What Causes Nightmares?

The mechanisms involved in the origin of dreams remain one of the great unknowns in science. Dreams are typically viewed as fascinating, but largely irrelevant, mental epiphenomena of the sleeping brain with questionable functional relevance. Despite the many dreams people experience each night, most are lost into oblivion. But not all. Nightmares are remembered intensely and often reappear on subsequent nights.

Nightmares are vivid, distressing dreams that usually cause the sleeper to awaken abruptly with a clear recall of the details. Fortunately, only a small percentage of the general population experience weekly nightmares. Psychologists believe that nightmares may be an indicator of psychopathological processes given that they are more common in psychiatric populations. Children, who have more REM sleep each night than adults, report a greater frequency of nightmares, while the elderly, who have far less REM sleep each night than adults, report nightmares much less often. Overall, females report more nightmares than males.

Studies suggest that dreaming relies on multiple generators within the brain. Dreams are primarily visual; thus, it was once assumed that higher perceptual cortical systems play a critical role in dream generation. However, congenitally blind people report visual dreams. Thus, dreams must be originating from non-cortical structures. Decades of animal studies strongly support a role for bottom-up mechanisms involving brainstem structures, especially those involved in controlling the autonomic nervous system.

Nightmares are a characteristic symptom of PTSD. However, most people have idiopathic nightmares that are not related to a specific traumatic experience. Nightmares are reported more often by heavy smokers, people with type 2 diabetes mellitus, anxiety disorder, and constipation. The sleep of people with chronic nightmares is fragmented due to frequent awakenings and short periods of intense arousal that are associated with intensified cardiac activity that indicate activation of the sympathetic nervous system and attenuation of the parasympathetic nervous system.

A recent study investigated whether nightmares are caused by an altered autonomic regulation. Ordinarily, the sympathetic and parasympathetic systems are in a perfect Yin-Yang balance: as one increases in activity, the other decreases equally and symmetrically. This recent study monitored the parasympathetic nervous system activity during nightmares as well as while the subjects were awake and performing emotionally challenging tasks. The investigators hypothesized that the parasympathetic nervous system was less active during nightmares and while performing an emotionally challenging task.

The study confirmed that frequent nightmares were associated with emotional dysregulation and altered autonomic activity. Subjects who reported more nightmares exhibited enhanced heart rates during all sleep stages, as well as during the daytime, and significantly reduced parasympathetic activity. The increased heart rate during slow wave sleep is surprising given that this sleep period is usually dominated by the parasympathetic system.

The authors concluded that nightmares that are not directly caused by trauma are most likely induced by dysregulation of the parasympathetic nervous system. The dysregulation may be due to dysfunction of limbic structures, such as the hypothalamus (which might explain the prevalence in females due to hormone fluctuation) or the amygdala, that are known to directly control activity of the autonomic nervous system.

It is important to understand the cause of increased occurrence of nightmares since their frequency is linearly and statistically significantly associated with higher risk of cognitive decline amongst middle-aged adults, and higher risk of dementia amongst older adults.