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Nightmares are defined as very upsetting dreams that cause the dreamer to wake up. They are different from simply bad dreams, which are more common and less emotionally intense. The strong emotions that come with nightmares can linger after you're awake, such as when you wake up from being chased and it takes several minutes for your nervous system to calm down.

Researchers have examined the themes that show up in nightmares. In one major study, 331 participants wrote down every dream they remembered for 2 to 5 weeks, resulting in nearly 10,000 dream records (Robert & Zadra, 2014). Here are the most common themes that emerged. (The percentages total more than 100 because roughly half of nightmares involved two themes. These nightmares were "idiopathic," meaning they did not have a known trigger, such as trauma.)

1. Physical Aggression (48.6 percent). Nearly half of all nightmares involved a threatened or actual physical attack by another person—by far the most common nightmare theme. Examples include being kidnapped or assaulted.
2. Interpersonal Conflict (21.0 percent). The second-most common theme also involved negative experiences with another person, but without overt violence—for example, an intense argument or being insulted or humiliated.
3. Failure or Helplessness (16.2 percent). Nightmares with this theme involve being blocked in some way from reaching a goal. For example, you might make a horrible mistake that you can't recover from, or lose your ability to speak.
4. Being Chased (11.1 percent). Running from a threatening force is a classic nightmare theme. The pursuer (who might be bizarrely fast) gets closer and the terror builds as escape seems impossible.
5. Evil Presence (11.1 percent). Next was the presence of an evil force, such as a ghost or monster. These nightmares might also be about a sensed presence with no visible creature, or about being possessed by the evil presence.
6. Health-Related Concerns and Death (9.1 percent). You or someone you love is terribly sick or injured, or dies in the nightmare. A classic health-related nightmare is that your teeth are breaking or falling out.
7. Accidents (8.7 percent). This variety of nightmare can involve a car crash, falling off a cliff, or any other unintentional act that leads to danger or harm. Most people have been startled awake in the middle of a falling nightmare.
8. Apprehension/Worry (8.7 percent). Sometimes you might not know what the threat is, but you have a growing sense of unease. For example, I've woken up from a nightmare in which I suddenly realized that Something Very Bad was about to happen, but there was no specific feared outcome that I could point to.
9. Insects/Vermin (6.7 percent). The animals you find gross or dirty in real life, such as rats, flies, spiders, or snakes, can show up in your terrifying dreams. Sometimes the horror comes from an infestation, such as a trash can full of maggots, while other times you'll dream of being bitten or stung.
10. Disaster/Calamity (5.5 percent). A fire or a flood would fall into this category, along with bigger disasters like tidal waves or tornadoes.
11. Environmental Abnormality (4.7 percent). Finally, some nightmares are about implausible events around the dreamer that couldn't happen in real life. For example, you might dream that your bedroom dresser suddenly doubles in size. The bizarreness of these abnormalities contributes to the feeling of terror.

While none of these types of nightmares is pleasant, the authors of the study offer one possible consolation: Nightmares may function to prepare you to handle real-life threats. By confronting the threats in the safe (though terrifying) setting of dreams, your mind could be helping you to rehearse for problems that actually arise. However, this explanation is unlikely to account for all nightmares—for example, why would you need to practice responding to bedroom furniture that spontaneously grows? More research is needed to understand the possible connections between nightmare content and real-life threats.