Remembering personal experiences like these depends on episodic memory, which allows us to recall events from our lives. These memories help form the foundation of long-term memory, whether they involve a childhood milestone, such as the first day of elementary school, or something that happened just minutes earlier, like parking your car.

This ability tends to weaken as we grow older. The sights, sounds, and other details associated with past experiences can become increasingly difficult to retrieve. Scientists still do not fully understand why this happens. Although some memory decline is a common part of aging, the changes can become far more severe in people developing Alzheimer's disease.

A new study from researchers at the University of California, Berkeley, offers important clues about what may be happening inside the aging brain. Neuroscientist Omer Sharon and his colleagues, working under the direction of former Berkeley sleep researcher Matthew Walker, discovered a connection between disrupted brain waves during deep sleep, declining memory performance, and the accumulation of tau, a protein associated with Alzheimer's disease.

The findings suggest that tau buildup in the brain's frontal cortex is linked to changes in the coordinated electrical activity that helps preserve memories overnight.

"This is the first time we've shown this relationship between tau and how it messes with memory consolidation by attenuating traveling slow waves that originate in the frontal cortex," said Sharon, a postdoctoral researcher at UC Berkeley's Center for Human Sleep Science.

The research was published in Nature Neuroscience.

How Deep Sleep Helps the Brain Store Memories

Sleep involves several distinct stages, each associated with different patterns of brain activity. Rapid eye movement sleep is particularly recognizable because the eyes move beneath closed eyelids and most dreaming takes place during this stage. However, another stage, known as non-REM sleep, is especially important for maintaining healthy brain function.

During this deeper stage of sleep, when dreaming is limited or absent, large groups of neurons briefly become inactive and then active again in a coordinated sequence. These synchronized changes produce slow electrical waves that spread across the brain, typically beginning in the frontal cortex.

"Each slow wave reflects an enormous population of neurons switching off and back on together," Sharon said. "Our study shows that for memory, it's critical that these events cascade in sequence across large parts of the brain. This coordinated shutdown has been observed only during deep sleep, and may be one reason why we sleep."

The researchers found that this carefully organized activity becomes less coordinated as tau accumulates in the aging brain. Instead of traveling together across broad regions, the waves become more scattered and cover shorter distances. These changes are associated with weaker memory consolidation, declining cognitive abilities, and Alzheimer's disease.

Alzheimer's Protein Linked to Disrupted Brain Waves

To investigate the relationship, the scientists monitored sleeping brain activity using electroencephalograms (EEGs). They compared cognitively healthy young adults in their early 20s with older participants ranging from their mid-60s to mid-70s.

The differences were striking. In younger participants, groups of slow waves moved together across distances roughly equivalent to the width of a hand on the scalp. Among older adults, the waves typically traveled less far and appeared more independently, rather than moving in coordinated groups.

The team also examined whether these changes were associated with tau accumulation. Working with Dr. William Jagust, a UC Berkeley professor emeritus of public health and neuroscience, the researchers used positron emission tomography (PET) scans to examine the brains of older participants. This imaging technique uses radioactive tracers introduced into the bloodstream to reveal biological activity and the distribution of specific substances.

The scans showed that greater tau buildup in the frontal cortex was associated with a breakdown in the longer traveling waves normally seen during deep sleep.

Importantly, these changes were present in people who had not developed Alzheimer's disease.

"These people did not have Alzheimer's," Sharon said. "They had tau in their brains, but with subclinical impacts. Their memory decline was within the normal range for their age."

Sharon explained that early Alzheimer's related changes can vary considerably in severity. The amount of tau present in the brain is associated with cognitive performance, even when a person's memory abilities remain within the expected range for their age.

Mysterious "Lonely Waves" Linked to Memory Loss

The researchers wanted to determine whether the altered sleep patterns were associated with actual differences in how well people remembered information.

To test this, participants learned pairs of associated words in the evening. The following day, they were asked to recall what they had learned, allowing the scientists to evaluate how effectively the memories had been retained overnight.

Participants whose slow brain waves traveled shorter distances and appeared more isolated generally remembered fewer word associations. The researchers called these disconnected patterns "lonely waves."

The team also tracked a subset of participants over several years to see how their brains changed with time. Among those whose frontal cortex tau levels increased, the slow waves became less coordinated, and their ability to retain newly learned information overnight declined.

These observations provide evidence of a relationship between tau accumulation, changes in deep sleep brain activity, and worsening episodic memory. However, they do not establish that tau buildup directly causes memory loss or that disrupted waves are the mechanism responsible.

Additional Tests Strengthen the Connection

Because PET scans are costly and require specialized facilities, the scientists sought another way to investigate the relationship between Alzheimer's related proteins and sleep activity.

They collaborated with neurologist Yo-El Ju at Washington University in St. Louis to analyze spinal fluid from a separate group of older adults. The goal was to determine whether measurements of Alzheimer's associated proteins would reveal patterns similar to those seen with brain imaging.

Unlike PET scans, the spinal fluid measurements could not identify tau accumulation specifically within the frontal cortex. Nevertheless, the results pointed in the same direction.

People whose sleeping brains produced more isolated slow waves had higher ratios of tau to amyloid, another protein closely associated with Alzheimer's disease, in their spinal fluid.

The consistency between these independent groups and testing methods strengthened the researchers' confidence in the findings.

"It's collected in a totally different place and in a different cohort, so I think that adds to the strength of the evidence," Sharon said.

Together, the sleep recordings, brain scans, spinal fluid analyses, and memory assessments suggest that tau accumulation is associated with a particular disruption of brain activity during deep sleep.

"This longitudinal correlation suggests that Alzheimer's disease pathology is associated with a disruption of this main sleep feature," Sharon said.

What Deep Sleep Reveals About the Aging Brain

Although memory often declines with age, the new research indicates that the process may involve more than simply growing older. Changes associated with Alzheimer's disease, particularly tau accumulation in regions responsible for generating widespread slow waves, may help explain why some people experience greater difficulty retaining memories.

"It's not just about age," Sharon said. "It's how much pathology you have in frontal areas where global waves originate."

Sleep quality also appears to be part of the picture. Although it can be difficult to define or measure precisely, the organization and movement of slow brain waves may provide important indicators of how effectively the brain performs its nighttime memory functions.

The findings raise the possibility that changes in deep sleep activity could eventually help researchers better understand or identify early memory problems. They also point toward potential targets for future treatments aimed at protecting memory as people age.

For now, however, a central question remains unanswered. Scientists still need to determine whether accumulating tau contributes to the breakdown of synchronized sleep waves, whether changes in those waves influence tau accumulation, or whether the relationship involves additional processes.

"We can see tau and lonely waves rising together over time," Sharon said. "What we cannot yet say is which leads -- and that question will shape how we design interventions."

The study's additional co-authors include Xi Chen, James Westphal, Chelsea Brown, and Vyoma D. Shah of UC Berkeley, along with Jason Dude of Washington University in St. Louis.

Funding came from grants awarded by the National Institute on Aging at the National Institutes of Health. Sharon also received support through a Glenn Foundation for Medical Research Postdoctoral Fellowship and a Weill Neurohub Fellowship.