Daytime sleep intensified memories of emotional trauma and anxiety
Scientists from the Ural Federal University and the University of Tübingen (Germany) have studied the effect of sleep on the formation and translation of primary memories of something scary into long-term memory. The neurobiologists found that sleeping during the day enhances memory of disturbing and frightening events, but a similar memory enhancement effect is also observed after a period of quiet wakefulness. The findings will be useful for developing rehabilitation strategies for people who have been emotionally traumatised by disasters, warfare, and violence.
Fear-related disorders, such as panic disorder, post-traumatic stress disorder (PTSD) or specific phobias, occur against the background of the stress suffered. They are characterised by persistent feelings of anxiety and fear, which may increase over time.
Memory consolidation is the process of transferring primary memories into a stable long-term form. Studies of declarative (memory for facts), episodic (memory for life events), and procedural (memory for skills) memory show that sleep after learning has a positive effect. A person's sleep cycle is typically about 1.5 hours. People sleep four to six cycles at night, if they also resort to rest in the form of daytime naps, they rarely sleep more than one full cycle during the day.
The researchers' study is published in the journal Cognitive, Affective, & Behavioral Neuroscience. Memory consolidation - that is, the transfer of memories from short-term memory to long-term memory - occurs primarily during sleep. Research shows that sleep after learning can have positive effects beyond passive wakefulness. This occurs by reactivating important memories, which is possibly also reflected in dreams.
In this case, the positive effect of sleep can be observed even years later. However, there are currently no studies that analyse whether sleep enhances the effect of remembering emotionally difficult events. Therefore, the scientists decided to find out how sleep affects the memory of the fear experienced by a person.
"Understanding what effect sleep has in situations where emotional trauma occurs is important for forming effective behavioural strategies with survivors of some disaster, people with panic disorder or post-traumatic stress disorder. If we were to find that the effects of sleep on fear memory are similar to other types of memory, such as episodic memory (memory of life events), then it would be more useful for survivors to stay awake after trauma.
In our experiments, we determined that a two-hour nap during the day enhanced the anxiety memories we elicited from subjects just before bedtime. However, a similar effect was observed when compared to a period of quiet wakefulness - watching an emotionally neutral film or computer game," says co-author of the paper Yuri Pavlov, a researcher at the teaching and research laboratory of UrFU and the Institute of Medical Psychology and Behavioural Neurobiology at the University of Tübingen.
Before and after the sleep period, participants underwent a fear conditioning paradigm, namely listening to an irritant sound. Participants in the experiment first heard a neutral sound, and then it was periodically combined with noise, the anticipation of which causes anxiety and fear. The scientists note that many rated this stimulus as more unpleasant than even electric shocks, also commonly used in fear research.
"To form emotions and conditioned reflexes, it is often sufficient to combine a neutral stimulus (conditioned stimulus) many times with a stimulus that naturally elicits an emotion (unconditional stimulus). Subsequently, the neutral stimulus initiates an equally strong emotional response on its own. In our study, we combined a neutral sound, a conditioned stimulus that initially elicits no response in the body, and a noise, an unconditioned stimulus that predicts the occurrence of a very loud sound that people find extremely unpleasant. As a result of current research, we show that the fear response only to neutral sound is formed and intensifies after sleep, "- explains Yuri Pavlov.
The fear response was studied using electroencephalography before and after sleep or an equal period of wakefulness in 18 healthy young adults. The researchers are now taking the study to the clinic, where they plan to test patients in a vegetative state and a state of minimal consciousness to determine how sleep will affect their anxiety levels and the formation of fear memories. They also note that further research is needed on the effects of longer sleep periods.