Evidence is growing that the brain uses sleep time to consolidate memories acquired during the day.
The brain uses sleep to store memories
Scientists who measured the brain signals of rats found distinctive patterns of activity in certain areas of the brain during sleep.
Their analysis suggests that the signals are "reverberations" lasting up to 48 hours after a novel experience.
The finding, at Duke University Medical Centre, will help scientists hunting genes key to memory formation.
The research team used a network of 100 recording electrodes placed in the brains of their rats, in four regions of the brain areas traditionally linked with memory.
The rats were put in dark environments which they then proceeded to explore. Various different objects were placed in the environment for them to "find".
The scientists then looked for signs of activity during a sleep phases called slow-wave sleep and "rapid eye movement" sleep.
The former is a deep dreamless sleep while REM sleep is linked to dreaming.
After the rats had been looking at an unfamiliar environment, there was a distinctive pattern of brainwave activity, particularly in slow-wave sleep.
This differed from the brainwave patterns that followed activity in a familiar environment.
Previous studies had found that during REM sleep, genes are activated that appear to have a role in memory consolidation.
Dr Sidarta Ribeiro, one of the study authors, said: "We're proposing that the two stages play separate and complementary roles in memory consolidation.
"Periods of slow-wave sleep are very long and produce a recall and probably amplification of memory traces.
"Ensuing episodes of REM sleep, which are very short, trigger the expression of genes to store what was processed during slow-wave sleep."
The next stage of their research is to conduct experiments over longer periods - and perhaps genetically modify the mice in order to work out which genes are key to memory storage.