http://www.bbc.co.uk/news/health-24567412
James Gallagher
The brain uses sleep to wash away the waste toxins built up during a hard day's thinking, researchers have shown.
The US team believe the "waste removal system" is one of the fundamental reasons for sleep.
Their study, in the journal Science, showed brain cells shrink during sleep to open up the gaps between neurons and allow fluid to wash the brain clean.
They also suggest that failing to clear away some toxic proteins may play a role in brain disorders.
One big question for sleep researchers is why do animals sleep at all when it leaves them vulnerable to predators?
It has been shown to have a big role in the fixing of memories in the brain and learning, but a team at the University of Rochester Medical Centre believe that "housework" may be one of the primary reasons for sleep.
"The brain only has limited energy at its disposal and it appears that it must choose between two different functional states - awake and aware or asleep and cleaning up," said researcher Dr Maiken Nedergaard.
"You can think of it like having a house party. You can either entertain the guests or clean up the house, but you can't really do both at the same time."
Plumbing
Their findings build on last year's discovery of the brain's own network of plumbing pipes - known as the glymphatic system - which carry waste material out of the brain.
Scientists, who imaged the brains of mice, showed that the glymphatic system became 10-times more active when the mice were asleep.
Cells in the brain, probably the glial cells which keep nerve cells alive, shrink during sleep. This increases the size of the interstitial space, the gaps between brain tissue, allowing more fluid to be pumped in and wash the toxins away.
Dr Nedergaard said this was a "vital" function for staying alive, but did not appear to be possible while the mind was awake.
She told the BBC: "This is purely speculation, but it looks like the brain is losing a lot of energy when pumping water across the brain and that is probably incompatible with processing information."
She added that the true significance of the findings would be known only after human studies, but doing similar experiments in an MRI machine would be relatively easy.
Commenting on the research Dr Neil Stanley, an independent sleep expert, said: "This is a very interesting study that shows sleep is essential downtime to do some housekeeping to flush out neurotoxins.
"There is good data on memory and learning, the psychological reason for sleep. But this is the actual physical and chemical reason for sleep, something is happening which is important."
Dr Raphaelle Winsky-Sommerer, a lecturer in sleep at Surrey University, said: "It's not surprising, our whole physiology is changing during sleep.
"The novelty is the role of the interstitial space, but I think it's an added piece of the puzzle not the whole mechanism.
"The significance is that, yet again, it shows sleep may contribute to the restoration of brain cell function and may have protective effects."
Many conditions which lead to the loss of brain cells such as Alzheimer's or Parkinson's disease are characterised by the build-up of damaged proteins in the brain.
The researchers suggest that problems with the brain's cleaning mechanism may contribute to such diseases, but caution more research is needed.
The charity Alzheimer's Research UK said more research would be needed to see whether damage to the brain's waste clearance system could lead to diseases like dementia, but the findings offered a "potential new avenue for investigation".
Boosting sleep 'may slow memory rot'
James Gallagher
It may be possible to slow the decline in memory and learning as we age by tackling poor sleep, researchers hope.
Their study, in the journal Nature Neuroscience, has revealed an intimate relationship between an ageing brain, sleep and memory.
Experiments showed that changes in the ageing brain damaged the quality of deep sleep, this in turn hampered the ability to store memories.
Scientists want to test ways of boosting sleep to halt memory decline.
Wisdom may come with age, but both the brain and the body take the strain of time. Many people will be aware that both their memory and sleep are not as good as in their earlier years.
Researchers at the University of California, Berkeley, said it was unknown whether changes in the brain, sleep and memory were all separate signs of ageing or deeply connected.
A good kip
They performed a series of experiments on 36 people - an even split of those in their twenties and their seventies.
First the team showed that they could use the state of a region of the brain, called the medial prefronal cortex, to predict how much deep, or slow-wave, sleep a person would have.
That part of the brain is essential for entering deep sleep, but with age the region degrades.
Secondly, they showed that the amount of deep sleep could be used to predict how well people would do on memory tests.
The younger patients getting loads of good quality sleep performed better on tests than their older colleagues who had worse quality sleep.
One of the researchers, Dr Matthew Walker told the BBC: "Taken all together, the deterioration of the brain leads to the deterioration of sleep to the deterioration of memory."
"Slow wave sleep is critically important for cementing new memories you've recently learned. It's like clicking the save button.
"It's especially depressing as I continue to get older, but there might be a silver lining."
Researchers are not able to restore the ageing section of the brain, but they believe they can do something about sleep.
It is possible to boost the quality of sleep by stimulating the right region of the brain with electricity during the night.
The researchers said this had been shown to boost memory performance in young people and they wanted to begin experiments on elderly patients too.
"You don't have to restore brain cells to restore sleep," said Dr Walker who described their aim as "jump-starting" the system.
Dementia
In patients with dementia, the symptoms of brain cells dying, bad sleep and memory loss are far far worse than in normal ageing.
Some studies have suggested a link between and dementia. A report,published in the journal Science Translational Medicine, suggested problems sleeping may be an early sign of Alzheimer's.
Dr Simon Ridley, from the charity Alzheimer's Research UK, said further studies were needed to confirm the link.
"Increasing evidence has linked changes in sleep to memory problems and dementia, but it's not clear whether these changes might be a cause or consequence.
"The people studied here were followed for a very short period, and one next step could be to investigate whether a lack of 'slow-wave' sleep may also be linked to a long-term decline in memory."
The myth of the eight-hour sleep
Stephanie Hegarty
We often worry about lying awake in the middle of the night - but it could be good for you. A growing body of evidence from both science and history suggests that the eight-hour sleep may be unnatural.
In the early 1990s, psychiatrist Thomas Wehr conducted an experiment in which a group of people were plunged into darkness for 14 hours every day for a month.
It took some time for their sleep to regulate but by the fourth week the subjects had settled into a very distinct sleeping pattern. They slept first for four hours, then woke for one or two hours before falling into a second four-hour sleep.
Though sleep scientists were impressed by the study, among the general public the idea that we must sleep for eight consecutive hours persists.
In 2001, historian Roger Ekirch of Virginia Tech published a seminal paper, drawn from 16 years of research, revealing a wealth of historical evidence that humans used to sleep in two distinct chunks.
His book At Day's Close: Night in Times Past, published four years later, unearths more than 500 references to a segmented sleeping pattern - in diaries, court records, medical books and literature, from Homer's Odyssey to an anthropological account of modern tribes in Nigeria.
Much like the experience of Wehr's subjects, these references describe a first sleep which began about two hours after dusk, followed by waking period of one or two hours and then a second sleep.
"It's not just the number of references - it is the way they refer to it, as if it was common knowledge," Ekirch says.
During this waking period people were quite active. They often got up, went to the toilet or smoked tobacco and some even visited neighbours. Most people stayed in bed, read, wrote and often prayed. Countless prayer manuals from the late 15th Century offered special prayers for the hours in between sleeps.
And these hours weren't entirely solitary - people often chatted to bed-fellows or had sex.
A doctor's manual from 16th Century France even advised couples that the best time to conceive was not at the end of a long day's labour but "after the first sleep", when "they have more enjoyment" and "do it better".
Ekirch found that references to the first and second sleep started to disappear during the late 17th Century. This started among the urban upper classes in northern Europe and over the course of the next 200 years filtered down to the rest of Western society.
By the 1920s the idea of a first and second sleep had receded entirely from our social consciousness.
He attributes the initial shift to improvements in street lighting, domestic lighting and a surge in coffee houses - which were sometimes open all night. As the night became a place for legitimate activity and as that activity increased, the length of time people could dedicate to rest dwindled.
When segmented sleep was the norm
- "He knew this, even in the horror with which he started from his first sleep, and threw up the window to dispel it by the presence of some object, beyond the room, which had not been, as it were, the witness of his dream." Charles Dickens, Barnaby Rudge (1840)
- "Don Quixote followed nature, and being satisfied with his first sleep, did not solicit more. As for Sancho, he never wanted a second, for the first lasted him from night to morning." Miguel Cervantes, Don Quixote (1615)
- "And at the wakening of your first sleepe You shall have a hott drinke made, And at the wakening of your next sleepe Your sorrowes will have a slake." Early English ballad, Old Robin of Portingale
- The Tiv tribe in Nigeria employ the terms "first sleep" and "second sleep" to refer to specific periods of the night
In his new book, Evening's Empire, historianCraig Koslofsky puts forward an account of how this happened.
"Associations with night before the 17th Century were not good," he says. The night was a place populated by people of disrepute - criminals, prostitutes and drunks.
"Even the wealthy, who could afford candlelight, had better things to spend their money on. There was no prestige or social value associated with staying up all night."
That changed in the wake of the Reformation and the counter-Reformation. Protestants and Catholics became accustomed to holding secret services at night, during periods of persecution. If earlier the night had belonged to reprobates, now respectable people became accustomed to exploiting the hours of darkness.
This trend migrated to the social sphere too, but only for those who could afford to live by candlelight. With the advent of street lighting, however, socialising at night began to filter down through the classes.
In 1667, Paris became the first city in the world to light its streets, using wax candles in glass lamps. It was followed by Lille in the same year and Amsterdam two years later, where a much more efficient oil-powered lamp was developed.
London didn't join their ranks until 1684 but by the end of the century, more than 50 of Europe's major towns and cities were lit at night.
Night became fashionable and spending hours lying in bed was considered a waste of time.
"People were becoming increasingly time-conscious and sensitive to efficiency, certainly before the 19th Century," says Roger Ekirch. "But the industrial revolution intensified that attitude by leaps and bounds."
Strong evidence of this shifting attitude is contained in a medical journal from 1829 which urged parents to force their children out of a pattern of first and second sleep.
"If no disease or accident there intervene, they will need no further repose than that obtained in their first sleep, which custom will have caused to terminate by itself just at the usual hour.
"And then, if they turn upon their ear to take a second nap, they will be taught to look upon it as an intemperance not at all redounding to their credit."
Today, most people seem to have adapted quite well to the eight-hour sleep, but Ekirch believes many sleeping problems may have roots in the human body's natural preference for segmented sleep as well as the ubiquity of artificial light.
The condition first appears in literature at the end of the 19th Century, at the same time as accounts of segmented sleep disappear.This could be the root of a condition called sleep maintenance insomnia, where people wake during the night and have trouble getting back to sleep, he suggests.
"For most of evolution we slept a certain way," says sleep psychologist Gregg Jacobs. "Waking up during the night is part of normal human physiology."
The idea that we must sleep in a consolidated block could be damaging, he says, if it makes people who wake up at night anxious, as this anxiety can itself prohibit sleeps and is likely to seep into waking life too.
Russell Foster, a professor of circadian [body clock] neuroscience at Oxford, shares this point of view.
"Many people wake up at night and panic," he says. "I tell them that what they are experiencing is a throwback to the bi-modal sleep pattern."
A small city like Leipzig in central Germany employed 100 men to tend to 700 lamps
Stages of sleep
Every 60-100 minutes we go through a cycle of four stages of sleep
- Stage 1 is a drowsy, relaxed state between being awake and sleeping - breathing slows, muscles relax, heart rate drops
- Stage 2 is slightly deeper sleep - you may feel awake and this means that, on many nights, you may be asleep and not know it
- Stage 3 and Stage 4, or Deep Sleep - it is very hard to wake up from Deep Sleep because this is when there is the lowest amount of activity in your body
- After Deep Sleep, we go back to Stage 2 for a few minutes, and then enter Dream Sleep - also called REM (rapid eye movement) sleep - which, as its name suggests, is when you dream
In a full sleep cycle, a person goes through all the stages of sleep from one to four, then back down through stages three and two, before entering dream sleep
Source: Gregg Jacobs
But the majority of doctors still fail to acknowledge that a consolidated eight-hour sleep may be unnatural.
"Over 30% of the medical problems that doctors are faced with stem directly or indirectly from sleep. But sleep has been ignored in medical training and there are very few centres where sleep is studied," he says.
Jacobs suggests that the waking period between sleeps, when people were forced into periods of rest and relaxation, could have played an important part in the human capacity to regulate stress naturally.
In many historic accounts, Ekirch found that people used the time to meditate on their dreams.
"Today we spend less time doing those things," says Dr Jacobs. "It's not a coincidence that, in modern life, the number of people who report anxiety, stress, depression, alcoholism and drug abuse has gone up."
So the next time you wake up in the middle of the night, think of your pre-industrial ancestors and relax. Lying awake could be good for you.
Craig Koslofsky and Russell Foster appeared on The Forum from the BBC World Service. Listen to the programmehere.
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