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.
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
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 Center 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.
Dr 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
