Poor sleep may be an indication of increased risk for Alzheimer’s disease, a new study of older people suggests.

Researchers studied 101 cognitively normal people, average age 63, who completed well-validated sleep questionnaires. They analyzed their spinal fluid for the presence of indicators of the plaques and tangles that are characteristic of Alzheimer’s. The study is in Neurology.

After controlling for age, a family history of Alzheimer’s, the ApoE gene that increases Alzheimer’s risk and other factors, they found that poor sleep quality, sleep problems and daytime sleepiness were associated with increased spinal fluid indicators of Alzheimer’s disease.

The reason for the association is unclear, but at least one animal study found that during sleep the brain’s capacity to clear toxins like beta amyloid, the toxic protein that forms plaques in the brains of those with Alzheimer’s, improves. It may be that poor sleep interferes with this process in people, too.