REM sleep has been associated with the consolidation and formation of memory, including memories tagged by our emotions. Short-term memories are converted into long-term memories once the hippocampus becomes involved. Memories are then consolidated between neocortical regions to avoid redundancy and save time. More specifically, indexing by the hippocampus allows related events such as visual and auditory experiences to be stapled together so that the hippocampus no longer needs to be involved.
When compared to slow wave sleep, REM studies show that emotions are enhanced during this portion of the sleep cycle. Memories tagged with emotions are then retained in the long-term storage area of our brain in vivid detail. There is debate as to whether REM is participating in the consolidation or formation of emotional memories. A recent study supports the latter. In other words, wherein emotions are involved, REM serves to preserve rather than reduce emotions.
Sleep during the REM stage increases the number of memory recall events. The brain prioritizes memories for consolidation based on the emotions associated with them so that emotions serve to modulate the formation of new memories. False memories are formed in this way also. In theory, sleep deprivation can cause future, or veridical, memories to become attached with false events incorrectly or with greater distortion.
Sleep deprivation can also affect negative memory consolidation and formation. For instance, a recent study on fear in rats gives insight into human anxiety. Rats who are trained with lights-on in the morning display less occurrences of frightful freezing than do rats who are trained with lights-off during nighttime. In other words, lights-off rats are not able to cue their fear correctly, which is comparable to humans who are unable to control anxiety due to interrupted sleep.
Another primary organ associated with memory and fear is the amygdala, which not surprisingly is situated next to the hippocampus. A significant number of mood disorders have a sleep abnormality element associated with them. An overactive amygdala in which adrenergic neurotransmitters are not suppressed by REM sleep is believed to cause excessive display of subjective emotions in these cases. Anxiety, for instance, could be considered a subjective emotion when the setting is not appropriate. For these reasons, regular sleep is undoubtedly an essential component in the comprehensive treatment of mood disorders.
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