How does sleep length affect dementia risk?

Evan Walker
Evan Walker TheMediTary.Com |
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New research adds to the evidence that sleep hygiene plays an important role in brain health. Image credit: Johner Images/Getty Images.
  • Sleeping for more or less than the optimal duration of 7 to 9 hours is associated with an increased risk of dementia and stroke.
  • Brain imaging biomarkers that measure structural changes in the brain using magnetic resonance imaging (MRI) scans can predict the onset of stroke or dementia years before they occur.
  • A new study shows that suboptimal sleep duration is associated with an increased presence of brain biomarkers of dementia and stroke in healthy middle-aged individuals.
  • This study further underscores the role of suboptimal sleep duration in brain health and the need to improve sleep hygiene.

Experts recommend that adults get between 7 and 9 hours of sleep each night. Indeed, studies have shown that sleeping 7 to 8 hours a night is favorably associated with health while shorter or longer sleep durations are linked with adverse health outcomes. Suboptimal sleep duration is also a risk factor for stroke and dementia.

A new study published in the Journal of the American Heart Association now shows that healthy individuals aged 40 to 69 years sleeping more or less than the optimal duration were more likely to show elevated expression of brain biomarkers of stroke and dementia.

Study author Dr. Santiago Clocchiatti-Tuozzo, a postdoctoral fellow at the Yale School of Medicine, noted that these results further support the role of sleep in long-term health.

Dr. Clocchiatti-Tuozzo explained to Medical News Today that, “[b]ecause these magnetic resonance imaging markers of poor brain health are known to precede the occurrence of stroke and dementia by many years, there may be merit in assessing and managing abnormal sleep duration among asymptomatic middle‐aged adults.”

“Both short and long sleep durations may potentially be modifiable risk factors for magnetic resonance imaging markers of poor brain health,” added Dr. Clocchiatti-Tuozzo.

Studies have consistently shown that insufficient or excessive sleep is linked to an increased risk of conditions such as diabetes, cardiovascular disease, high blood pressure, and depression.

The link between suboptimal sleep with metabolic and cardiovascular diseases has led the American Heart Association to include sleep among the eight key components that constitute Life’s Essential 8.

Life’s Essential 8 consists of measurable components such as sleep, diet, and physical exercise that influence the risk of cardiovascular disease.

Studies have shown that suboptimal sleep duration is associated with an increased risk of cardiovascular conditions like stroke and coronary artery disease.

Similarly, shorter sleep duration in midlife is also associated with an increased risk of cognitive decline and dementia later in life.

Dr. José Morales, a vascular neurologist and neurointerventional surgeon at Pacific Neuroscience Institute in Santa Monica, CA, noted that studies conducted so far have not shown a link between sleep duration and brain health.

Dr. Morales, who was not involved in the current study, explained:

“Long sleep duration has in the past been associated with increased risk of mortality and morbidity. However, these historical findings did not explicitly implicate compromised brain health.”

In the present study, the researchers examined whether suboptimal sleep duration was associated with brain health in healthy individuals without stroke or dementia aged over 40.

Stroke and dementia are preceded by structural changes in the brain’s white matter, which is made up of nerve fibers or axons that transmit information between nerve cells.

These changes in the structure of white matter include white matter hyperintensities and fractional anisotropy, which can be detected using magnetic resonance imaging (MRI).

White matter hyperintensities appear as bright white spots during MRI scans. White matter hyperintensities are more frequent with increasing age and are often caused by damage to the white matter owing to the disease of small blood vessels in the brain.

Fractional anisotropy is a measure of white matter integrity. Decreased white matter integrity and white matter hyperintensities are observed several years before a stroke or onset of dementia, thus serving as biomarkers for these conditions.

The present study consisted of 39,771 individuals enrolled in the UK Biobank study between 2006 and 2010. The UK Biobank is a long-term population-based study designed to understand the role of genetic and environmental factors in disease development.

These participants were aged 40 to 69 at the time of enrollment (baseline) in the UK Biobank study. The researchers used MRI to scan the participants’ brains for changes in white matter at an average of 9 years after enrollment.

The researchers used self-reported sleep duration data collected at the time of enrollment and at the end of the follow-up period, that is, during the brain scans. Sleep duration was calculated by averaging the total time spent sleeping daily, including time spent napping during the daytime.

Individuals sleeping between seven and less than nine hours daily were categorized as sleeping for the optimal duration. Participants receiving less than 7 hours or in excess of 9 hours of sleep were classified as sleeping for an inadequate or excessive duration, respectively.

Individuals with stroke or dementia were excluded from the study due to the brain damage associated with these conditions.

Based on the sleep duration interviews conducted at the time of enrollment, the researchers found that the participants who slept less or more than the optimal duration were more likely to show greater signs of damage to white matter than those sleeping for an optimal duration.

Specifically, suboptimal sleep duration was associated with a larger volume of white matter hyperintensities (lesions) and a greater decline in white matter integrity.

Similar results were obtained in a follow-up analysis when the sleep duration at the time of the brain scans was used to assess the association between sleep and white matter damage.

In addition, the association between sleep duration and white matter damage was also observed after adjusting the analysis for several variables, including age, sex, body mass index, and cardiovascular risk factors, such as diabetes, high blood pressure, and smoking.

The study thus shows that suboptimal sleep duration could potentially have an adverse impact on brain health. However, whether this link is causal needs to be established.

Dr. Clocchiatti-Tuozzo noted that “[b]etter understanding of the directionality and mechanisms of the association between suboptimal sleep and poor brain health is needed, as well as clinical trials aimed at unveiling whether early interventions on sleep duration during middle age can potentially benefit brain health later in life.”

The large number of participants included in this brain imaging study was a major strength of the study. Such a large sample size is generally uncommon for imaging studies.

Dr. Balaji Krishnaiah, Neurology Medical Director at Methodist Le Bonheur Healthcare in Memphis, TN, not involved in the current study, commented that:

“The study is one of the largest population health studies globally, with nearly 40,000 middle-aged participants from the UK Biobank, adding robustness to the findings. The prospective nature of the study, assessing sleep duration approximately 9 years before neuroimaging, provides temporal depth to the results.”

Nevertheless, one of the study’s major limitations was its observational design. Dr. Clocchiatti-Tuozzo said that: “Due to the observational nature of our study, we are precluded from making causal inferences between sleep duration and poor brain health. In addition, the directionality of such a causal relationship remains indeterminate.”

“The generalizability of our results is limited by the characteristics of the UK Biobank study, which is being conducted in a single European country and may be prone to healthy volunteer bias, where individuals who are willing and able to participate in a study tend to be healthier than the source population,” he added.

Dr. Morales also noted: “It is a retrospective study using self-reported sleep durations, and the latter may not be reliable data inputs. Future research should incorporate objective measures of sleep duration (rather than self-reported) — and more importantly sleep quality — for correlation with these imaging biomarkers.”

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