What are Nightmares and Why Do They Occur?

By Anthoni Oisin, BScReviewed by Sophia Coveney

How are bad dreams and disturbed sleep implicated in disease?
Parkinson’s disease, dreams, and sleep
Neurodegenerative dementia and sleep architecture
The future of neuroscientific dream research
References
Further reading

Dreams are events that occur during some stages of sleep. They consist of imagery, and at times sensations, that are experienced by the dreamer during one stage of sleep. Nightmares are unpleasant or distressing dreams. They are a type of parasomnia, and can also be referred to as dysphoric dream imagery.

The stage of sleep associated with dreams and nightmares is referred to as Rapid Eye Movement (REM) Sleep, which makes up around 25% of the sleep cycle. This stage of sleep is not considered to be restful due to the relatively high levels of associated neurological and physiological activity.

Dreaming occurs across several parts of the brain. The amygdala is associated with emotional production while dreaming and the hippocampus is the memory region active during dreams, similar to their functions during states of wakefulness. The pons activates the rapid eye movements that occur during REM sleep through activity in the paramedian pontine reticular formation or conjugate gaze center.

There are multiple models of understanding dreams and nightmares, including neurobiological, cognitive, evolutionary and psychodynamic models, in addition to the affective network dysfunction model.

Such models suggest that the function of dreams and nightmares could provide a number of different purposes, such as fear extinction, mood regulation and representations of pathological symptoms.

How are bad dreams and disturbed sleep implicated in disease?

Nightmares can be associated with a number of neuropsychiatric and neurodegenerative disorders. This is because dreams can be indicative of wider symptoms or particular processes occurring within the brain. Nightmares are also, at times, a side effect of the medication used to treat some neurodegenerative disorders.

Not all dysphoric dream experiences are indicative of brain health, however, as they can also have a range of non-disease-related etiologies. For instance, a common understanding of bad dreams is that it is a response to powerful emotion, as opposed to it being indicative of problematic brain health.

REM Sleep Behavior Disorder is a form of parasomnia wherein the dreamer physically “acts out” some part of the experience of their dream. This can occur in the case of both positive dreams and dysphoric dreams or nightmares.

REM Sleep Behavior Disorder is thought to be an early indicator for a number of neurodegenerative disorders, including Dementia with Lewy Bodies, Multiple System Atrophy, and Parkinson’s disease. Such disorders are all characterized as a-synucleinopathies.

A-synucleinopathies are a group of neurodegenerative diseases that relate to atypical or abnormal massing of a-synuclein aggregates in relevant cells, such as nerve fibers, glial cells, and neuron cells. REM Sleep Behavior Disorder may be an early indicator for the later occurrence of a-synucleinopathy.

Longitudinal cohort studies suggest that there is a relationship between REM Sleep Behavior Disorder and a-synucleinopathy. Over 80% of those who develop REM Sleep Behavior Disorder also progress to develop a neurodegenerative disorder that is an a-synucleinopathy. However, the exact causation between the two, and thus the wider impact of the disorder on brain health, has not yet been fully established.

Parkinson’s disease, dreams, and sleep

Parkinson’s disease is a neurodegenerative disorder with various presentations and etiologies. The symptoms vary, but the cardinal manifestations are motor-related. Diagnostic criteria for the disorder require the presence of bradykinesia together with rigidity, tremor, or both during rest.

Nightmares are not a diagnostic symptom of Parkinson’s disease, although research has indicated that patients with the disorder may experience a higher volume of unpleasant dreams than those without Parkinson’s. Researchers in New Delhi found that 32% of a sample of participants diagnosed with Parkinson’s disease reported experiencing nightmares, compared to 5% of the control group, who were not diagnosed with Parkinson’s.

The association between Parkinson’s disease and dreams is not wholly understood. However, some parasomniac disorders appear to relate to the disorder due to the abnormal levels of a-synuclein aggregates found in neural cells among those with Parkinson’s disease.

Neurodegenerative dementia and sleep architecture

Dementia is a grouping of memory-related neurodegenerative disorders. Some types of dementia include Alzheimer’s disease, Huntington’s disease, vascular dementia, dementia with Lewy bodies, and frontotemporal dementia.

REM Sleep Behavior Disorder is associated with Lewy body dementia and other disorders with similar pathologies. A study using polysomnographic methods, meaning that the participants were monitored while sleeping, found both a characteristic and pathological association between dementia with Lewy bodies and REM Sleep Behavior Disorder.

Nightmares and nighttime behavioral disorders are associated with a number of types of dementia, including Alzheimer’s disease. This is thought to relate to the overarching changes in the organization of sleep, known as sleep architecture, of those with dementia. For instance, those with dementia can experience different rates of particular stages of sleep.

The future of neuroscientific dream research

There is still a great deal to be understood about the relationship between bad dreams and brain health. While sleep research has been undertaken for a long time, the neuroscience that underpins sleep and dreaming still leaves some questions unanswered.

When examining the literature, it appears that a correlation could be drawn between dreams and neurodegenerative disease, but the extent of the relationship is not yet comprehended. With further research, the relationship could offer valuable clinical information in both diagnostic and treatment-related contexts.

Pharmacological treatments, such as clonazepam, have been trialed for the management of disturbed sleep among those with Rapid Eye Movement-related sleep disorders.

However, this has not been trialed longitudinally with dementia patients to demonstrate safety and efficacy. This is a gap in the literature that could be addressed in the future of the field.

References

• Siclari F, Valli K, & Arnulf I (2020). Dreams and nightmares in healthy adults and in patients with sleep and neurological disorders. The Lancet Neurology, 19(10), 849-859.
• Hu MT (2020). REM sleep behavior disorder (RBD). Neurobiology of Disease, 143, 104996.
• Nielsen T & Levin R (2007). Nightmares: a new neurocognitive model. Sleep medicine reviews, 11(4), 295-310.
• Patel AK, Reddy V & Araujo JF (2022). Physiology, sleep stages. In StatPearls [Internet]. StatPearls Publishing.
• McCann H, Stevens CH, Cartwright H, et al. (2014). α-Synucleinopathy phenotypes. Parkinsonism & related disorders, 20, S62-S67.
• Kumar S, Bhatia M & Behari M (2002). Sleep disorders in Parkinson's disease. Movement disorders, 17(4), 775-781.
• Bloem BR, Okun MS & Klein C (2021). Parkinson's disease. The Lancet, 397(10291), 2284-2303.
• Boeve BF, Silber MH, Ferman TJ, et al. (1998). REM sleep behavior disorder and degenerative dementia: an association likely reflecting Lewy body disease. Neurology, 51(2), 363-370.
• McCurry SM & Ancoli-Israel S (2003). Sleep dysfunction in Alzheimer’s disease and other dementias. Current treatment options in neurology, 5(3), 261-272.

Further Reading

• All Nightmares Content
• What are Nightmares?