sleep

Do We Sleep to Clean Our Brains?

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The purpose of sleep remains unknown. Using state-of-the-art in vivo two-photon imaging to directly compare two arousal states in the same mouse, Xie et al., in a paper titled “Sleep Drives Metabolite Clearance from the Adult Brain” published in Science, found that metabolic waste products of neural activity were cleared out of the sleeping brain at a faster rate than during the awake state. This finding suggests an explanation for how sleep serves a restorative function, in addition to its well-described effects on memory consolidation.

From the abstract:

The conservation of sleep across all animal species suggests that sleep serves a vital function. We here report that sleep has a critical function in ensuring metabolic homeostasis. Using real-time assessments of tetramethylammonium diffusion and two-photon imaging in live mice, we show that natural sleep or anesthesia are associated with a 60% increase in the interstitial space, resulting in a striking increase in convective exchange of cerebrospinal fluid with interstitial fluid. In turn, convective fluxes of interstitial fluid increased the rate of β-amyloid clearance during sleep. Thus, the restorative function of sleep may be a consequence of the enhanced removal of potentially neurotoxic waste products that accumulate in the awake central nervous system.

So: we sleep to clean our brains (as the authors of the paper want us to believe).

This research is tentative, as it’s only been found to be the case in mice. The Guardian provides more commentary:

Maiken Nedergaard, who led the study at the University of Rochester, said the discovery might explain why sleep is crucial for all living organisms. “I think we have discovered why we sleep,” Nedergaard said. “We sleep to clean our brains.”

Writing in the journal Science, Nedergaard describes how brain cells in mice shrank when they slept, making the space between them on average 60% greater. This made the cerebral spinal fluid in the animals’ brains flow ten times faster than when the mice were awake.

The scientists then checked how well mice cleared toxins from their brains by injecting traces of proteins that are implicated in Alzheimer’s disease. These amyloid beta proteins were removed faster from the brains of sleeping mice, they found.

Nedergaard believes the clean-up process is more active during sleep because it takes too much energy to pump fluid around the brain when awake. “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,” she said in a statement.

According to the scientist, the cerebral spinal fluid flushes the brain’s waste products into what she calls the “glymphatic system” which carries it down through the body and ultimately to the liver where it is broken down.

The importance of replicating this work is obvious, lest it be considered cargo cult science.

The Role of Sleep in Brain Repair and Growth

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The purpose of sleep is not very well understood. I’ve been fascinated with the topic for a number of years, so I am pretty excited when there’s new developments in the field of sleep research.

A new study sheds light on the role sleep plays in the the ability of the brain’s cells to grow and repair themselves. Preliminary research suggests that sleep replenishes a type of brain cells that go on to make an insulating material known as myelin, which protects our brain’s circuitry.

The research, published in The Journal of Neuroscience, was conducted in mice that were either allowed to sleep, or forced to stay awake. Researchers looked particularly at how sleep affected gene activity of cells called oligodendrocytes, which play a role in the production of myelin. Myelin covers brain and spinal cord nerve cell projections as a sort of “insulation”; researchers explained that it is integral to the movement of electrical impulses from cell to cell.

The study shows that sleep seems to turn on genes known to play a part in the formation of myelin. Conversely, lack of sleep was linked with the activation of genes associated with cell stress and death.

Dr Chiara Cirelli and colleagues from the University of Wisconsin, where the study was conducted, explained:

For a long time, sleep researchers focused on how the activity of nerve cells differs when animals are awake versus when they are asleep.

Now it is clear that the way other supporting cells in the nervous system operate also changes significantly depending on whether the animal is asleep or awake.

 

On the Optimal Time for Napping

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The Wall Street Journal summarizes the benefits of napping and enumerates the time you should nap for:

For a quick boost of alertness, experts say a 10-to-20-minute power nap is adequate for getting back to work in a pinch.

For cognitive memory processing, however, a 60-minute nap may do more good, Dr. Mednick said. Including slow-wave sleep helps with remembering facts, places and faces. The downside: some grogginess upon waking.

Finally, the 90-minute nap will likely involve a full cycle of sleep, which aids creativity and emotional and procedural memory, such as learning how to ride a bike. Waking up after REM sleep usually means a minimal amount of sleep inertia, Dr. Mednick said.

My favorite part of the article was this tip about taking a 10 to 15 minute nap by holding a pen/pencil in your hand:

Jonathan Brandl is a Newton, Mass.-based consultant who works from home. Up at 5 a.m. to hit the gym, he finds himself fading around 2 p.m. His solution is a fast snooze in a comfy chair in his den. His trick for waking up: He holds a pen or pencil in his hand, which usually falls about 10 to 15 minutes into his nap, waking him up.

Sign me up.

A Brief History of Sleep

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From a very interesting Wall Street Journal piece on sleep, we learn some history about how humans used to get two sleeping chunks at night:

So why is sleep, which seems so simple, becoming so problematic? Much of the problem can be traced to the revolutionary device that’s probably hanging above your head right now: the light bulb. Before this electrically illuminated age, our ancestors slept in two distinct chunks each night. The so-called first sleep took place not long after the sun went down and lasted until a little after midnight. A person would then wake up for an hour or so before heading back to the so-called second sleep.

It was a fact of life that was once as common as breakfast—and one which might have remained forgotten had it not been for the research of a Virginia Tech history professor named A. Roger Ekirch, who spent nearly 20 years in the 1980s and ’90s investigating the history of the night. As Prof. Ekirch leafed through documents ranging from property records to primers on how to spot a ghost, he kept noticing strange references to sleep. In “The Canterbury Tales,” for instance, one of the characters in “The Squire’s Tale” wakes up in the early morning following her “first sleep” and then goes back to bed. A 15th-century medical book, meanwhile, advised readers to spend their “first sleep” on the right side and after that to lie on their left. A cleric in England wrote that the time between the first and second sleep was the best time for serious study.

The time between the two bouts of sleep was a natural and expected part of the night, and depending on your needs, was spent praying, reading, contemplating your dreams or having sex. The last one was perhaps the most popular. A noted 16th-century French physician named Laurent Joubert concluded that plowmen, artisans and others who worked with their hands were able to conceive more children because they waited until after their first sleep, when their energy was replenished, to make love.

The phrase is “segmented sleep” and it can be reproduced:

Studies show that this type of sleep is so ingrained in our nature that it will reappear if given a chance. Experimental subjects sequestered from artificial lights have tended to ease into this rhythm. What’s more, cultures without artificial light still sleep this way. In the 1960s, anthropologists studying the Tiv culture in central Nigeria found that group members not only practiced segmented sleep, but also used roughly the same terms to describe it.

Fascinating.