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.
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.
‘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.
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.
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 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 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 easy to rouse.
But the second is the deepest kind of sleep you experience.
In this new study, published in the journal Science Advances, Dr Nedergaard found that deep non-REM sleep is ‘optimal’ for the function of the glymphatic system.
For the new study, researchers conducted experiments with mice that were anesthetized using six different combinations of drugs.
While the animals were under sedation, the researchers tracked brain electrical activity, cardiovascular activity, and the cleansing flow of CSF through the brain.
They saw that a combination of the drugs ketamine and xylazine (K/X) most closely replicated the slow and steady electrical activity in the brain and slow heart rate associated with deep non-REM sleep.
And the electrical activity in the brains of mice administered K/X appeared to be optimal for function of the glymphatic system.
Study first author Dr Lauren Hablitz said: ‘The synchronized waves of neural activity during deep slow-wave sleep, specifically firing patterns that move from front of the brain to the back, coincide with what we know about the flow of CSF in the glymphatic system.
‘It appears that the chemicals involved in the firing of neurons, namely ions, drive a process of osmosis which helps pull the fluid through brain tissue.’
The study further bolsters the link between sleep, ageing, and Alzheimer’s.
It is known that as we age it becomes more difficult to consistently achieve deep non-REM sleep, and the study reinforces the importance of deep sleep to the proper function of the glymphatic system.
The researchers said that their findings also show that the glymphatic system can be manipulated by enhancing sleep, a finding that may point to potential clinical approaches, such as sleep therapy or other methods to boost the quality of sleep of those at risk.
They said the study also sheds light on the cognitive difficulties that older patients often experience after surgery and suggests classes of drugs that could be used to avoid the problem.
Mice in the study that were exposed to anaesthetics that did not induce slow brain activity saw diminished glymphatic activity.
Study co-author Dr Tuomas Lilius, of the University of Copenhagen in Denmark, added: ‘Cognitive impairment after anaesthesia and surgery is a major problem.
‘A significant percentage of elderly patients that undergo surgery experience a postoperative period of delirium or have a new or worsened cognitive impairment at discharge.’