The findings may help to explain how that process may go wrong in people with learning difficulties.
Sleep spindles or bursts of brain activity while we are asleep play a vital role in strengthening new memories, say researchers who found a way to decode and even enhance these brain waves.
The findings may lead to new ways to help people remember things better, researchers said.
Scientists have long known that sleep spindles - sudden bursts of oscillatory brain activity - play an important role in the formation and retention of new memories.
Sleep spindles are half-second to two-second bursts of brain activity that occur during deep sleep, and can be visualised and measured on an electroencephalogram (EEG).
Earlier studies have shown that the number of spindles that occur during the night could predict a person's memory the next day.
However, many questions about the link between sleep spindles and how a person's recently acquired information is 'reactivated' and strengthened during sleep remained.
Researchers from University of Birmingham and University of York in the UK demonstrated that there is a particular pattern of brain activity that supports this reactivation process.
The study, published in the journal Current Biology, has also shown that the content of reactivated memories can be decoded for brain activation patterns at the time that spindles occur.
The team devised an experiment in which people learned to associate particular words with particular objects and scenes.
Some study participants then took a 90-minute nap after their study session, whereas others stayed awake.
While people napped, researchers cued those associative memories and unfamiliar words. The team monitored the participants' brain activity during sleep using an EEG machine.
The results showed them that sleep spindles occurred when memories were reactivated by presenting the associated words.
Researchers were able to differentiate the brain signals associated with reactivated objects and scenes. This demonstrates that spindles produce a specific code for the content of reactivated memories - a process that may underpin our ability to remember more after sleep.
"While it has been shown previously that targeted memory reactivation can boost memory consolidation during sleep, we have now showed that sleep spindles might represent the key underlying mechanism," said Bernhard Staresina from University of Birmingham.
"Thus, direct induction of sleep spindles - for example, by stimulating the brain with electrodes - perhaps combined with targeted memory reactivation, may enable us to further improve memory performance while we sleep," said Staresina.
"Our data suggest that spindles facilitate processing of relevant memory features during sleep and that this process boosts memory consolidation," he said.
"We are quite certain that memories are reactivated in the brain during sleep, but we do not know the neural processes that underpin this phenomenon," said Scott Cairney from the University of York.
"Sleep spindles have been linked to the benefits of sleep for memory in previous research, so we wanted to investigate whether these brain waves mediate reactivation," said Cairney.
"If they support memory reactivation, we further reasoned that it could be possible to decipher memory signals at the time that these spindles took place," he added.
The findings may help to explain how that process may go wrong in people with learning difficulties, according to the researchers.
It might also lead to the development of effective interventions designed to boost memory for important information.