Changing Planet

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


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.





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

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Forty years in U.S., UK, and South African media gives David Braun global perspective and experience across multiple storytelling platforms. His coverage of science, nature, politics, and technology has been published/broadcast by the BBC, CNN, NPR, AP, UPI, National Geographic, TechWeb, De Telegraaf, Travel World, and Argus South African Newspapers. He has published two books and won several journalism awards. He has 120,000 followers on social media. David Braun edits the National Geographic Society blog, hosting a global discussion on issues resonating with the Society's mission and initiatives. He also directs the Society side of the Fulbright-National Geographic Digital Storytelling Fellowship, awarded to Americans seeking the opportunity to spend nine months abroad, engaging local communities and sharing stories from the field with a global audience. Follow David on Facebook  Twitter  LinkedIn
  • PhilTerry

    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:
    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.”
    Creator, Darwin150 Facebook campaign and lecture series
    On our way to 1 million, help us get there

  • dogs

    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.

  • Denzer

    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

  • brian

    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.

  • Brian

    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.

  • Azure

    It could be simply the reason of conserving energy. That our bodies incorporated sleep as a way of conserving energy so that we don’t always need to expend so much energy to hunt for food through the ages. I mean the seasons and climate may have something to do with it as well through the different periods of evolution as well. The simple evidence is if our bodies spend too much time moving around, and the mind is awake too long, the activity is often going to decline. Muscles weaken, our mental activity decreases. The source of energy for living breathing life is food, nutrients, but that is going to take energy to find and attain. Sleep gives our minds and bodies a chance to rest, and to take it off of spending time and energy to feed.

  • Tennessee Individual Health Insurance

    Sleep is one important part(primary) of our life that we all constantly see as secondary.
    I love this article !!

  • doug l

    It’s unlikely animals would survive a world without sleep. One universal aspect of sleep is that it happens to animals at those times when being awake would actually be a disadvantage such as it would for human primate ancestors whose sensory array has evolved to find ripe fruit in trees…wandering around at night when it couldnt locate fruit visually anyway, and while moving about drawing attention to itself when nocturnal predators look for unwitting prey. It’s good that we have the occasional bout of amnesia in our collective social structure or a lucky predator could approach un-detected and without a neighboring insomniac making an alarm.

  • lbrother

    is the sleep help our body get rest? and brain as well.the most straight answer is conserving energy.
    even machine need stop to be maintained.

  • Koykos

    I enjoyed reading this article. thanks!

  • Rob

    My question is if sleep is universal. Every living thing on earth sleeps in a 24 hour cycle because of the rotation of the earth; daylight and nighttime. Would animals on other planets sleep? Would they sleep on a cycle comparative to their planets rotation around their sun? If the human species continues to evolve for millions of more years, would no sleep evolve as a better thing for the species? Do aliens sleep?

    • David Braun

      All good questions. There’s a lot we don’t know about this.

  • aubri

    why can’t we invent a pill or some kind of device that stimulates the release of the hormones typical during sleep so we can rejuvenate and repair so that we can add more waking hours to our lives? is this not possible? is anyone trying to do this? I hate sleeping 8 hours, it’s such a waste of time but yet i feel terrible if i don’t get those 8-9 hours. I imagine remaining relatively still while our brains are releasing the chemicals from the pills or the device, but we can stay awake and perhaps get some things done, or relax while its taking place and dramatically reduce our sleeping time. We can sleep like dolphins.

  • Dave Coombs


    Funny you should mention that…
    I’ve been taking a new suppliment called Protandim for three months now. I’ve always required at least 7-8 hours sleep, and 51 days ago my sleep schedule suddenly and dramatically changed. Since then I’ve averaged only 5 hours a night and feel fine! Protandim is a very strong antioxidant (stimulates the body’s own enzymes to reduce free radicals). Since this sort of cellular repair is something my body might be doing while I sleep, it’s possible that Protandim’s contribution in that area is causing me to need less sleep. Just my guess, but anyway the result has been wonderful for me, lots more free time…

    Best of luck,

  • ravi prajapati

    i also want some accurate reason for we got sleepy action in our life.

  • edwin castro

    i think we felt more sleepy at nighttime than daytime because at the dark side of our planet we called night, the power of the sun didn’t affecting our body system very our body shutting down, we called travelling very far away from our sun is impossible..we all just go sleeping when we go very far from the sun.our brains never sleep that’s why we think,we called it dreaming even our body laid rest.

  • donald becquer

    i think sleep is a process of life the brain refreshes . What i mean is brain defragments the memory then it transfes to the subconcious mind all the things in a compressed manner it transfers memory to new nodes which grow more when we sleep . All these reallocation of memory can be done only if all other parts of our body are at rest . Thats why all parts are at rest exept some important process like respiration and heart pumping even the process of excretion slows down . Also this is the reason that brain gets awake when this process is disturbed . Certain senses like hearing,touch,will be working as a protective mechanism but as the evolution leaps forward for every millions of years all these things might change.
    We can be awake for much more time if we dont allow new thoughts enter our brain so it wont have any need reallocate the memory
    This was actually proved there is an indian monk who had been awake for several months when he meditates

  • Colette Bembenek

    This 4 hour – 2 part program, “Why Do We Sleep”? was so
    thorough and fascinating. I was experiencing sleep
    deprivation for a week and couldn’t explain the damage
    it does to the human body to my friends. Now I can with
    the provocatively total information provided by this series.
    Where can I purchase this particular program?

  • Joel

    Sleeping has an rejuvenating effect so i think that is one of the main reasons but i see many people commenting that it seems like a waste of life.
    This is where Lucid dreaming steps in to the game. You can choose those hours to be full of “waste” or full of adventures and anything you may desire.

  • Ashleigh

    As for why we feel sleepy IN THE NIGHT TIME, look up Seasonal Affective Disorder 🙂

  • sarvesh angadi

    sleep is a thing which occurs to refresh it is a way in which cosmic radiation enter to our body and then we are enough stable to work out day.if we do not sleep then that thing happens in day that is dreaming.

  • Jonah

    I think sleep is required by the universe. It is a balance that is required as energy is finite and redistributed accordingly. We are all part of the life force and cannot all share this force at the same level simultaneously. The Universe regulates this energy and has created a balance between sleep states and awake states. It is kind of like the energy company. You can fight it all you want but if you don’t pay your bill you will get shut off at some point.. In humans seems like it is around 11 days. The universe isn’t as generous as the utility companies 😉

  • mursal

    I wonder if anyone can explain it medically

  • Ricardo Locatelli

    Why can’t we just Yolo it and not sleep? I know most people can’t because they don’t have mad swag, but I think I can. Please help me out!

  • Tim McKay

    Why do people sleep. Psychologists and Neurological scientists are still unsure of it or not in consensus of what sleep is or why organisms sleep. People and animals possess senses. These senses take in extraoridnary amounts of information every second, every millisecond. Some information we take in we are not even aware of at the time, but it is being recorded, perceived, sensed and imprinted on our conscious, subconcious, or whatever form or state of concsiousness humans choose to define or describe. Not that anything in the universe is really separate anyway, but humans separate things into distinct and unique categories to help themselves understand them. The truth is the truth and facts are facts whether people perceive them or not. But, regardless, all of this information coming in via our senses requires extaordinary amounts of energy also. This energy is decoded, re-encoded, stored, and processed by the various regions of our brains. The brain is an organic structure and like any organic structure it only possess so much energy at any one time. Muscles can be flexed and unflexed only so many times before they become exhausted. And although, not permanently exhausted, these muscles and brain tissue matter do need repleneshment and rest. Sleep is the time in which our brains need not process new information taken in via our senses. In other words, are senses are given a break. We close our eyes, ears, taste, and tactile sensations in order rejuvenate those energies and muscles and grey matter and as a result we lie in an altered consciousness state. The brain is still very active during sleep. And, even if the same areas of the brain are being utilized during sleep, they are being utilized in different ways and/or using different types of energy or different types of contraction/expansion. While we are diurnal, other animals are nocturnal. We take in the majority of our information in through our visual cortex and it needs to be light (daytime) for us to perceive all of the colors and reflections of light and shadow to make the most use of the physical information around us. Most nocturnal animals have adaptations that allow them either much better vision at night or a more elevated sense of hearing, or touch, or smell/taste to make use of the physical information in their environment (navigation and or information recording). These animals’ brain activations require a different area of the brain to be most active and also have their limits in regards to energy consumption. Like diurnal species, they also have to repelenish those energies from time to time. For species that have better vision at night, daylight would hurt or damage their eyesight, which is adapted to life at night. Therefore, they have “learned” to sleep during the day to make up for the energy expleted during their waking state. There also exist animals that are neither diurnal or nocturnal and their senses, coupled with their survival instincts may dictate when or where or how they are able to sleep and replenish the specific energies that they need to that correlates with the sense(s) of which they are most reliant. Organisms/lifeforms without the sense of sight, hearing, touch, or taste/smell, should not have any distinct patterns of sleep or energy replenishment. In the case of those specimens, they would sleep or rest in between times of contraction or expansion or whenever the energy was depleted to the extent of needing rest.

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