The contents of 10,000 nightmares and bad dreams have been analysed by scientists to establish the effect they have on the sufferer.
Experts at the University of Montreal discovered that nightmares have a greater emotional impact on people than bad dreams because the former generally involve a physical threat. Bad dreams are more centred on interpersonal conflicts.
Findings of the study showed that fear is mostly absent in bad dreams, but appears in a third of nightmares. In bad dreams, people tend to feel sadness, confusion, guilt and disgust.
Authors Geneviève Robert and Antonio Zadra said: "Physical aggression is the most frequently reported theme in nightmares. Moreover, nightmares become so intense they will wake you up. Bad dreams, on the other hand, are especially haunted by interpersonal conflicts."
Genevieve Robert added: "Death, health concerns and threats are common themes in nightmares. But it would be wrong to think that they characterise all nightmares. Sometimes, it is the feeling of a threat or an ominous atmosphere that causes the person to awaken.
"I'm thinking of one narrative, in which the person saw an owl on a branch and was absolutely terrified."
The authors analysed narratives of 10,000 dreams, including 431 bad dreams and 253 nightmares. They found nightmares in men were more likely to contain natural disasters such as floods, earthquakes and war compared to women, who tended to have nightmares about personal conflicts.
Participants were asked to write down their dreams in a diary. One example read: "I'm in a closet. A strip of white cloth is forcing me to crouch. Instead of clothes hanging, there are large and grotesquely shaped stuffed animals like cats and dogs with grimacing teeth and bulging eyes. They're hanging and wiggling towards me. I feel trapped and frightened."
What dreams are remains a mystery to science. Some believe they are memories from the day being processed, while others think they are a disruption of the nervous system.
The source of a recurring nightmare may involve a traumatic event, while consumption of drugs and alcohol can explain the frequency or intensity of nightmares.
Zadra said: "Nightmares are not a disease in themselves but can be a problem for the individual who anticipates them or who is greatly distressed by their nightmares. People who have frequent nightmares may fear falling asleep – and being plunged into their worst dreams. Some nightmares are repeated every night. People who are awakened by their nightmares cannot get back to sleep, which creates artificial insomnia."
Explaining what more there is to understand about dreams, he added: "Almost everything. Through this research, we can better assert that dreams, bad dreams, and nightmares are part of the same emotional and neurocognitive process. How and which one? It remains to be determined."