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They say you need to get eight hours' sleep to prevent all kinds of diseases, from diabetes to dementia.
But new research shows it's all for nothing if you're not getting quality sleep.
Fact - Tests - Mice - Hours - Quality
In fact, tests on mice showed those who got fewer hours but higher quality sleep had a lower risk of dementia than those who tossed and turned for longer.
Study lead author Professor Maiken Nedergaard, of University of Rochester Medical Center (URMC), said: 'Sleep is critical to the function of the brain's waste removal system and this study shows that the deeper the sleep the better.
Findings - Evidence - Quality - Sleep - Deprivation
'These findings also add to the increasingly clear evidence that quality of sleep or sleep deprivation can predict the onset of Alzheimer's and dementia.'
The study is a crucial development in sleep science, building on Dr Nedergaard's discovery in 2012 of the 'glymphatic system', the brain's unique process of removing waste while we sleep.
Paper - System - Blood - Vessels - Pumps
That paper identified a system of plumbing which 'piggybacks' on blood vessels and pumps cerebral spinal fluid (CSF) through brain tissue to wash away waste.
Her next paper showed that the system primarily works while we sleep.
Findings - Link - Research - Dementia - Research
The findings provided a link between sleep research and dementia research.
We knew dementia seemed to have its seeds in build-ups of plaque in the brain, clumps of waste.
Dr - Nedergaard - Function - Brain - Waste
Dr Nedergaard suggested that poor function of our brain's waste disposal may exacerbate that.
Her hunch, shared by others in the field, led to a boom in research into how we sleep. There are different types of sleep: non-REM, and REM.
REM - Sleep - Lasts - Minutes - Time
REM sleep lasts for 10 minutes at a time at various points during your shut-eye. Those are the moments when you experience vivid dreams and your heart rate speeds up.
The first kind of non-REM sleep is when you're barely asleep and...
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