What are nightmares?
Alexandre Lemyre, M.A., Ph.D.
The content of this article was last updated on February 1, 2022.
Introduction
Until my early twenties, nightmares have been an omnipresent theme in my life. From my childhood, to my adolescence, to my first years as a university student, I have had episodes where I had nightmares quite frequently. In fact, I choose to do my Ph.D. thesis on dreaming largely because of a Eureka! moment after waking up from a nightmare in the middle of the night, where I thought I had found the very reason why we experience nightmares (for more on that topic, see another article on the theoretical models explaining nightmares)! During my master’s and doctoral studies, I have had the opportunity to conduct several studies on dreaming and nightmares. Here, I want to help you elucidate what nightmares are exactly. Naturally, the nature of nightmares can vary from a night to another, and from a person to another. However, there are some patterns that are worth exploring.
The definition of nightmares
The American Psychiatric Association (2013, p. 404) defines nightmares as “extended, extremely dysphoric, and well-remembered dreams that usually involve efforts to avoid threats to survival, security, or physical integrity.” Nightmares should not be confounded with sleep terrors, which are defined as “abrupt terror arousals from sleep, usually beginning with a panicky scream” (American Psychiatric Association, 2013, p. 399). According to the American Psychiatric Association (2013, p. 406), there are at least three main differences between nightmares and sleep terrors:
1) Nightmares generally occur during the second half of the night, whereas night terrors usually occur during the first half of the night. This is because most nightmares (and dreams) occur in Rapid Eye Movement (REM) sleep, which is most prominent in the second half of the night, whereas night terrors occur from non-REM sleep (especially deep sleep, also called “slow wave sleep” or “Stage 3 sleep”), which is most prominent in the first half of the night (Carskadon & Dement, 2005; Nielsen, 2000).
2) Nightmares are well remembered upon awakening, whereas night terrors are not accompanied with a clear memory of a dream (if there is such memory, it is very fragmented). In fact, after a night terror, the individual often does not awaken fully, but returns to sleep and keep no memory of the sleep terror episode upon awakening in the morning.
3) After a nightmare, the individual is rapidly oriented and fully awake. On the contrary, during a sleep terror, the individual is in a state of incomplete arousal. The person is not fully oriented in their environment and mostly unresponsive when others attempt to interact with them.
The prevalence of nightmares
The term “prevalence” refers to the proportion of a particular population (for instance, children, adolescents, or adults) that is affected by a specific condition, in this case, nightmares. A review of the scientific literature (Gauchat et al., 2014) focused on the prevalence of nightmares in children and adolescents. Because the studies published on this topic report estimates of nightmare prevalence that vary greatly, it is impossible to report exact numbers. In most studies, roughly 40% to 60% of children aged 5 to 10-year-old experience nightmares at least on occasions, and similar estimates were obtained for children and adolescents aged 10 to 18 years old. The prevalence of weekly nightmares in these two age groups varies between 3% and 12% in most studies (Gauchat et al., 2014).
In the adult population, about 2% to 5% of individuals experience at least one nightmare per week (Sandman et al., 2013; Schredl, 2013). However, adults with a mental disorder are much more likely to present this sleep difficulty (Lemyre et al., 2019). In the best study conducted on nightmares in adults with a mental disorder (van Schagen et al., 2017), the percentage of individuals who experienced at least one nightmare per week was 31% for personality disorders, 37% for mood disorder (including depression), 16% for anxiety disorder (e.g., phobias, panic disorder, generalized anxiety disorder), and 67% for posttraumatic stress disorder.
Based on a meta-analysis of 98 samples of healthy individuals (i.e., individuals that were not selected based on the presence of a mental disorder; Schredl & Reinhard, 2011), women tend to experience more nightmares than men in three age groups: adolescents (10-17 years of age), young adults (18-29 years of age), and middle-aged adults (30-59 years old); this gender difference is not present for children (10 years of age or younger) or older adults (60 years of age or older). Based on an analysis of 7 samples healthy individuals mainly comprised of adolescents and young adults, women also experience more distress (i.e., more consequences) from their nightmares compared to men (Schredl & Reinhard, 2011).
Distinguishing idiopathic nightmares and posttraumatic nightmares
Nightmares can be divided in two types: idiopathic nightmares and posttraumatic nightmares. Idiopathic nightmares are not linked to a past traumatic experience in any obvious way. In contrast, posttraumatic nightmares share some similarity with a past trauma. More precisely, the theme, setting or content of the nightmare is linked to the trauma, and/or the main emotion experienced in the nightmare is the same as the one that was experienced during the trauma (Spoormaker & Montgomery, 2008). According to a representative study of the adult population in the United States, 68% of individuals who develop a posttraumatic stress disorder following a traumatic experience suffers from posttraumatic nightmares (Milanak et al., 2019).
Replicative posttraumatic nightmares are usually viewed as the most severe form of nightmares. These nightmares largely “replay” a past traumatic experience, as if the person was experiencing it for the first time (Levin & Nielsen, 2007). However, a careful analysis of replicative posttraumatic nightmares shows that the replay is rarely, if ever, exact: there seems to always be at least one significant difference between the replicative posttraumatic nightmare and the actual traumatic experience (Hartmann, 2010).
Distinguishing nightmares and bad dreams
We sometimes differentiate nightmares from bad dreams based on an awakening criterion. More precisely, nightmares are defined as highly dysphoric (i.e., highly unpleasant) dreams that trigger awakening, whereas bad dreams are defined as highly dysphoric dreams that do not trigger awakening. This distinction has some appeal, as people can generally tell with great confidence whether a dysphoric dream woke them up (Blagrove & Haywood, 2006). In practice, however, this distinction is rarely used. In the dream literature, when the term “nightmare” is used, it generally encompasses all highly dysphoric dreams, whether or not they awaken the dreamer.
The emotional intensity of nightmares and bad dreams
Zadra et al. (2006) conducted a study on emotions in nightmares and bad dreams. The study’s sample was comprised of 90 university students. The participants completed a dream journal for a period of four weeks in which they reported their nightmares and bad dreams (distinguished based on the awakening criterion). They rated the emotional intensity of each nightmare and bad dream on a nine-point scale from very weak (1) to very intense (9). The mean emotional intensity for nightmares was 7.95, whereas the mean emotional intensity for bad dreams was 7.24 (for participants who had experienced at least one nightmare and one bad dream) or 7.06 (for participants who had experienced at least one bad dream, but no nightmare). While nightmares were slightly more emotionally intense than bad dreams on average, about 46%-47% of bad dreams had an equal or greater emotional intensity than the mean intensity of nightmares. In other words, almost half of all bad dreams were at least as intense as the average nightmare (Zadra et al., 2006).
In another study, Robert and Zadra (2014) analyzed 253 nightmares and 431 bad dreams reported by 331 undergraduate students. The participants kept a dream journal over the course of two to five weeks in which they reported their nightmares and bad dreams (distinguished based on the awakening criterion). They rated the emotional intensity of each nightmare and bad dream on a five-point scale. The mean emotional intensity of nightmares was 4.36, whereas the mean emotional intensity of bad dreams was 3.48 (Robert & Zadra, 2014). In summary, both nightmares and bad dreams are perceived as having a high emotional intensity (not surprisingly), and nightmares are more emotionally intense than bad dreams on average.
The nature of emotions in nightmares and bad dreams
The two studies that were presented in the previous section (Robert & Zadra, 2014; Zadra et al., 2006) also analyzed the nature of the main emotions experienced in nightmares and bad dreams. Below, I present the proportion of nightmares and bad dreams that contain each emotion as their main emotion (the symbol “~” means “roughly”):
Fear
70% of nightmares, ~52% of bad dreams (Zadra et al., 2006)
65% of nightmares, 45% of bad dreams (Robert & Zadra, 2014)
Frustration
7% of nightmares, ~6% of bad dreams (Zadra et al., 2006)
3% of nightmares, 4% of bad dreams (Robert & Zadra, 2014)
Anger
6% of nightmares, ~8% of bad dreams (Zadra et al., 2006)
5% of nightmares, 7% of bad dreams (Robert & Zadra, 2014)
Guilt
5% of nightmares, ~1% of bad dreams (Zadra et al., 2006)
2% of nightmares, 4% of bad dreams (Robert & Zadra, 2014)
Sadness
3% of nightmares, ~10% of bad dreams (Zadra et al., 2006)
7% of nightmares, 12% of bad dreams (Robert & Zadra, 2014)
Confusion
1% of nightmares, ~5% of bad dreams (Zadra et al., 2006)
3% of nightmares, 7% of bad dreams (Robert & Zadra, 2014)
Disgust
0% of nightmares, ~3% of bad dreams (Zadra et al., 2006)
3% of nightmares, 5% of bad dreams (Robert & Zadra, 2014)
Positive emotions (due to a positive ending of the nightmare or bad dream)
This category was not used in the study by Zadra et al. (2006)
4% of nightmares, 4% of bad dreams (Robert & Zadra, 2014)
Other emotions
8% of nightmares, ~14% of bad dreams (Zadra et al., 2006)
9% of nightmares, 12% of bad dreams (Robert & Zadra, 2014)
Themes in nightmares and bad dreams
In the study by Robert and Zadra (2014) that I described above, the authors constructed a list of 12 nightmare themes based on the scientific literature. They used these themes to categorize 253 nightmares and 431 bad dreams. The percentage of nightmares and bad dreams featuring each theme was as follows (the same nightmare or bad dream could feature more than one theme):
- Physical aggression (49% of nightmares, 21% of bad dreams)
- Interpersonal conflicts (21% of nightmares, 35% of bad dreams)
- Failure or helplessness (16% of nightmares, 18% of bad dreams)
- Health-related concerns and death (9% of nightmares, 14% of bad dreams)
- Apprehension/worry (9% of nightmares, 14% of bad dreams)
- Being chased (11% of nightmares, 6% of bad dreams)
- Evil force (11% of nightmares, 5% of bad dreams)
- Accidents (9% of nightmares, 5% of bad dreams)
- Disaster and calamity (6% of nightmares, 6% of bad dreams)
- Insects (7% of nightmares, 4% of bad dreams)
- Environmental abnormality (5% of nightmares, 4% of bad dreams)
- Other themes (7% of nightmares, 10% of bad dreams)
The source of threat and the dreamer’s response in fear-inducing nightmares and bad dreams
In another study, McNamara et al. (2015) selected 436 fear-inducing nightmares and bad dreams (i.e., nightmares and bad dreams in which the dreamer experience fear) among a huge database of 49.000 dream reports. The source of threat in these nightmares and bad dreams were as follows:
- Environement or circomstances (38%)
- Unfamiliar male (17%)
- Supernatural agent (e.g., ghost or demon, 12%)
- Pest or animal (10%)
- Familiar male (5%)
- Unfamiliar female (2%)
- Familiar female (2%)
- Unspecified (15%).
In these nightmares, the dreamer usually fled, hide, or simply felt trapped and helpless; rarely, the dreamer fought back the threat (McNamara et al., 2015).
Offender nightmares
Mathes et al. (2018) report the results of two studies focusing on a type of nightmares which we rarely hear of: offender nightmares. An offender nightmare is a nightmare in which the dreamer is acting mainly as an active offender. In the first study, 39 participants who experienced at least one nightmare per month kept a dream diary for 28 days. During this period, a total of 145 nightmares were reported. According to the participants’ evaluation of their own nightmare(s), 26 were offender nightmares (representing 18% of all nightmares), 49 were victim nightmares, and 70 could not be clearly categorized as offender‑nightmares or victim nightmares. The aggressive actions were accidental for only three of the 26 offender nightmares; in the other 23 nightmares, the aggression consisted of “killing a character” (12 nightmares), hurting a character (8 nightmares), and attacking a character verbally (3 nightmares). In the second study, 60 participants who experienced nightmares at least monthly completed a dream diary for 28 days. Seventeen of them (28% of the sample) had at least one offender nightmare during this four-week period (Mathes et al., 2018).
Autonomic (physiological) activity of the sleeper during nightmares
In waking, when we are scared, the whole body reacts, and these responses are regulated (largely uncontrollably) by the autonomic nervous system. Does the sleeping body react in a similar way when fear is triggered by a nightmare? This is a difficult question to answer, because measures of autonomic arousal are traditionally performed in a laboratory, and nightmares are notoriously difficult to study in the laboratory (for reasons that are yet to be elucidated, individuals rarely experience nightmares when sleeping in a laboratory).
To the best of my knowledge, only one study has satisfactorily assessed autonomic arousal during nightmare episodes (Paul et al., 2019). The authors of this study used a method called “ambulatory polysomnographic recording” along with measures of skin conductance and saliva cortisol. This allowed them to measure autonomic activity in 19 nightmare sufferers and 19 good sleepers while they slept at home for three nights. The good sleepers reported 74 dream reports, whereas nightmare sufferers reported 70 dream reports and 13 nightmare reports. For the analyses, the authors considered autonomic activity during the last five minutes of rapid-eye movement (REM) sleep prior to awakening from a dream or a nightmare. Several autonomic measures (e.g., electrodermal measures, heart rate, breathing cycle length) showed that nightmares are accompanied with increased autonomic activity compared to normal dreams. This was apparent to a greater extent when comparing the nightmares and the normal dreams of the nightmare sufferers, and to a lesser extent when comparing the nightmares of the nightmare sufferers and the normal dreams of the good sleepers (Paul et al., 2019). This finding suggests that strong negative emotions (especially fear) in nightmares are accompanied with autonomic arousal in the sleeping body.
Summary and Conclusion
In summary, nightmares are prolonged, highly dysphoric and well-remembered dreams. They are not to be confounded with sleep terrors, which constitute a completely different phenomenon. Nightmare can occur at all ages, from childhood, to adolescence, to adulthood. Women generally experience more nightmares and more consequences from their nightmares than do men. Nightmares can be divided in two types: posttraumatic nightmares that are linked to a past traumatic experience, and idiopathic nightmares, which are not linked to such experience. Replicative posttraumatic nightmares not only share similarity with the trauma, but “replays” this trauma to a large extent. Nightmares can also be differentiated from bad dreams based on an awakening criterion. When this criterion is used, we observe that nightmares are slightly more emotionally intense than bad dreams, in addition to being more likely to contain fear as their primary emotion. Nightmares and bad dreams can feature several themes, but physical aggression, interpersonal conflicts and failure or helplessness are the most common. In nightmares that trigger fear in the dreamer, the source of the threat is generally an environmental circumstance or an unfamiliar male, and the dreamer generally flee, hide, or fell helpless or trapped. Interestingly, the dreamer can take the role of an offender in an appreciable proportion of nightmares. Finally, a clever study has shown that nightmares experienced at home tend to be associated with increased autonomic arousal in the sleeping body. Overall, nightmares constitute a complex phenomenon that can take many shapes and forms.
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