A new study has shown that sleep deprivation can inhibit the brain's ability to suppress unwanted memories and intrusive thoughts.
Scientists at the University of York, in collaboration with the University of East Anglia, have shown that sleep deprivation interferes with the ability of the prefrontal area of the brain to restrict the retrieval of memories that would have otherwise been suppressed.
Dr Scott Cairney from the University of York said: "Memories of unpleasant experiences often intrude into our conscious mind in response to reminders, but tend to be fleeting and can be put out of the mind again, but we have previously shown that the brain's ability to suppress such intrusive memories is contingent on obtaining restful sleep.
"Suppression is a very clever function of the brain as it weakens all of the connecting traces of the memory, thereby inhibiting us from joining up all the dots to retrieve the full picture of the experience when it is triggered by an external stimulus."
To understand how the brain does this, the team used functional magnetic resonance imaging (fMRI) scans to investigate the brain activity of 85 healthy adults, half of which had experienced a healthy night sleep in the sleep lab, and the other half stayed awake all night.
They were asked to look at faces, which they had previously seen paired with images of scenes, some of which were emotionally negative, such as a picture of a car crash or a fight. For each face they were asked to either recall the scene associated with it, or suppress the memory of the scene.
When attempting to suppress the scene images, the well-rested participants showed more activation in the right dorsolateral prefrontal cortex - a brain region that controls thoughts, actions, and emotions - compared to those who stayed awake all night.
The rested participants also showed reduced activity in the hippocampus - a brain region involved in memory retrieval - during attempts to suppress unwanted memories, demonstrating that they could 'shut down' the retrieval operations that underpin emerging intrusive thoughts.
They also found that individuals who obtained more rapid eye movement (REM) sleep, were better able to engage the right dorsolateral prefrontal cortex during memory suppression, pointing to a role for REM sleep in restoring the mechanisms in the brain that can help prevent unwanted memories from entering conscious thought.
Dr Cairney explains: "The participants who were sleep deprived were unable to engage the area of the brain that helps us suppress unwanted memories. Consequently, they could not quash memory related processes in the hippocampus that give rise to intrusive thoughts.
"This is really important to our understanding of mental health issues as it is well documented that those who suffer with anxiety, depression or PTSD, also have difficulty with sleep. Now that we have better understanding of the the mechanisms in the brain that can help restrict negative memories and thoughts, we can perhaps work on more targeted treatments and behavioural therapies that help with improving sleep and as a result support the brain in doing what it has so cleverly adapted to doing, allowing us to lead mentally fit lives."
'Memory control deficits in the sleep-deprived human brain' is published in the journal Proceedings of the National Academy of Sciences .
