NGS photo by David Boyer
"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.
NGS photo by J. Baylor Roberts
"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.
NGS photo by Anthony Stewart
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.
NGS photo by Chris Johns
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.
NGS photo by W. E. Garrett
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."
NGS photo by James L. Stanfield
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."
NGS photo by Joe Scherschel
You might also be interested in:
Dolphins can stay sharp and alert, monitoring their environment for days on end without getting the least bit tired because they send half their brains to sleep while the other half remains conscious, researchers have learned.
Sex Evolved as an Escape From Parasites, Study Suggests
Why is sex the dominant form of reproduction on the planet? Scientists think they know why--and it all has to do with evasion of parasites.
What Others Had to Say
Added by PhilTerry on August 29, 2009
This article shows another reason nothing in biology can be explained without evolution.
Even sleep is adaptive (of course - that would be expected) across species and over the course of a human's lifetime.
As David says;
"This Darwinian perspective can explain age-related changes in human sleep patterns as well," he said.
Sleep is one of those topics that the general public is quite interested in - and it's a good teaching topic for the power of evolutionary biology.
Variation and adaptation will be covered by Jonathan Weiner at Columbia, Pulitzer-Prize winning author of "Beak of the Finch." He gives the second of four online and live lectures for our fall Darwin150 lecture series.
You can sign-up free for his lecture (and attend live or over the phone) here:
http://DarwinLecture2.Eventbrite.com
I'll put a link to this article on the Darwin150 Facebook page, our all-volunteer group which now has 250,000 members on the way to 1 million in time to celebrate 150 years since the publication of Darwin's "Origin of Species."
Thanks,
Phil
Creator, Darwin150 Facebook campaign and lecture series
On our way to 1 million, help us get there
http://www.facebook.com/group.php?gid=53320310123
http://www.Darwin150.org
Added by sockyee on August 30, 2009
Actually I'm quite surprised after reading the article here. When I was searching around for info on whether a fish does need to sleep, immediately my thoughts link my question to the scientific explanation mentioned here. Well, it baffles me as well because when fish does go to sleep, not sure whether doing this is necessary at the expense of survival and falling prey to predators.
SockYee
Everything about Aquarium Pet Fish
Added by dogs on September 7, 2009
If humans did not have to sleep imagine how much more we could get done. My dogs sleep so much for only living 13 to 18 years on avg.
Added by zabiullah rezai on October 7, 2009
All the info is very effective.I really liked it.
Added by Denzer on October 22, 2009
The brain sends a signal to the body to sleep so as to function better when awake in the day
As we are now living longer the brain may change the sleep patterns
Added by brian on November 1, 2009
The explanation offered for evolving sleep seem contrived. Evolving to rest while aware would be safer and more productive yet we must sleep and dream.
Something is yet to be discovered as the logic used to explain sleep doesn't address why, except to try and make it fit into evolution.
Added by Brian on November 13, 2009
So based on this information and evolutionary theory (true or not), one could assume that eventually persons that live in the industrialized countries of the world are not often found in damger and generally have plenty to eat and are progresivly doing less and less. Based on this could the desendants of these persons eventually achieve a status that does not require sleep or maybe only rest or very little sleep? These perosns are not in danger and they lack few resources and life in general is or should be less stressful. I don't particularlly agree with this thinking but I'll take it out for a walk here and see where it goes.