"We know that people with Alzheimer's disease often report depression and various sleep problems, like insomnia," said study author Abbas Dehghan, Ph.D., of Imperial College London in the United Kingdom. "We wanted to find out if there are causal relationships between different sleep patterns and depression and Alzheimer's."

Disturbed sleep patterns do not cause Alzheimer's disease but people who are at high genetic risk of developing Alzheimer's disease may be more likely to be a "morning person," have shorter sleep duration and other measures of sleep disturbance and are less likely to have insomnia, according to a study published in the August 19, 2020, online issue of Neurology.

"We know that people with Alzheimer's disease often report depression and various sleep problems, like insomnia," said study author Abbas Dehghan, Ph.D., of Imperial College London in the United Kingdom. "We wanted to find out if there are causal relationships between different sleep patterns and depression and Alzheimer's."

To evaluate the relationship between different sleep patterns, major depressive disorder, and Alzheimer's disease, researchers analysed the result of different genetic studies collected from databases that included: 21,982 people diagnosed with Alzheimer's disease who were compared to 41,944 without Alzheimer's disease; 9,240 with major depressive disorder who were compared to 9,519 without major depressive disorder; and 446,118 people with measurements of sleep-related characteristics.

Risk of Alzheimer's was determined based on genetic studies where Alzheimer's was diagnosed by autopsy or clinical examination.

Researchers analysed the genetic information using a study design called Mendelian randomisation that can determine if there is cause and effect.

Researchers found no evidence that sleep-related characteristics caused Alzheimer's disease. They also found no evidence of cause and effect between major depressive disorder and Alzheimer's.

Researchers did find a small association between the following: people with twice the genetic risk for Alzheimer's disease were 1% more likely to call themselves "morning people" compared to people at lower genetic risk; and people with twice the genetic risk of Alzheimer's had a 1% lower risk of insomnia. However, this effect of this association is small and shows only a possible link, not cause and effect.

A limitation of the study was that most of the people in the study were of European ancestry, so the results may not apply to the people of different ethnicity.

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