What Causes Nightmares

Understanding nightmares

A nightmare is a scary, frightful dream that causes the individual to wake up abruptly in a state of unease. When we awake from a nightmare, there are distressing physiological symptoms such as increased heart rate, cold sweat, shortness of breath and strong negative emotions. This is what separates a nightmare from a bad dream. 

The strong and emotionally unpleasant effect of nightmares is thought to be precipitated by stressful events where negative affect may be a trigger.¹ Negative affectivity refers to the experience of a negative emotional response to a stressful situation. Understanding nightmares and their triggers is important to ensure restorative sleep and brain recovery, and is useful to our understanding of their relation with mood disorders, sleep disorders and other psychiatric issues.

Classification and prevalence

Everyone experiences sporadic nightmares, but experiencing frequent nightmares is considerably rarer. Nightmares affect less than 5% of the overall population, with more females affected than males, and a higher prevalence in childhood compared to adulthood.² Nightmares have two main classifications: emotional or post-traumatic, and idiopathic. Both of these groups are characterised by awakening during REM sleep with clear recall of disturbing mental activity. Post-traumatic nightmares may directly replicate the traumatic event, or have emotions related to the trauma. These nightmares have more severe arousal effects of increased heart rate and shortness of breath. Post-traumatic nightmares lead to more abrupt awakenings, stronger aggression, and more elevated helplessness than idiopathic nightmares. Idiopathic nightmares are unrelated to any specific trauma. The cause for idiopathic nightmares is unknown, but they are thought to originate from early adverse experiences and can depict more imaginative stories. Nightmares are distinct from night terrors which involve fear-based centres which arise from non-REM sleep and do not have clear mental recall. 

Understanding sleep factors

Sleep takes place in cycles with various stages occurring multiple times a night. These are split into two main types: Non-Rapid Eye Movement (NREM) and Rapid Eye Movement (REM). Both these stages form a sleep cycle that typically lasts 90 minutes and repeats 4-6 times during a night’s sleep. The dreaming state of sleep occurs during REM sleep and is therefore the stage of sleep which is most associated with the occurrence of nightmares.3 During REM sleep, the most vivid dreaming occurs, breathing becomes more shallow and heart rate and blood pressure increase. The brain also becomes more active with high-frequency amplitude waves that resemble the same patterns that occur when awake. This heightened activity during REM means that disruptions to REM sleep can increase the likelihood of nightmares. Waking up during REM sleep makes it more likely you will remember your dreams and nightmares.

Areas of the brain involved in nightmares

During the occurrence of nightmares, specific areas of the brain may be overactive or underactive which may provide insights into the neurobiological causes of nightmares/ Some of the key areas involved are the amygdala, hippocampus, prefrontal cortex and thalamus.

Amygdala

The amygdala is highly active during REM and can increase the fear response in nightmares. The amygdala processes emotions such as fear and anxiety and regulates the brain's stress response. In individuals with negative affect and high stress, the amygdala stays overactive during sleep.

Hippocampus

The hippocampus is involved in consolidating and forming memories. During REM, the hippocampus integrates and processes emotional memories. When this process involves traumatic or difficult memories, nightmares can occur. In individuals with PTSD, hippocampus processing is impacted and the brain may struggle to properly integrate traumatic memories resulting in nightmares which replay the traumatic event as a means for consolidation.

Prefrontal cortex

The prefrontal cortex is involved in logical decision-making, emotional regulation and impulse control. During REM sleep, there is reduced activity in the prefrontal cortex. This means there is less logical reasoning and emotional regulation. This reduces the brain's ability to distinguish the nightmare from reality and contributes to the frightening nature of nightmares.

Thalamus

The thalamus is involved in processing and transmitting sensory information to different parts of the brain. The thalamus integrates sensory experiences into dreams and this integration can amp up the intensity of nightmares. In individuals with negative affect, anxiety or PTSD the thalamus becomes overactive.

Psychological factors

Stress and anxiety

High levels of hormones such as cortisol can disrupt sleep and trigger nightmares. Traumatic life events can also cause nightmares as the brain attempts to process and consolidate these experiences when asleep.

Mental health conditions

Mental health conditions like depression, PTSD and anxiety disorders are often linked to frequent nightmares. The more pronounced the conditions are, the increased severity of nightmares. Some psychoanalysts theorise that nightmares are a mechanism for the brain to process unresolved trauma, conflicting emotions and fears. As we sleep, our brain works these issues through, replaying and integrating them which can result in distressing nightmares. This is the brain’s attempt to integrate challenging emotions.

Environmental and lifestyle factors 

Sleep environment

Having a good sleep routine and sleep hygiene is a good way to prevent nightmares. A poor sleep environment which is noisy, bright or uncomfortable can contribute to nightmares. These factors can disrupt sleep and the REM cycles, leading to nightmares. Getting sunlight early in the day and going to sleep at the same time every day, reducing screen time before bed and relaxation techniques can help to ensure a good night’s rest, and mitigate the risk of disruption of REM.

Substance use

Alcohol, drugs and certain medications have disruptive effects on sleep patterns, and as a consequence, can increase the frequency of nightmares. Alcohol and certain prescription drugs such as antidepressants and antihistamines can disrupt REM patterns and lead to nightmares. Withdrawal from alcohol, marijuana and stimulant drugs can also lead to unpleasant nightmares.

Mitigating nightmares

Behavioural and psychological interventions

For treating individuals with recurrent nightmares, imagery rehearsal therapy (IRT) is a useful tool. During IRT, the recurrent nightmare is reimagined into a more pleasant scenario, and this is rehearsed before bed.

Medical treatments

In individuals with recurrent nightmares and PTSD, medications may help reduce nightmares. It is important to consult with your healthcare provider before taking any new medications.

Summary

• Nightmares are frightful dreams that cause you to awaken suddenly. They have a higher prevalence in young children and in women than men
• There are a multitude of factors which contribute to the increased frequency of nightmares and interventions to mitigate the incidence
• Implementing good sleep hygiene and environment can reduce the incidence of nightmares