Scientists may have uncovered why some people naturally lose their ability to make new memories as they get older.
Using brain scans, University of Arizona scientists found differences in how older rats "replayed" their actions in the brain during sleep.
They say this night-time process might help them remember better the next day.
Not all experts agree. A UK sleep expert said there could be other reasons why older people are less able to form memories while asleep.
The University of Arizona finding, reported in the Journal of Neuroscience, is the latest to investigate the role of an area of the brain called the hippocampus.
Studies suggest that it plays an important part in learning and memory, particularly in "episodic" memory - the ability to recall events, and the smell, sight and even tastes associated with them.
In the experiment, 22 rats - half young, and half old - were all given food rewards in return for successfully navigating mazes.
First, scientists measured activity within the hippocampus while they were in the maze, and then later, while the rats slept.
In the younger rats, the neural patterns closely matched those recorded during the maze activity itself, suggesting that they may have been "replaying" the action as part of the process of memory consolidation.
However, in most of the older rats, the pattern produced did not match the original.
More evidence supporting the notion that these neural patterns might have a link to laying down memory came the following day, when the animals were put back in the same mazes.
Those whose neural patterns in sleep had been the best match performed better.
Dr Carol Barnes, who led the study, said: "This is the first study to suggest that an animal's ability to perform a spatial memory task may be related to the brain's ability to perform memory consolidation during sleep."
Dr Michael Hasselmo, from Boston University, said that the study might "inspire" the development of new drugs to enhance this process during sleep.
But Dr John Groeger, from the Sleep Research Centre at the University of Surrey, said that the precise role of the hippocampus in the storing and recall of memory was yet to be proven.
He said: "It would be extremely difficult to test this theory in humans, because there are wide differences in the sleep patterns of older and younger people.
"On average, older people will not only sleep for between 1.5 and two hours less each night, but also have a completely different sleep architecture, in terms of the amount of each type of sleep they have.
"There could be other reasons for differences in the ability to consolidate memory."
He added that it was quite logical that hippocampal activity in older people would be different, due to the natural shrinkage of the ageing human brain.