The question: It's no secret that our memories fade with age, but the reason for the deterioration remains a puzzle to neuroscientists. Previous studies have suggested that the prefrontal cortex, which sits just behind the forehead and helps moderate sleep quality, is somehow related because it shrinks as we get older. Other research from the 1970s links stage 4 "slow-wave" sleep to a person's ability to convert new information to memories, but a connection was never explicitly outlined, at least until now. For this new study, published in the journal Nature Neuroscience, researchers sought to link the size of a person's prefrontal cortex with their ability to get a good night's rest and, ultimately, to their ability to memorize new words. Researchers asked: Is it possible that our brain's shrinking sleep centers are responsible for mental sluggishness in old age?   

How it was tested: The study had three main stages:

1. Gauge brain size. Researchers from the University of California, Berkeley, enlisted 19 people of retirement age and 18 people in their early 20s. Both groups were evaluated for the size of their medial prefrontal cortexes.

2. Bring on the tests. Next, researchers had the two groups study a list of strange word combinations like "action-siblis" and "arm-reconver." Choosing nonwords is important, says The New York Times, because "one type of memory that declines with age is for new, previously unseen information." After a half hour of studying, participants were tested on their ability to recall the words. 

3. Participants were put to bed. While dreaming, an electroencephalogram machine, or EEG, was used to analyze each person's quality of sleep, particularly the amount of slow-wave sleep — which is when the brain is thought to move memories from short-term to long-term storage. When they woke up the next morning, participants were again tested on what they had learned the previous day.

The outcome: The 20-year-olds, all in all, not only had larger prefrontal cortexes (and thus slept better), but they unsurprisingly outperformed the older group across the board:

1. On average, the prefrontal cortexes of the older group were one-third smaller than those of the twenty-somethings.

2. When tested on the nonwords, the younger group outscored the older group by a significant 25 percent.

3. While sleeping, the older group experienced just a quarter of the amount of slow-wave sleep the younger group did. In the morning, the younger group outscored the older group on the word test by a whopping 55 percent.

What the experts say: "Slow-wave sleep is critically important for cementing new memories you've recently learned. It's like clicking the save button," researcher Dr. Matthew Walker tells the BBC. "Taken all together, the deterioration of the brain leads to the deterioration of sleep to the deterioration of memory." Our analysis "showed that the differences were due not to changes in capacity for memories, but to differences in sleep quality," Bryce A. Mander, a postdoctoral fellow at Berkeley and the lead author of the study, tells The New York Times

The lesson: Regularly getting a good night's sleep is crucial to keeping your mind sharp as you age. Regular physical exercise definitely helps, and "can improve sleep," says Dr. Paller, director of the cognitive neuroscience program at Northwestern University, who has not involved in the study. Neuroscientists in Germany are also working on electrical-stimulation techniques that can enhance deep sleep, with early results suggesting their technique can double a participant's memory overnight. The message is clear: Remember to make getting a good night's rest a priority.