New findings reveal a connection between sleep and memory, and shed light on why forgetfulness is common in the elderly.
Our brains naturally deteriorate with age. Sleep quality—specifically the slow-wave activity that occurs during deep sleep—also decreases as we get older. Previous research found that slow waves are generated in a brain region called the medial prefrontal cortex (mPFC), which exhibits age-related deterioration.
A team of neuroscientists led by Drs. Bryce Mander and Matthew Walker at the University of California, Berkeley, set out to explore whether age-related changes in sleep and brain structure are linked to impaired memory. Their study included 18 healthy young adults (ages 18 to 25) and 15 healthy older adults (ages 61 to 81).
Before going to sleep, the subjects memorized and were tested on 120 word pairs. While they slept, their brain activity was measured using an electroencephalogram. After 8 hours of sleep, the subjects were tested on the same word pairs, this time while undergoing functional MRI (fMRI) scans to measure changes in brain activity. The study, funded by NIH’s National Institute on Aging (NIA), appeared online on January 27, 2013, in Nature Neuroscience.
Memory performance in older adults was significantly worse than in their younger counterparts. Older adults also had significantly less slow-wave activity. Brain structures differed between the age groups as well, with the most degeneration in the older group in the mPFC. Interestingly, reduced mPFC volume was associated with lower slow-wave activity, regardless of age.
To confirm that diminished memory retention in older adults was sleep-dependent, the researchers had participants perform the same word-pair memory task after an 8-hour period of wakefulness. Older adults still performed worse on the memory tasks than the younger group. However, while sleep improved memory retention for the younger group, this overnight sleep benefit was markedly impaired in the older adults.
Older adults, the fMRI scans revealed, relied more heavily on their hippocampus, a brain region crucial for memory formation, to perform memory tasks. Younger adults, on the other hand, relied more on the mPFC.
Taken together, these findings suggest that, as we age, changes in the mPFC reduce slow-wave activity during sleep, which contributes to a decline in establishing long-term memory. As slow-wave activity wanes, the brain must rely more heavily for memory tasks on the hippocampus, a structure designed for short-term memory storage.
“When we are young, we have deep sleep that helps the brain store and retain new facts and information,” says Walker. “But as we get older, the quality of our sleep deteriorates and prevents those memories from being saved by the brain at night.”
This study helps explain the relationship between brain deterioration, sleep disruption and memory loss as we get older. The findings may give researchers insight into designing new approaches for treatment.
—by Meghan Mott, Ph.D.