BioEd Online

Deterioration of a specific brain region impairs sleep quality in older adults, leading to poorer memory retention, according to new research published today in Nature Neuroscience¹. 

Ageing is associated with the gradual loss of brain cells, sleep disturbances, and declining memory function, but how these factors are related to each other is unclear.

Neuroscientist Bryce Mander of the University of California, Berkeley, and his colleagues recruited 18 healthy young adults and 15 older adults, all with normal mental function, and asked them to memorise a list of word pairs.

The participants were asked to recall the words ten minutes later, and then left to sleep overnight, while the researchers recorded the electrical activity of their brains. The next morning, they recalled the list again while having their brains scanned.

In keeping with earlier studies, the older adults performed worse on the memory test, and showed significant reductions in the slow brain waves associated with deep sleep, compared to the younger ones.

The extent of deep sleep disruption was related to the degree of memory impairment, with those exhibiting the least slow wave activity performing the worst. These differences were in turn associated with a reduction of grey matter in the medial prefrontal cortex (mPFC).

“We’ve known for decades that sleep is disrupted in older adults, but we didn't know why,” says Mander. “Our findings show that brain deterioration, bad memory and bad sleep are not independent, but instead are significantly inter-related.”

It’s well established that sleep strengthens newly formed memories, and slow brain waves are thought to enhance the transfer of information from the hippocampus, a brain structure that is critical to memory formation, to other parts of the brain for long-term storage.   

The new findings therefore suggest that deterioration of the mPFC diminishes the slow waves that occur during deep sleep, such that older adults are less able than younger ones to solidify new facts while they sleep. 

“This study provides valuable insight into the relationship between sleep and cognition,” says Roxanne Sterniczuk, a neurophysiologist at Dalhousie University in Halifax, Canada, “but caution should be taken that the observed brain abnormalities may represent changes indicative of early neurodegeneration.”

“The inability to examine the participants’ brains for pathology is a major limitation,” she adds. “It would be interesting to follow the older adults over time, or add a dementia group and compare the differences.”

Last year, Sterniczuk and her colleagues presented preliminary findings showing that sleep disturbances accurately predict a subsequent diagnosis of Alzheimer’s Disease.

“Sleep disruption is even more pronounced in Alzheimer's,” says Mander, “so a good next step will be to see if sleep disruption in these populations is associated with their memory symptoms. If so, targeting sleep may reduce some of their deficits.”