Subtle shifts in how you sleep could be an early indicator of your dementia risk, a study suggests.

Everyone has micro awakenings while they sleep, very short periods - usually just a few seconds - when the brain becomes partially awake.

Part of a healthy sleep cycle, people typically experience dozens of these every night and they normally do not remember them. 

Now, however, researchers in Europe are warning that having more micro awakenings at night could be a warning sign that someone is at higher risk of cognitive decline.

In a study tracking 500 adults for up to seven years, scientists found that middle-aged adults who had more micro awakenings were more likely to also have genetic markers that raise their risk of Alzheimer's disease. 

This included genetic markers such as the APOE4 gene, which is linked to a 60 percent higher risk of an Alzheimer's diagnosis by age 85.

'These micro awakenings are therefore not insignificant,' said co-author Dr Puneet Talwar, a sleep researcher at the University of Liege, Belgium.

'Certain profiles could promote the accumulation of proteins involved in Alzheimer's disease and be associated with increased vulnerability.'

About seven million Americans are living with Alzheimer's, estimates suggest, but that number is expected to nearly double over the next 24 years. 

Scientists have warned repeatedly that sleep disruption, poor quality sleep and daytime sleepiness may all raise the risk of Alzheimer's disease, possibly because it can cause harmful proteins to build-up in the brain. It was not clear how the genetic markers may disrupt sleep. 

The team behind the paper said their research may have revealed a potential method for detecting Alzheimer's risk years before symptoms appear. Typically, the condition is not diagnosed until someone turns 65 years or older. 

In the study, published in the journal Sleep, scientists tracked 453 adults with an average age of 22 years and 87 middle-aged adults with an average age of 59 years. 

Overall, younger adults slept for about seven hours and 30 minutes per night while middle-aged adults slept for about six hours and 30 minutes per night. 

The younger participants experienced about 250 micro awakenings every night while older participants experienced about 180 micro awakenings. 

After analyzing the data, the researchers said that those experiencing more micro awakenings every night were more likely to have genes that raised their risk of suffering from Alzheimer's. 

They also used brain scans to analyze the locus coeruleus, a region in the brainstem that controls the sleep-wake cycle.

The team found that sleep quality, the ability to fall asleep and sleep depth, was dependent on the proper functioning of the locus coeruleus. 

It was also important for rapid-eye movement sleep, the phase of sleep in which dreaming and memory consolidation occurs.

Previous research has also shown that it is one of the first places where abnormal protein deposits appear. 

'Sleep could become an accessible marker for the early identification of vulnerable individuals,' said Gilles Vandewalle, a sleep researcher and co-author of the study. 

And Lucie Leroux, head of French-speaking activities at the Stop Alzheimer’s Foundation, which funded the research, added: 'This research shows that sleep is not only an indicator of health, but also a potential lever for intervention.