Lack of "dreaming" sleep is linked to a higher risk of Alzheimer's disease, a study has found.
The finding is based on data from a US sleep study involving 321 participants over the age of 60 whose progress was monitored for 12 years.
Every percentage reduction in rapid eye movement (REM) sleep - the sleep phase during which most dreaming occurs - was associated with a 9pc higher chance of developing any type of dementia and an 8pc greater risk of Alzheimer's.
Lead researcher Dr Matthew Pase, from Boston University School of Medicine in the US, said: "Different stages of sleep may differentially affect key features of Alzheimer's disease.Our findings implicate REM sleep mechanisms as predictors of dementia."
While it is common for dementia patients to suffer disturbed sleep, whether this is a result of the condition or plays a role in causing it remains unclear.
Earlier this year the same team found that people who regularly sleep more than nine hours per night are twice as likely to develop dementia as people who sleep fewer hours.
The new research appears in the journal 'Neurology'.
Dr Alison Evans, from the charity Alzheimer's Research UK, said: "It is impossible to tell from this study whether disturbed REM sleep could be causing increased dementia risk or whether it's an early consequence of disease processes already under way in the brain. Larger studies involving more detailed testing will be necessary to better understand the complex relationship between sleep and dementia."
Meanwhile, thousands of cases of dementia could be prevented by increasing levels of lithium in tap water, another major study has suggested.
Scientists at the University of Copenhagen compared dementia rates to the natural quantities of lithium in water for more than 800,000 people in Denmark, from areas occupied by nearly half of the population.
They found that in places where lithium was highest, the dementia rate fell by 17pc compared with those with the lowest levels. Although researchers warn that the link could be due to other environmental factors, they say that it is worth investigating.
It was sitting on a bench under a tree in the grounds of Chatsworth House after a dog walk that Jane Clarke first broached the subject of dementia with her dad. A retired teacher, Brian was only 64, but Jane - a nutritionist who, by then, had worked with dementia patients for 10 years - was beginning to notice a change in him.