Optimizing emotion regulation and memory with sleep

Research Summary
Bryce A. Mander, PhD
Contributing writer for Baby Sleep News

Like a healthy diet and sufficient exercise, restorative sleep is a fundamental pillar of health. Not a single organ system that has been studied is impervious to the ravages of insufficient sleep, and the symptoms of many medical and mental health disorders are exacerbated by sleep disturbance. The brain is no exception, with a multitude of cognitive and emotional impairments associated with inadequate sleep quality. These impairments are not only due to the consequences of a tired, overworked brain, but also the lack of sleep makes it difficult for the brain to continuously optimize thoughts, feelings, and behaviors. This is particularly relevant for autism spectrum disorder (ASD), as its cognitive, emotional, and behavioral symptoms often coexist with sleep disturbance. 


In humans, there are two types of sleep: non-rapid-eye-movement (NREM) sleep and rapid-eye-movement (REM) sleep.  Each is associated with dramatically different states of brain activity and chemistry, which cycle repeatedly throughout the night. The cognitive benefits of sleep have largely been tied to NREM sleep, while emotional benefits are largely tied to REM sleep. NREM sleep, particularly its deepest stage called slow wave sleep, supports the long-term retention of memories. EEG measurements of brain activity during NREM sleep show specific features called slow waves and sleep spindles. In subjects who have learned new information, these features of deep NREM sleep predict the degree that new information is remembered over hours, days, and even years. Increasing those same deep NREM sleep features in both young and older adults improves memory retention. Further, total sleep deprivation and selective suppression of deep NREM sleep impairs memory retention. These findings support the largely held view that deep NREM sleep is a necessary part of a process that permanently cements newly acquired memories into the brain. Some evidence suggests that REM sleep also supports this process if memories are emotional, with emotionally charged memories being preferentially saved over non-emotionally charged memories.   


Beyond emotional memory, a series of studies now show that REM sleep is critical for emotional regulation in general. Sleep disturbance results in an increase in the physiological and behavioral response to emotional events, a decrease in the ability to accurately interpret emotional scenes and faces, and an impaired ability to bring emotional responses under control. This is thought to be due to the fact that the amygdala, a part of the brain that responds to negatively charged, emotionally significant events, is overactive in a sleep-deprived state. Under well-rested conditions, brain activity in the prefrontal cortex keeps amygdala activity from getting out of control. Unfortunately, the prefrontal cortex is particularly vulnerable to sleep loss and decreases communication with the amygdala following sleep deprivation. This, coupled with the selective erosion of all but negatively charged emotional memories after sleep loss, may explain why sleep disorders such as insomnia increase the risk of developing both post-traumatic stress disorder (PTSD) and depression. Essentially, sleep disturbance casts a pessimistic hue on past and present experiences. 


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