[Matt Walker of UC-Berkeley] and his colleagues uncovered key evidence for why this should be so. The team showed a set of increasingly disturbing images to people who had slept normally and people deprived of sleep for 35 hours. In the sleep-deprived group, the gruesome images produced 60 per cent more activity in the amygdala - a primitive, emotionally reactive part of the brain - than in well-rested people. Further scans revealed that in those deprived of sleep the amygdala was failing to communicate with the prefrontal lobe, which normally controls and sends inhibitory signals down to the emotional brain. "The reason we don't blow our top when someone says something we don't like is because we have a highly developed prefrontal cortex, which acts as an emotional brake," says Walker. A loss of communication between the amygdala and the prefrontal lobe is one way that sleep loss could create psychiatric symptoms, he thinks. "In a number of psychiatric disorders, such as depression, it has been demonstrated that the frontal lobe's activity becomes disrupted. There's also preliminary evidence [of this] for ADHD and post-traumatic stress disorder," Walker says.
In another strand of research, evidence is growing that sleep - and dreaming, REM sleep, in particular - helps the brain to process memories. Disrupt this mechanism, and you could end up with psychological problems such as PTSD.
In August 2008, [Robert Stickgold] and colleagues reported that when people are presented with pictures of an emotional or neutral object or scene, their memory for these scenes decreases during the day. After a night's sleep, they forget pretty much everything except the things that roused their emotions, for which their memories stay the same, or even improve (Psychological Science, vol 19, p 781). Cast your mind back, says Walker, and you will appreciate that almost all of your memories are emotional ones. He thinks this is because emotions act as a red flag for important things that we should be remembering. But, crucially, if you recall them now you don't re-experience the visceral reaction that you had at the time. Somehow, the brain has retained the memory while stripping away the visceral emotion. Both Stickgold and Walker believe this stripping process occurs during REM sleep.
They note that during REM, production of serotonin and noradrenalin shuts down in the brain. Noradrenalin is the neurochemical associated with stress, fear and the flight response; it translates to adrenalin in the body. Serotonin modulates anger and aggression. "You get this beautiful biological theatre during REM sleep, where the brain can go back over experiences it has learned in days past, but can do so in a situation where there are none of these hyping-up neurochemicals," Walker says. So although dreams can be highly emotional, he thinks that they gradually erode the emotional edges of memories.
Wednesday, February 25, 2009
I think I've got this problem. Recent research by separate groups of psychologists suggests that bad sleep habits cause psychological disorders, not the other way around.