The findings show that the brain flushes out toxic material, and suggest a new biological purpose for sleep and indicate that waste disposal may underlie its restorative properties.
There could also be far reaching implications for understanding and treating diseases such as Alzheimer's.
"This study shows that the brain has different functional states when asleep and when awake," said US researcher Dr Maiken Nedergaard, from the University of Rochester. "In fact, the restorative nature of sleep appears to be the result of the active clearance of the by-products of neural activity that accumulate during wakefulness."
The findings, published in the journal Science, show that the brain's unique method of cleansing itself - known as the glymphatic system - is highly active during sleep.
As we slumber, it clears away toxins that would otherwise build up and trigger neurological disorders such as Alzheimer's disease.
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If you find yourself hungry all day (and not because you skipped breakfast or have recently amped up your gym routine) it might be because you've been skimping on sleep. Research presented at the 2010 meeting of the Society for the Study of Ingestive Behavior linked little shuteye with <a href="http://www.huffingtonpost.com/2012/07/10/sleep-hunger-deprivation-_n_1659954.html">higher levels of the hormone ghrelin</a>, the same one that triggers hunger, HuffPost reported. This uptick in the hunger hormone seems to lead to not only increased snacking, but also a hankering for <a href="http://www.sciencedaily.com/releases/2004/12/041206210355.htm">high-carb, high-calorie foods</a>, according to a 2004 study, which may help explain why people who don't get enough sleep are at a greater risk of obesity.
Ever find yourself tearing up over an embarrassing TV commercial? While women might be quick to blame PMS, it could be a lack of sleep sending your emotions into overdrive. A 2007 study found that sleep-deprived brains were <a href="http://usatoday30.usatoday.com/news/health/2007-10-22-sleep-deprivation-brain_N.htm">60 percent more reactive</a> to negative and disturbing images, <em>USA Today</em> reported. "It's almost as though, without sleep, the brain had reverted back to more primitive patterns of activity, in that it was <a href="http://berkeley.edu/news/media/releases/2007/10/22_sleeploss.shtml">unable to put emotional experiences into context</a> and produce controlled, appropriate responses," Matthew Walker, senior author of the study, said in a statement.
You're Forgetful Or Unfocused
You might be tempted to blame your trouble focusing on your age or stress or your overflowing email inbox, but a lack of sleep could be the true culprit. Too few hours in dreamland has been linked to a <a href="http://www.webmd.com/sleep-disorders/excessive-sleepiness-10/emotions-cognitive">whole host of cognitive problems</a>, like difficulty focusing and paying attention, confusion, lower alertness and concentration, forgetfulness and trouble learning, WebMD reports. So next time you find yourself forgetting where you put your keys, consider how much sleep you got last night.
You Can't Shake That Cold
If you keep coming down with the sniffles -- or can't seem to kick that never-ending case -- you might want to assess your sleep schedule. A 2009 study found that people who sleep fewer than seven hours each night have almost <a href="http://articles.latimes.com/2009/jan/17/science/sci-sleep17">three times the risk of catching a cold</a> than people who slept for at least eight hours, the <em>LA Times</em> reported.
You're Clumsier Than Usual
First you knock the alarm clock off the dresser, then you spill the milk as you're pouring your cereal, then you stub your toe on the way out the door -- you've become a klutz overnight. Researchers don't know exactly why, but sleepy people seem to <a href="http://www.prevention.com/amisleepdeprived/list/5.shtml">"have slower and less precise motor skills,"</a> Clete Kushida, M.D., Ph.D., director of Stanford University Center for Human Sleep Research told <em>Prevention</em>. Reflexes are dulled, balance and depth perception can be a little wonky and since you may also have trouble focusing, reaction time can be slowed, meaning you can't quite catch the egg carton before it hits the floor.
You've Lost That Lovin' Feeling
If you or your partner just can't get in the mood, and stress or an underlying health problem isn't to blame, you might want to spend some extra time between the sheets -- sleeping. Both men and women who don't get their 40 winks experience a <a href="http://www.webmd.com/sleep-disorders/excessive-sleepiness-10/10-results-sleep-loss">decreased sex drive</a> and less interest in doing the deed, WebMD reports. A lack of sleep can also <a href="http://www.everydayhealth.com/erectile-dysfunction/causes-of-low-libido.aspx">elevate levels of cortisol</a>, the stress hormone, according to Everyday Health, which doesn't help in the bedroom either.
The scientists also found that during sleep the brain's cells reduce in size to allow waste to be removed more effectively.
The purpose of sleep has vexed both philosophers and scientists since ancient Greek times.
From an evolutionary perspective, sleep is a puzzle. Virtually every animal species, from fruit flies to whales and humans needs some form of sleep. Yet being asleep has significant drawbacks, such as leaving an animal at the mercy of predators, and using up valuable time that could be better spent foraging or looking for mates.
Recent research has shown that sleep can help the brain store and consolidate memories, but these benefits are not thought to outweigh its disadvantages. This has led scientists to suspect that sleep must have a more essential biological function.
The new findings hinge on the discovery last year of a previously unknown waste disposal system unique to the brain.
In other parts of the body, the lymphatic system gets rid of cellular waste. But this mechanism does not extend to the brain, which is a closed "fortress" protected by a complex system of molecular gateways called the Blood Brain Barrier.
Mouse studies revealed how the brain's glymphatic waste disposal system works, by pumping cerebral spinal fluid (CSF) through brain tissue and flushing toxins into the blood circulation and liver.
Scientists speculated that the cleaning process may not be compatible with functions the brain must perform while awake and actively processing information.
This was confirmed by experiments in mice which showed that the glymphatic system was almost 10 times more active during sleep. The studies also showed that the sleeping brain cleared away significantly more amyloid-beta - a toxic protein linked to Alzheimer's - than the wakeful brain.
"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 Dr 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."
Another startling discovery was that cells in the brain shrink by 60% during sleep. The contraction creates more space between the cells and allows CSF to wash more freely through brain tissue. When we are awake, the brain's cells are closer together, restricting the flow of CSF.
The hormone noradrenaline was also found to be less active during sleep. Normally the hormone is released in bursts when the brain needs to become more alert, typically in response to fear. Noradrenaline may serve as a "master regulator" controlling the contraction and expansion of brain cells during sleep-wake cycles, the scientists believe.
"These findings have significant implications for treating 'dirty brain' disease like Alzheimer's," said Dr Nedergaard. "Understanding precisely how and when the brain activates the glymphatic system and clears waste is a critical first step in efforts to potentially modulate this system and make it work more efficiently."
Dr Simon Ridley, from the charity Alzheimer's Research UK, said: "We all know that a bad night's sleep can affect our memory and thinking skills in the short term, and this early stage research suggests that sleep may be crucial for clearing potentially harmful material from the brain.
"It would be important to determine whether continued problems with sleep may have long-term effects on the brain's ability to clear away excess proteins.
"Much more research would be needed to confirm whether a breakdown of the brain's waste clearance is to blame for diseases like Alzheimer's."