Scientists talked about the connection of deep sleep with memory impairment

Disturbances in the propagation of slow waves during deep sleep are associated with the accumulation of tau protein in the brain and memory impairment, neuroscientists at the University of California at Berkeley have found. How the brain's work changes during sleep and what it can mean for studying Alzheimer's disease — in the "Izvestia" material.

How does a dream relate to memories?

Episodic memory helps to remember where the car was parked near the store, who the conversation took place with yesterday, or how the first day of school went. It preserves information about the events of our lives along with the circumstances in which they occurred.

With age, such memories may become less detailed, and new information may be remembered worse. However, the severity of these changes varies: some people retain a good memory, while others face noticeable difficulties.

A study by Omer Sharon, a researcher at the Center for the Study of Human Sleep at the University of California at Berkeley, and his colleagues led by Matthew Walker is devoted to one of the possible explanations for these differences. Scientists have studied how the propagation of slow waves during sleep is associated with the consolidation of memories and the accumulation of a protein involved in the development of Alzheimer's disease.

What happens in the brain during deep sleep

A night's sleep consists of several repetitive cycles. They alternate between sleep with rapid eye movements, or REM sleep, and sleep without rapid eye movements, or NREM sleep. The latter includes three stages, of which the third corresponds to deep sleep. Therefore, all NREM sleep cannot be considered equally deep.

During deep sleep, large groups of neurons consistently switch between periods of reduced activity and activation. On an electroencephalogram, such changes manifest themselves in the form of slow waves.

The researchers drew attention to the sequence of these events. The wave can start in the frontal areas and spread to other areas of the cortex. According to Sharon, it is important for memory that changes in activity consistently cover significant areas of the brain. Thus, scientists were interested not only in the presence of slow waves or their severity, but also in how consistently they arise and how far they spread.

Why scientists called the waves "lonely"

The researchers called the local, weakly spreading events "lone waves." This term describes the features of the electrical activity of the brain. In older participants, such waves were more likely to remain confined to separate areas, instead of consistently spreading through the cortex. Scientists have tested whether this feature is related to how well a person retains new information overnight.

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In the evening, participants were given tasks to memorize word associations, and the next day they were checked how much material they remembered. People whose slow waves propagated worse and remained local more often retained less studied information.

The results linked three observations: accumulation of tau protein, changes in sleep waves, and deterioration in nighttime memory retention. At the same time, the study revealed a statistical relationship between them, rather than proving a causal sequence.

What role does tau protein play?

Tau protein is present in healthy neurons and helps maintain their internal structure. In Alzheimer's disease, it undergoes pathological changes, separates from structures that it normally stabilizes, and can form clusters inside cells.

Another characteristic feature of the disease is the accumulation of beta—amyloid. These protein changes are important areas of research in Alzheimer's disease. In the new work, the accumulation of pathological tau protein in the frontal areas, where propagating slow waves often occur, was important.

At the same time, the elderly participants in the main group did not have a diagnosis of Alzheimer's disease. According to Sharon, the detected memory changes remained within the age range.

These observations help to study the changes that may occur before the appearance of pronounced clinical disorders. However, they do not mean that every participant with tau protein accumulation will necessarily develop dementia later.

What did the repeated examinations show?

Some of the participants were re-examined several years later. In people who had increased accumulation of tau protein in the frontal areas during this time, the consistency of slow waves worsened. At the same time, the ability to preserve the studied material overnight decreased.

Repeated measurements made it possible to trace the changes in the same people. This complemented the initial comparison of young and older participants.

However, even a joint change in indicators over time does not establish which process begins first. The accumulation of abnormal protein may be associated with sleep disorders, but the direction of this relationship has yet to be determined.

Why did you need a cerebrospinal fluid analysis?

For additional verification, the researchers turned to an independent group of elderly people. Together with Yo-El Ju, a neurologist at Washington University in St. Louis, they compared the characteristics of sleep waves with indicators of cerebrospinal fluid.

In this group, more local slow waves were associated with an increased ratio of tau protein to beta-amyloid. A similar pattern appeared with a different method of assessing protein changes in other participants.

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The methods provided different information. PET allowed us to assess the distribution of tau protein across brain regions, while analysis of cerebrospinal fluid did not show exactly where it accumulated.

According to Sharon, the coincidence of the results in the independent group strengthens the evidence base. However, it also remains evidence of a connection rather than a definitive explanation of its mechanism.

Is it possible to protect memory by improving sleep

The study shows why it is important to take into account the sleep pattern when studying memory: the length of a night's rest does not describe all the features of the brain during this period. However, the authors did not test whether any treatment could restore the propagation of slow waves, reduce the accumulation of tau protein, or prevent Alzheimer's disease. Separate intervention studies are needed for such conclusions.

The general guidelines for sleep duration remain in place: most adults are recommended to sleep between seven and nine hours per night. This guideline relates to maintaining overall health and is not an established way of preventing dementia in the new work.

It is also important to assess how much memory changes affect daily life. According to the US National Institute of Aging, episodic forgetfulness can accompany normal aging. Difficulties that prevent you from doing your usual things, navigating, or using familiar objects require discussion with your doctor.