For decades scientists have known that the ability to remember newly learned information declines with age, but it was not clear why. A new study may provide part of the answer.

The report, posted online Sunday by the journal Nature Neuroscience, suggests that structural brain changes occurring naturally over time interfere with sleep quality, which in turn blunts the ability to store memories for the long term.

Previous research found that the prefrontal cortex, the brain region behind the forehead, tends to lose volume with age, and that part of this region helps sustain quality sleep, which is critical to consolidating new memories. But the new experiment, led by researchers at the University of California, Berkeley, is the first to link structural changes directly with sleep-related memory problems.

The findings suggest that one way to slow memory decline in aging adults is to improve sleep, specifically the so-called slow-wave phase, which constitutes about a quarter of a normal night's slumber.

In the study, a research team in California took brain images from 19 people of retirement age and 18 in their early 20s. It found that a brain area called the medial prefrontal cortex, roughly behind the middle of the forehead, was about a third smaller on average in the older group than in the younger one - a difference due to natural atrophy over time, previous research suggests.

Before bedtime, the team had the two groups study a long list of words paired with nonsense syllables. The team used such nonwords because one type of memory that declines with age is for new, previously unseen information.

After training on the pairs, the participants took a test on some of them. The young group outscored the older group by about 25 percent.

Then everyone went to bed. The older group got only about a quarter of the amount of high-quality slow-wave sleep that the younger group did, as measured by the shape and consistency of electrical waves on an electroencephalogram machine, or EEG.

On a second test, given in the morning, the younger group outscored the older group by about 55 percent. The estimated amount of atrophy in each person roughly predicted the difference between his or her morning and evening scores, the study found.

"The analysis showed that the differences were due not to changes in capacity for memories, but to differences in sleep quality," said Bryce A. Mander, a postdoctoral fellow at the University of California, Berkeley, and the lead author of the study.