Poor sleep quality may signal the risk of Alzheimer's disease in older adults, a study suggests.

People with Alzheimer's tend to wake up tired and their nights become even less refreshing as memory loss and other symptoms worsen. 

However, the reason was not fully understood.

The study, led by the Washington University in St. Louis found that older adults who sleep poorly or have less slow-wave sleep -- deep sleep needed to consolidate memories and wake up feeling refreshed -- have higher levels of tau -- a toxic brain protein.

Tau has also been linked to brain damage and cognitive decline.

"Measuring how people sleep may be a non-invasive way to screen for Alzheimer's disease before or just as people begin to develop problems with memory and thinking," said lead author Brendan Lucey, Assistant Professor from the varsity. 

Moreover, the findings, published in the journal Science Translational Medicine, showed that it was not the total amount of sleep that was linked to tau, but the slow-wave sleep, which reflects quality of sleep. 

The people with increased tau pathology were actually sleeping more at night and napping more in the day, but they weren't getting as good quality sleep.

"What's interesting is that we saw this inverse relationship between decreased slow-wave sleep and more tau protein in people who were either cognitively normal or very mildly impaired, meaning that reduced slow-wave activity may be a marker for the transition between normal and impaired," Lucey added.

For the study, the team studied 119 people aged 60 or older among which almost 80 per cent were cognitively normal and the remainder were very mildly impaired.

Up to two decades before Alzheimer's symptoms of memory loss and confusion appear, amyloid beta protein begins to collect into plaques in the brain. Tangles of tau appear later, followed by decline of key brain areas. Only then do people start showing unmistakable signs of cognitive decline.

The challenge is finding people on track to develop Alzheimer's before such brain changes undermine their ability to think clearly. For that, sleep may be a handy marker, the researchers said.