When the brain is triggered to recall a memory during sleep, memories are made stronger.
By associating odor with memories, researchers show that sleep helps keep memories strong.
The finding could be applied to help people address traumatic memories.
A new study claims the best way to remember something important is to remember it while you are asleep.
The research in this week's Nature Neuroscience, claims reactivating newly learned memories during sleep rather than when awake does a better job of strengthening the memory trace.
The work, led by Björn Rasch of the University of Basel in Switzerland could have clinical implications for treating disorders such as post traumatic stress disorder (PTSD).
The research involved training subjects in a spatial memory task to learn an object's location in a grid. At the same time an odor was released -- the idea being to associate the memory with the smell.
One group then went to sleep, while the second group stayed awake.
After about 20 minutes, while the sleeping group's brain wave patterns were in slow wave sleep (SWS), both groups were subjected to the same odor again to reactivate the memory. Another 20 minutes later, the sleeping group were woken, and both groups were given a similar task, but the objects were in different locations and without the odor.
Half an hour later they were asked to recall the position of the cards from the original task.
Rasch and colleagues found both groups were correct about 60 percent of the time without any odor assistance.
But once the odor was added to the test, the non-sleeping group were only correct about 42 percent of the time, compared to the sleeping group's 84 percent.
"As we expected, reactivation during waking destabilized memories. In contrast, reactivation during SWS immediately stabilized memories, thereby directly increasing their resistance to interference," the researchers write.
The researchers also performed functional magnetic resonance imaging (fMRI) tests, which showed memory reactivation by odor presentation during wakeful periods involved patterns of brain activity different to those seen during sleep.
"The results of our fMRI study indicate that during reactivation in the wake state, mainly the lateral [pre-frontal cortex] is activated. This brain region is implicated in the control of memory retrieval with respect to the actual contex," they write.
In contrast, the researchers saw activation in the hippocampus and neocortex regions of the brain in the SWS group.
According to the study, reactivation of a memory returns it to a labile (unstable) state allowing it to be reinforced or updated. But this process could in some cases produce faulty or distorted memories.
The researchers speculate that reactivation during SWS reinforces the memory without labilization.
The transfer of memories from the hippocampus to the neocortex could protect "these memories against interference from information subsequently encoded in the hippocampus," the researchers write.
"Alternatively, reactivations during sleep might lead to a transient destabilization of memory traces, but with restabilization occurring at a much faster rate during sleep."
The researchers suggest the finding has implications for the treatment of patients with panic disorder or PTSD.
"Our results strongly suggest that contextual cues presented during wakefulness might be capable of reactivating and destabilizing unwanted and maladaptive memories in a psychotherapeutic setting," they write.
"Subsequent reactivation of the newly learned concepts during ensuing SWS could then help to consolidate the desired therapeutic effects for the long term."