I miss two nights, three nights of good sleep, I can become unwell quite quickly," he said.
Sleeping pills are a useful method of dealing with sleep problems, Benjamin says. But he found that simpler options such as not using his phone and avoiding social media before bed are also ways to get a good night's sleep. He follows a bedtime routine, and if he is stressed, writing in a journal helps him deal with his day before going to sleep.
What nightmares tell us about our health
The average human has at least one nightmare a week, said Bill Fish, a sleep science coach and co-founder of the online sleep resource Tuck, but whether they remember it is a different story. "If someone has frequent nightmares, that's not normal and something that should be treated."
Nightmares are a core symptom of PTSD, probably because the difficulty with traumatic events lies in people's inability to process them, says Neil Greenberg, professor of defense mental health at King's College London. "So what happens is that their mind unconsciously tries to play around with them, tries to make sense of them," Greenberg explained.
This results in "the unconscious bit of the brain, which may come to the front of the mind when someone is asleep," including material from the traumatic event that may come out as a nightmare.
Not all nightmares are indicative of a mental health problem, Greenberg warns. People have to look not just at their nightmares but at other symptoms, "which make up together the recipe of a particular mental health problem."
Studies have found other mental health issues also associated with frequent nightmares, such as bipolar disorder and schizophrenia.
Monitoring REM sleep
There are four stages of sleep. The first three are characterized by slow eye movement and lead up to the final stage, rapid eye movement or REM.
Dreams occur in the REM stage, when skeletal muscles are nearly paralyzed.
A long-term follow-up of people with REM sleep behavior disorder showed that most developed conditions such as Parkinson's disease, Lewy body dementia or multiple system atrophy. Another recent study from a research team at the University of Toronto reported that 80% of REM sleep behavior disorder patients developed these conditions.
The study says postmortem evidence shows that some people with the disorder have degeneration in the areas of the brain that control REM sleep. The disorder is potentially caused by the same mechanisms that underlie neurological diseases, it concludes.
REM sleep is also strongly tied to emotional health. "By looking at how quickly REM occurs, you can tell a lot about a person, including whether they might get depression in a few weeks' time," Ellis said.
A usual sleep cycle is 100 minutes, and REM comes between the 90- and 100-minute marks. But "if we see REM coming in much earlier than that -- 40, 50, 60 minutes -- generally that's an indicator that somebody might end up with depression in a few weeks' time," he said.
Researchers theorize that if REM comes in too quickly, people's brains don't have the time to organize their memories and determine which ones are needed, "so it all becomes a little bit messy," Ellis said.
But, according to Fish, lifestyle also has a role to play in what you dream.
The consequence of change
If you start to get frequent nightmares, "the first thing a doctor is going to ask is, 'has there been a change in your lifestyle?' " Fish said. Things like new medications, changes to diet, stress or trauma can all cause an uptick in unpleasant dreams.
Any changes to your body, like cutting out certain foods, are also likely to play into sleep patterns, he added.
Nightmares are also common in people with chronic pain, possibly because they tend to wake more and therefore report more nightmares, Ellis said.
Breathing problems, like sleep apnea or asthma, are also linked to nightmares.
"Essentially, it's changes your body isn't accustomed to -- and your brain either -- and as you sleep, your brain is kind of letting that out in the form of dreams and a lot of times nightmares," Fish said.
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