Why do we Sleep? Scientists are Still Trying to Find Out by National Geographic

We spend a third of our lives asleep, but sleep researchers still don't know why. Some researchers regard sleep as one of the greatest unsolved mysteries of science, even though all animals do it in one form or another.

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"Theories range from brain 'maintenance'--including memory consolidation and pruning--to reversing damage from oxidative stress suffered while awake, to promoting longevity," says a statement released this week by the University of California in Los Angeles. "None of these theories are well established, and many are mutually exclusive."
A new analysis by Jerome Siegel, UCLA professor of psychiatry and director of the Center for Sleep Research at the Semel Institute for Neuroscience and Human Behavior at UCLA and the Sepulveda Veterans Affairs Medical Center, has concluded that sleep's primary function is to increase animals' efficiency and minimize their risk by regulating the duration and timing of their behavior, the UCLA statement said.
The research appears in the online edition of the journal Nature Reviews Neuroscience.

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"Sleep has normally been viewed as something negative for survival because sleeping animals may be vulnerable to predation and they can't perform the behaviors that ensure survival," Siegel said. These behaviors include eating, procreating, caring for family members, monitoring the environment for danger and scouting for prey.

"So it's been thought that sleep must serve some as-yet unidentified physiological or neural function that can't be accomplished when animals are awake," he said. But after monitoring the sleep times of a broad range of animals--from the platypus and the walrus to the echidna--the team led by Siegel concluded that sleep itself is highly adaptive, "much like the inactive states seen in a wide range of species, starting with plants and simple microorganisms."
"These species have dormant states as opposed to sleep--even though in many cases they do not have nervous systems," UCLA noted.
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That challenges the idea that sleep is for the brain, Siegel said.
"We see sleep as lying on a continuum that ranges from these dormant states like torpor and hibernation, on to periods of continuous activity without any sleep, such as during migration, where birds can fly for days on end without stopping," he said.

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Hibernation is one example of an activity that regulates behavior for survival. A small animal can't migrate to a warmer climate in winter, Siegel said. "So it hibernates, effectively cutting its energy consumption and thus its need for food, remaining secure from predators by burrowing underground."
Sleep duration, then, is determined in each species by the time requirements of eating, the cost-benefit relations between activity and risk, migration needs, care of young, and other factors, the research team said.
"However, unlike hibernation and torpor," Siegel said, "sleep is rapidly reversible--that is, animals can wake up quickly, a unique mammalian adaptation that allows for a relatively quick response to sensory signals."
Humans fit into this analysis as well.

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What is most remarkable about sleep, according to Siegel, is not the unresponsiveness or vulnerability it creates but rather the ability to reduce body and brain metabolism while still allowing a high level of responsiveness to the environment.
"The often cited example is that of a parent arousing at a baby's whimper but sleeping through a thunderstorm. That dramatizes the ability of the sleeping human brain to continuously process sensory signals and trigger complete awakening to significant stimuli within a few hundred milliseconds."

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In humans, the brain constitutes, on average, just 2 percent of total body weight but consumes 20 percent of the energy used during quiet waking, so these savings have considerable adaptive significance, UCLA said.

"Besides conserving energy, sleep also invokes survival benefits for humans."

Besides conserving energy, sleep also invokes survival benefits for humans, including, according to Siegel, "a reduced risk of injury, reduced resource consumption and, from an evolutionary standpoint, reduced risk of detection by predators."
"This Darwinian perspective can explain age-related changes in human sleep patterns as well," he said.
"We sleep more deeply when we are young, because we have a high metabolic rate that is greatly reduced during sleep, but also because there are people to protect us.
"Our sleep patterns change when we are older, though, because that metabolic rate reduces and we are now the ones doing the alerting and protecting from dangers."

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