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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 Do the Lions and Cowboys Always Play on Thanksgiving?
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Because it's tradition! But how did this tradition begin?

Every year since 1934, the Detroit Lions have taken the field for a Thanksgiving game, no matter how bad their record has been. It all goes back to when the Lions were still a fairly young franchise. The team started in 1929 in Portsmouth, Ohio, as the Spartans. Portsmouth, while surely a lovely town, wasn't quite big enough to support a pro team in the young NFL. Detroit radio station owner George A. Richards bought the Spartans and moved the team to Detroit in 1934.

Although Richards's new squad was a solid team, they were playing second fiddle in Detroit to the Hank Greenberg-led Tigers, who had gone 101-53 to win the 1934 American League Pennant. In the early weeks of the 1934 season, the biggest crowd the Lions could draw for a game was a relatively paltry 15,000. Desperate for a marketing trick to get Detroit excited about its fledgling football franchise, Richards hit on the idea of playing a game on Thanksgiving. Since Richards's WJR was one of the bigger radio stations in the country, he had considerable clout with his network and convinced NBC to broadcast a Thanksgiving game on 94 stations nationwide.

The move worked brilliantly. The undefeated Chicago Bears rolled into town as defending NFL champions, and since the Lions had only one loss, the winner of the first Thanksgiving game would take the NFL's Western Division. The Lions not only sold out their 26,000-seat stadium, they also had to turn fans away at the gate. Even though the juggernaut Bears won that game, the tradition took hold, and the Lions have been playing on Thanksgiving ever since.

This year, the Lions host the Minnesota Vikings.

HOW 'BOUT THEM COWBOYS?


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The Cowboys, too, jumped on the opportunity to play on Thanksgiving as an extra little bump for their popularity. When the chance to take the field on Thanksgiving arose in 1966, it might not have been a huge benefit for the Cowboys. Sure, the Lions had filled their stadium for their Thanksgiving games, but that was no assurance that Texans would warm to holiday football so quickly.

Cowboys general manager Tex Schramm, though, was something of a marketing genius; among his other achievements was the creation of the Dallas Cowboys Cheerleaders.

Schramm saw the Thanksgiving Day game as a great way to get the team some national publicity even as it struggled under young head coach Tom Landry. Schramm signed the Cowboys up for the game even though the NFL was worried that the fans might just not show up—the league guaranteed the team a certain gate revenue in case nobody bought tickets. But the fans showed up in droves, and the team broke its attendance record as 80,259 crammed into the Cotton Bowl. The Cowboys beat the Cleveland Browns 26-14 that day, and a second Thanksgiving pigskin tradition caught hold. Since 1966, the Cowboys have missed having Thanksgiving games only twice.

Dallas will take on the Los Angeles Chargers on Thursday.

WHAT'S WITH THE NIGHT GAME?


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In 2006, because 6-plus hours of holiday football was not sufficient, the NFL added a third game to the Thanksgiving lineup. This game is not assigned to a specific franchise—this year, the Washington Redskins will welcome the New York Giants.

Re-running this 2008 article a few days before the games is our Thanksgiving tradition.

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Big Questions
Why Don't We Eat Turkey Tails?
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Turkey sandwiches. Turkey soup. Roasted turkey. This year, Americans will consume roughly 245 million birds, with 46 million being prepared and presented on Thanksgiving. What we don’t eat will be repurposed into leftovers.

But there’s one part of the turkey that virtually no family will have on their table: the tail.

Despite our country’s obsession with fattening, dissecting, and searing turkeys, we almost inevitably pass up the fat-infused rear portion. According to Michael Carolan, professor of sociology and associate dean for research at the College for Liberal Arts at Colorado State University, that may have something to do with how Americans have traditionally perceived turkeys. Consumption was rare prior to World War II. When the birds were readily available, there was no demand for the tail because it had never been offered in the first place.

"Tails did and do not fit into what has become our culinary fascination with white meat," Carolan tells Mental Floss. "But also from a marketing [and] processor standpoint, if the consumer was just going to throw the tail away, or will not miss it if it was omitted, [suppliers] saw an opportunity to make additional money."

Indeed, the fact that Americans didn't have a taste for tail didn't prevent the poultry industry from moving on. Tails were being routed to Pacific Island consumers in the 1950s. Rich in protein and fat—a turkey tail is really a gland that produces oil used for grooming—suppliers were able to make use of the unwanted portion. And once consumers were exposed to it, they couldn't get enough.

“By 2007,” according to Carolan, “the average Samoan was consuming more than 44 pounds of turkey tails every year.” Perhaps not coincidentally, Samoans also have alarmingly high obesity rates of 75 percent. In an effort to stave off contributing factors, importing tails to the Islands was banned from 2007 until 2013, when it was argued that doing so violated World Trade Organization rules.

With tradition going hand-in-hand with commerce, poultry suppliers don’t really have a reason to try and change domestic consumer appetites for the tails. In preparing his research into the missing treat, Carolan says he had to search high and low before finally finding a source of tails at a Whole Foods that was about to discard them. "[You] can't expect the food to be accepted if people can't even find the piece!"

Unless the meat industry mounts a major campaign to shift American tastes, Thanksgiving will once again be filled with turkeys missing one of their juicier body parts.

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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