Scientists have long known that once we nod off, certain memories grow stronger. One recent theory suggests that forgetting, too, is an essential function of sleep [see “Sleep's Secret Repairs,” by Jason Castro; Scientific American Mind, May/June 2012]. Researchers now suspect that post-traumatic stress disorder (PTSD) may emerge from flaws in sleep's forgetting process. Two studies presented at the 2012 meeting of the Society for Neuroscience in New Orleans indicate that sleep might offer a window of opportunity for weakening memories and providing relief from lingering reminders of trauma.

Neuroscientists believe that during sleep, a memory-elimination routine cleans out obsolete information by physically weakening synapses, the junctions between communicating neurons. Gina Poe, a neuroscientist at the University of Michigan, found in mice that for synapses to lose strength, levels of the neurotransmitter noradrenaline must drop. Noradrenaline levels typically fall during REM sleep in rodents and humans, but in people with PTSD the amount stays high throughout sleep. Normalizing noradrenaline with pharmaceuticals, Poe says, “could absolutely be a key target to actually cure PTSD through normal sleep.”

In a separate experiment, researcher Asya Rolls of Stanford University hijacked memory remodeling in sleeping mice to make a traumatic association less scary. Rolls and her colleagues conditioned mice to fear the scent of jasmine flowers by pairing the smell with a foot shock. When the mice slept, they released a puff of jasmine. Under normal circumstances, the smell would reactivate and bolster the memory, a process that requires newly made structural proteins. The researchers gave some mice a drug that prevented the manufacture of these building blocks in a key fear-memory area. When these mice woke up, they no longer responded to the odor with fearful behavior, indicating that the memory had been successfully disrupted. The findings might someday translate to a new kind of sleep-based therapy in people whose traumatic experiences are tied to specific sounds and smells—such as the noise of a bomb going off—that can be presented to their sleeping brain.

Current treatments for PTSD—and other persistent negative memories—often rely on exposure therapy, which inoculates patients against their fear trigger by creating a new, safe memory that springs to mind more often than the old, frightening memory. But the old memory remains. To truly diminish its power, this research suggests, we must target the unconscious mind and help the brain forget.