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Why Do We Sleep?

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Why do we sleep?

Paul King:

A number of proposals and perspectives have emerged that, taken together, paint a compelling and converging picture for why sleep evolved and why it is now needed.

WHY SLEEP EVOLVED

The earliest differentiation into day versus night behavior in animals was probably driven by differences in temperature and the light available for vision. Day and night are further differentiated by the aggregate "behavior" of the ecosystem (other animals also have day/night cycles). As a result, the optimal day and night survival strategies are different, and animals would have adapted to synchronize their survival strategy to the 24-hour cycle.

Evidence is accumulating that complex nervous systems, and especially the brain, perform and benefit from internal maintenance activities. Some of these maintenance activities, such as "synaptic network stabilization," occur at the cellular level. Others, such as memory consolidation or the proposed memory transfer between brain areas, occur at the whole brain level.

The brain's internal maintenance disrupts optimal behavioral responsiveness. An animal can't be fully vigilant for predators while its brain is doing internal housekeeping. For example, memory consolidation requires information unrelated to the present moment to be accurately moved around the brain under the management of organized processes. Certainly the groggy state upon waking up is not optimal for fending off an attacker. For this reason, there is evolutionary pressure for the animal to postpone internal maintenance activities while engaging with the external environment (finding food, mates, etc.) so as to reduce risk of harm.

These postponed maintenance activities have to happen sometime. To optimize survival, the organism schedules a time for them when it will be safest, which includes finding a protected place to be and initiating all maintenance activities in parallel to get them out of the way. This leads to the temporal synchronization of all postponed maintenance activities.

Collectively, these four environmental and biological pressures lead to a bifurcation of external interactive behavior and internal maintenance activity that is synchronized with the 24-hour day/night cycle. They further encourage all postponed internal maintenance activities to be synchronized and performed in parallel—in other words, sleep.

Given the different day versus night ecosystems, it is natural that animals that have a survival edge during one phase of the 24-hour cycle would use the other phase for sleeping. "Diurnal" animals are specialized for engaging with the daytime environment, whereas "nocturnal" animals (cats, kangaroos, and owls, for example) are specialized for night and sleep during the day. However there are other patterns as well, such as "crepuscular" animals that are active at the day/night boundary (dawn or dusk).

WHY SLEEP IS NOW NEEDED

Given that a regular sleep period has formed, the question is: What goes on during sleep that is so important?

There appears to be no one single sleep activity that is the reason for sleep. Because the sleep/wake division of the 24-hour activity cycle has been around in animals for a long time—all known animals have a quiescent period—the sleep period has had hundreds of millions of years to acquire multiple uses.

At the cellular level, removing toxic radicals and strengthening or rebuilding tissues has been suggested.

In the brain, a number of uses for sleep have been identified or proposed [1], including:

  1. Restoring neurons biochemically
  2. Rescaling the connection strengths of synapses in the brain's neural networks to facilitate easier learning the next day. This might include rewiring activity (growing synapses).
  3. Consolidating (reorganizing and restructuring) memories. [3]
  4. Transferring memories from the memory-specialized fast-learning brain area (the hippocampus) to the higher-capacity more cognitively powerful area (the cerebral cortex). [4]

"At the whole animal behavioral level, the functions of sleep seem clear: energy is saved, performance is restored, and (in humans) affect becomes more positive. Such findings have led to the universal acknowledgement that sleep restores brain function." [1]

SLEEP AS A DIVERSE AND DECENTRALIZED PHENOMENON

A wide variety in sleep patterns can be found across animal species. Some animals (insects, fish) are quiescent but not fully "asleep" (inactive). In cetaceans (whales and dolphins), half of the brain goes to sleep at a time for one to two hours on an irregular schedule throughout the day and night. Bears, bats, and some rodents hibernate, which is a quiescent extended-sleep period lasting for weeks or months in the winter. Siegel (2008) reviews the phenomenon of sleep across animal species. [2]

Krueger et al (2008) reviews the functions and mechanisms of sleep in the brain and proposes that sleep is not a centralized process of the brain, as is normally thought, but is instead a decentralized intrinsic process of neural tissue. They propose that neural tissue initiates sleep onset in a decentralized fashion that propagates until it takes over the whole brain, in something like a social mass movement. The evolutionary pressure to partition activity into day/night divisions is also discussed. [1]

This chart shows the evolution of sleep in relation to the environment, metabolism, and the development of complex nervous systems. "R-A cycles" = Rest-Activity cycles. Sleep appears in the bottom two frames.


from Krueger et al (2008) [1]

This post originally appeared on Quora. Click here to view.

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Big Questions
Why Does the Queen Have Two Birthdays?
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On April 21, Her Majesty Queen Elizabeth II will turn 92 years old. To mark the occasion, there are usually a series of gun salutes around London: a 41 gun salute in Hyde Park, a 21 gun salute in Windsor Great Park, and a 62 gun salute at the Tower of London. For the most part, the monarch celebrates her big day privately. But on June 9, 2018, Her Majesty will parade through London as part of an opulent birthday celebration known as Trooping the Colour.

Queen Elizabeth, like many British monarchs before her, has two birthdays: the actual anniversary of the day she was born, and a separate day that is labeled her "official" birthday (usually the second Saturday in June). Why? Because April 21 is usually too cold for a proper parade.

The tradition started in 1748, with King George II, who had the misfortune of being born in chilly November. Rather than have his subjects risk catching colds, he combined his birthday celebration with the Trooping the Colour.

The parade itself had been part of British culture for almost a century by that time. At first it was strictly a military event, at which regiments displayed their flags—or "colours"—so that soldiers could familiarize themselves. But George was known as a formidable general after having led troops at the Battle of Dettingen in 1743, so the military celebration seemed a fitting occasion onto which to graft his warm-weather birthday. Edward VII, who also had a November birthday, was the first to standardize the June Trooping the Colour and launched a tradition of a monarchical review of the troops that drew crowds of onlookers.

Even now, the date of the "official" birthday varies year to year. For the first seven years of her reign, Elizabeth II held her official birthday on a Thursday but has since switched over to Saturdays. And while the date is tied to the Trooping the Colour in the UK, Commonwealth nations around the world have their own criteria, which generally involve recognizing it as a public holiday.

Australia started recognizing an official birthday back in 1788, and all the provinces (save one) observe the Queen's Birthday on the second Monday in June, with Western Australia holding its celebrations on the last Monday of September or the first Monday of October.

In Canada, the official birthday has been set to align with the actual birth date of Queen Victoria—May 24, 1819—since 1845, and as such they celebrate so-called Victoria Day on May 24 or the Monday before.

In New Zealand, it's the first Monday in June, and in the Falkland Islands the actual day of the Queen's birth is celebrated publicly.

All in all, just another reason it's great to be Queen.

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Big Questions
What Is the Meaning Behind "420"?
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Whether or not you’re a marijuana enthusiast, you’re probably aware that today is an unofficial holiday for those who are. April 20—4/20—is a day when pot smokers around the world come together to, well, smoke pot. Others use the day to push for legalization, holding marches and rallies.

But why the code 420? There are a lot of theories as to why that particular number was chosen, but most of them are wrong. You may have heard that 420 is police code for possession, or maybe it’s the penal code for marijuana use. Both are false. There is a California Senate Bill 420 that refers to the use of medical marijuana, but the bill was named for the code, not the other way around.

As far as anyone can tell, the phrase started with a bunch of high school students. Back in 1971, a group of kids at San Rafael High School in San Rafael, California, got in the habit of meeting at 4:20 to smoke after school. When they’d see each other in the hallways during the day, their shorthand was “420 Louis,” meaning, “Let’s meet at the Louis Pasteur statue at 4:20 to smoke.”

Somehow, the phrase caught on—and when the Grateful Dead eventually picked it up, "420" spread through the greater community like wildfire. What began as a silly code passed between classes is now a worldwide event for smokers and legalization activists everywhere—not a bad accomplishment for a bunch of high school stoners.

Have you got a Big Question you'd like us to answer? If so, let us know by emailing us at bigquestions@mentalfloss.com.

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