Why do we dream?

sleeping person

Sleep is a mixture of two behaviours that alternate throughout the night: deep or slow-wave sleep and REM (Rapid Eye Movement) sleep. While deep sleep is characterised by low brain activity (theta and delta waves), REM sleep is also called paradoxical sleep because even though the body is completely relaxed (no muscle tone), brain activity is high, similar to wakefulness (alpha and beta waves). If we wake up in REM sleep, it is very common for us to remember that we were dreaming.

Apparently, the activity of the cerebral cortex during the REM phase (first the visual areas, then the limbic areas) generates dreams, which are usually very visual and emotional. This activity is set in motion by the activation of ascending projections from brain stem centres induced by deep sleep, which ensures that we do not daydream. The same brainstem centres inhibit muscle contraction (remember the absence of muscle tone) and prevent us from expressing dreams and thus putting ourselves in danger.

This cortical activity seems to be related to the synaptic remodelling necessary for learning: the passage from short-term to long-term memory requires connections between neurons to appear and disappear. This remodelling cannot take place while we are awake, as it would dangerously alter our perception and mental functioning (it would cause us to hallucinate, for example). This is why it occurs during the REM phase, when we are sleeping. The result, hallucinations and alterations of perception and thought associated with this plasticity, would be dreams. Therefore, in exam periods, it is important not only to study, but also to sleep and dream.

Answered by Ferran Martinez-Garcia, full professor of Cellular Biology at the Predepartmental Unit of the Jaume I University in Castelló. He was full professor of Physiology at the Department of Cellular and Functional Biology and Physical Anthropology of the University of Valencia until 2014.

Question submitted by Jordi Renard Font.

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