The researchers followed 2,750 people aged 50 and older for about five and a half years. Every year, participants took memory tests. Many also had brain scans to check for amyloid plaques and small brain damages called white-matter hyperintensities, both linked to future cognitive problems.

People were considered to have chronic insomnia if their medical records showed at least two insomnia diagnoses a month apart. About 16% of the participants met this definition.

Those with chronic insomnia showed faster declines in memory and thinking. They were 40% more likely to develop mild cognitive impairment or dementia during the study.

Insomnia combined with shorter sleep was especially harmful. These participants performed as if they were four years older at the first test. They also had higher levels of amyloid plaques and white-matter damage.

On the other hand, insomniacs who slept more than usual had less white-matter damage than average.

Amyloid plaques and blood-vessel damage both matter. Alzheimer’s disease is not caused by amyloid alone. Studies show that clogged or leaky small blood vessels can speed up cognitive decline. Together, these problems can make brain decline worse.

White-matter damage affects communication between brain regions. Amyloid plaques affect the neurons themselves. Having both increases the risk of dementia.

The study also confirmed that people carrying the ApoE4 gene, a known risk factor for Alzheimer’s, declined faster. Chronic insomnia had a similar impact on the brain as carrying this gene.

In short, poor sleep is more than just tiring. It may push the brain toward dementia over time.