People who often take a nap during the day are more at risk of developing dementia, scientists believe.
Those who like a daytime snooze tended to have more tau proteins that form tangles in the brain, causing dementia.
And a lack of deep sleep fuels rogue proteins in the brain that destroy neurons, say scientists.
A study of more than 100 older people found those not getting enough “quality” shut-eye also had more tau, leading to memory loss and confusion.
It could lead to nocturnal habits being monitored to help identify patients most at risk of the devastating condition.
In particular it is deep, restorative slow wave sleep (SWS) that is essential. This declines naturally as we age – in men from their mid 30s and women during their 50s.
First author Professor Brendan Lucey, director of the Washington University Sleep Medicine Centre in St Louis, said: “The key is it wasn’t the total amount of sleep that was linked to tau.
“It was the slow-wave sleep – which reflects quality of sleep.
“The people with increased tau were actually sleeping more at night and napping more in the day – but they weren’t getting as good quality sleep.”
The finding published in Science Translational Medicine adds to a growing body of evidence linking poor sleep to Alzheimer’s and other forms of dementia.
Daytime napping alone was significantly associated with high levels of tau.
It means doctors could identify patients who could benefit from further testing simply by asking: “How much do you nap during the day?”
Last year a similar study by another US team found those who napped during the day – when they should have been awake – had almost three times as much amyloid beta.
This is another damaging protein that can trigger dementia by clumping together in grey matter and forming plaques.
Prof Lucey said fewer slow brain waves that occur during the most refreshing part of the sleep cycle is associated with high levels of the other toxic brain protein tau.
He said: “What’s interesting is 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 reduced slow-wave activity may be a marker for the transition between normal and impaired.
“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.”
Poor sleep is a hallmark of Alzheimer’s. People with the disease tend to wake up tired – and their nights become even less refreshing as memory loss and other symptoms worsen.
But the connections between restless nights and Alzheimer’s are not fully understood. Prof Lucey and colleagues believe they have uncovered part of the explanation.
Participants who had less SWS – that consolidates an memories and leaves us waking up feeling refreshed – had more tau.
This is a sign of Alzheimer’s and has been linked to brain damage and cognitive decline.
It suggests poor-quality sleep in later life could be a red flag for deteriorating brain health, said Prof Lucey.
Alzheimer’s affects an estimated 850,000 people in the UK and 5.7 million Americans – with the numbers expected to more than double by 2050. There is no cure.
But the illness starts slowly and silently. Up to two decades before the characteristic symptoms appear, amyloid beta begins to collect in the brain.
Then the tau tangles occur – followed by destruction 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.
So the researchers analysed 119 volunteers aged 60 and over – 80 percent of whom were mentally healthy with the remainder being mildly impaired.
Their sleep was monitored at home over the course of a normal week by them wearing portable skull caps in bed fitted with electrodes that measured their brain waves.
A watch-like sensor worn on the wrist tracked body movement and they also kept sleep logs – noting both night time sleep sessions and daytime napping.
Each participant produced at least two nights of data – with some having as many as six.
PET scans and spinal taps were used to measure levels of amyloid beta and tau and cerebrospinal fluid that bathes the brain and spinal cord, respectively.
Decreased SWS coincided with higher levels of tau in the brain and a higher tau-to-amyloid ratio in the cerebrospinal fluid.
This was after taking into account factors such as sex, age and movements while sleeping.
If future research bears out the results sleep monitoring may be an easy and affordable way to screen earlier for Alzheimer’s, said the researchers.
Prof Lucey said: “I don’t expect sleep monitoring to replace brain scans or cerebrospinal fluid analysis for identifying early signs of Alzheimer’s disease – but it could supplement them.
“It’s something that could be easily followed over time – and if someone’s sleep habits start changing that could be a sign for doctors to take a closer look at what might be going on in their brains.”
Brits are among the most sleep deprived people in the world with almost four in ten saying they do not get the recommended 7 to 8 hours a night.