Why nightmares often resemble real-life events - scientists' theory
Scientists have discovered that the brain can replay emotional events in dreams. This is especially accurate after an unpleasant experience - and it may explain why bad memories sometimes come back in dreams and resemble real events.
Important: the study does not directly prove that all nightmares are based on reality. Scientists did not study the content of dreams, but rather how the brain consolidates emotional memories during sleep.
Details
The study was conducted by experts from Neuroscience NeuroSU and the Institute of Biology Paris-Seine. They studied how two parts of the hippocampus, a region of the brain involved in memory formation, work during sleep.
The experiments were conducted on rats. The animals were either reliving an unpleasant experience or receiving a reward. After that, while they slept, scientists recorded the activity of neurons in different parts of the hippocampus using miniature electrodes.
It turned out that in sleep the brain re-activated neural patterns associated with recent experiences. In other words, it was as if the experience was being replayed in the brain - not like a film, but as a reworking of the same neural circuits.
The most important result: after an unpleasant experience, this "replay" was more accurate than after a positive one. The authors believe this mechanism may help the brain better anchor dangerous or stressful events.
Why it matters
Bad memories are often remembered more strongly than pleasant ones. New work shows a possible mechanism: during sleep, the brain may be particularly accurate in revisiting unpleasant experiences to link the event to emotion and context.
This could be useful from a survival perspective: if the event was dangerous, it's important for the brain to store it as a warning for the future.
But such a mechanism has a downside. If unpleasant or traumatic memories become too strong, they may come back again and again - including in dreams. The authors therefore believe that such processes may be important for understanding traumatic memories and conditions like PTSD.
Background
Sleep has long been considered an important stage of memory consolidation. Scientists previously knew that the brain can re-activate memories of places, actions and events during sleep. The new work adds an emotional layer to this picture: it shows how the brain can link an event to whether it was pleasant or unpleasant.
That said, transferring the findings to humans should be done with caution. The study was conducted on animals, not on patients with nightmares. It helps to understand the mechanism of memory, but is not a direct explanation for all nighttime fears or dreams.
Source
A study by Juan Facundo Morici and co-authors published in Nature Neuroscience in 2026: Dorsoventral hippocampus neural assemblies reactivate during sleep following an aversive experience. The authors studied how different parts of the hippocampus in rats coordinate during sleep after positive and unpleasant experiences.