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Humans (and most animals) sleep. The reasons aren't really clear but most people believe:

We have to sleep because it is essential to maintaining normal levels of cognitive skills such as speech, memory, innovative and flexible thinking. In other words, sleep plays a significant role in brain development.

Assuming a species had the same physical requirements (their brain had to offline for one reason or another) but didn't go comatose, hallucinate and suffer amnesia what other habits/patterns/routines could they evolve to serve the same function and allow normal cognitive functions?

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    $\begingroup$ You could evolve two brains, which take turns resting? I suppose it'd be arguable whether that counts as sleeping or not. $\endgroup$ – a CVn May 5 '15 at 8:38
  • $\begingroup$ Regular recharge-states with impaired physical and cognitive capabilities. Sort of resembling hypnagogia while being awake? $\endgroup$ – palako May 5 '15 at 8:44
  • $\begingroup$ @MichaelKjörling Interesting idea, would the two brains have distinct memories and personalities? $\endgroup$ – Styphon May 5 '15 at 10:33
  • $\begingroup$ Need for sleep that great most people have is a weird quirk; Actually for some sleep is not needed at all! $\endgroup$ – SF. May 5 '15 at 11:15
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    $\begingroup$ How about a species where half the brain 'sleeps' or rests while the other half remains active? The creature as a whole never sleeps. How would it evolve? An extremely dangerous environment where sleeping = death. $\endgroup$ – the_OTHER_DJMethaneMan May 5 '15 at 14:02
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I'm not sure if you'd count it as sleeping but meditating could be a way of shutting down part of the brain whilst still maintaining external awareness. In theory dreaming is our processing of memories from short term to long term memory. This could happen during meditation, where people re-live what they experienced as their memories transfer from short to long term.

If you're looking for other solutions that don't require people to "shut down" even so far as meditation then you would need to look at different structures of the brain. It's possible that a species could be constantly processing memories, or process them several times a day whilst being awake.

You could have a species that process memories straight away and that they need to rest for other reasons. An example of this might be that part of the brain produces a chemical, which when left to build up to certain levels becomes toxic. That part of the brain has to shut down for a period to allow the body to process the chemical. This could involve higher cognitive functions, anything from walking and talking to psionic powers. The species has to lose access to part of their mental capacity whilst their brain recharges, but other parts still go on. This could be automatically controlled, happen at a specific time of day or even when a certain threshold is reached. Or it could be triggered manually, when the species receives some sort of signal that they need to allow the body to process the chemical.

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    $\begingroup$ Your solution turns out to have a real-world basis: according to this TED talk, the brain accumulates waste while awake becuase the lymphatic system does not extend to the brain; but when the brain is asleep, spaces open up around the outside of the blood vessels to flush the waste products out of the brain. $\endgroup$ – 2012rcampion May 5 '15 at 14:50
  • $\begingroup$ @2012rcampion yea, I didn't know there was a TED talk about it but I remembered the brain did something like that, which is what I based it on. $\endgroup$ – Styphon May 5 '15 at 15:39
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    $\begingroup$ What a crappy design. We need to talk to the designer about this bug! $\endgroup$ – Jim2B May 6 '15 at 3:09
  • $\begingroup$ Growing should probably also be addressed, especially since when you sleep you grow the most. At least, that's what I've heard. $\endgroup$ – Xandar The Zenon Mar 1 '16 at 19:08
  • $\begingroup$ @Jim2B Apparently, "uptime" was not a design priority... for whatever reason. $\endgroup$ – xDaizu Jun 28 '17 at 8:45
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Fantasy Elfs

In most elf universes the elves are not sleeping, they have to spend a certain amount of every day meditating to keep their mental health and also a part of the day resting to recharge energy.

Dolphins

Dolphins can sleep with half their brain while the other half keeps the normal body functions alive. The dolphin is a voluntary breather that means the bodys autonome functions does not control the breathing but the dolphin has to choose to breathe. This means the dolphin is required to have a certain part of the brain functioning while sleeping.

Space Marines

Space marines from Games Workshop can battle for weeks without sleeping. The reason for this is they have computerized parts of the brain making them able to shutdown parts of the brain not needed for the current task, making it "sleep" for a certain time, while being awake and active.

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  • $\begingroup$ For the 'half brain sleeping', one might also want to dig into research awhile back on ducks in a row. $\endgroup$ – user487 May 5 '15 at 15:23
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I'm going to tackle this from a different perspective.

Imagine, if you will, an alien race composed of an organic metal. If we saw them in their true form, we'd say they'd look like humanoid robots; indeed, they blur the line between "Artificial Intelligence" and "Natural Intelligence"...their consciousness is maintained in a form they call a spark, but is this "spark" a highly advanced CPU/hard drive, or is it a brain?

Being composed of metal, members of this species have the ability to transform themselves into pretty much any metallic mechanical object that they encounter. As these alien robots visit earth, they typically assume the form of automobiles. When humans encounter these auto-robots, they are largely unaware of their sentience...let alone the great lengths to which these "robots in disguise" have gone, to protect them from the dangers in the galaxy (or to punish and enslave them).

Interestingly, even these highly-evolved robotic organisms also need sleep. While there is already some speculation as to why these machine people would need to sleep, consider for a moment the device on which you are reading this right now.

How many times has your computer, tablet, or phone, after being on constantly for days on end, just starts acting up? Perhaps it starts going slower than normal, maybe it's just doing some unexpected things. Applications start freezing. It stops detecting I/O devices. Network connections drop randomly.

Rebooting

Oftentimes, a good ol' fashioned reboot will fix your problems. Perhaps an essential driver has crashed; more likely, it has acquired a bunch of memory leaks during the time it's been awake. Restarting gives your computer and operating system a fresh slate when it comes back on, in terms of memory allocation/usage and loaded drivers. Is this not akin to sleeping? It seems like maybe our alien robots even need to do this every once in a while.

Other computer maintenance tasks that could be compared to the benefits and effects of sleeping, which may or may not apply to a race of sentient robotic beings:

Defragmentation

The physical storage of data will almost always become fragmented over the lifetime of the system. Occasional defragmentation helps to keep performance up (and to increase the lifetime of the actual moving parts of the computer).

Cache clearing

Sometimes, remote applications (typically websites) will create a local copy of its static components on your system, so that everytime you access it, you don't have to keep downloading the same static components. Occasionally, old cached files can cause interference with newer versions of the application, or even with other applications as well. You should clear your application caches every so often to maintain performance.

Anti-virus

Obviously, any networked system has a chance of becoming exploited or infected. It is always a good idea to take a few minutes every once in a while and scan and destroy any threats from malicious software.

One interesting question, though, is what these machines dream of when they're asleep.

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  • $\begingroup$ I have always found that my dreams are a cobbled-together narrative around bits and pieces of life experiences from (mostly) the last few days. I interpret this as having become consciously aware of my garbage reclamation algorithms at work. We can't store everything. While we sleep some unconscious agent assesses recent memories and if judged unimportant, it wipes them to reclaim the storage. $\endgroup$ – nigel222 Feb 10 '16 at 21:07
  • $\begingroup$ Sleep (in humans) surely is not a reboot. Defragmentation and garbage reclamation fit REM sleep and hardware maintenance fits with deep sleep. However, recovery from deepest unconsciousness or an epileptic seizure is uncannily like a computer rebooting after a powerdown or crash. $\endgroup$ – nigel222 Feb 10 '16 at 21:16
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It's been observed that sleep may be used to flush toxins out of the brain. If you assume that's the case, a species may have evolved a much quicker, more efficient way to flush the toxins and resume normal brain functioning. It could be like emptying the bladder, and would take only seconds. But like emptying the bladder, it may have to happen every few hours.

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The brain might take certain functions off-line for maintenance while keeping sensory awareness for danger, so he'd be seen as resting quietly and not paying attention, but could react if pestered.

Or if less global, just doing memory digesting could (only) impair long-term memory recall while going on, but still allow routine tasks.

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On its most basic level, sleep is the time when the body performs its 'maintenance routines'. As any life form goes through its life, it builds up cumulative damage, toxins, microbial invaders, and other minor problems that need to be cleaned up. For the vast majority of animals, it is most efficient for the animal to periodically go through a period of dormancy to perform all of its routine maintenance at once without the unpredictability of waking life getting in the way, which is why sleep is the most common solution among animals - particularly sleep during a time when the animal won't be able to work very efficiently anyway (at night for diurnal animals), or when it would be better off staying still and hiding from predators anyway (during the day for nocturnal animals).

But there's no reason it has to be this way. If there is a good reason why full sleep isn't an option (for instance, an environment so violently unpredictable that any period of dormancy would be fatal), this maintenance will be handled in other ways - resting one organ at a time, part of each organ at a time, or even patching things up while they are in operation. This is less efficient than sleep, but sometimes it's the only option.

We use sleep to maintain our brains and process information internally at the same time that we repair our bodies (because what would be the point of having the brain be awake when the body is not?), but an animal with a different body-maintenance method would naturally have a different brain-maintenance method. It might rest different parts when not in use (which might result in interesting personality swings), rest one half at a time, or just forego a specific resting phase altogether and perform maintenance on-the-fly, though this would probably slow down its thinking speed and reaction time overall.

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Actually it seems that our understanding of sleep is governed by experiences mad on mammals and especially with rats dying of long term sleep deprivation. However recent experiences duplicating the setup with pigeons showed that they do not suffer from sleep deprivation (and they were selected as they also have REM sleep): here and here.

Also a more recent study has shown that birds can actually sleep while flying: here.

All of which indicate that the need for humans to sleep might be a very mammal centric biological need and that other species though sleeping might not need it as much as us.

To answer your question, if the brain of your species has to go off, may be what it means is just that the individual just have a very reduced activity (similar to birds). Possibly no communication, little movement and no creative tasks being undertaken in that biological state.

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The Up-Down-Left-Right brain

This alien rests parts of their brain as well, ecept they are not identical halves, but specific functional areas. They do this in a daily cycle and while they can call on all functions when needed, this is prolong the rest time needed for the brain areas.

The alien cycle has four phases always going in the same sequence:

Down

This is the rest time of the most activity related part, making the creature slow and lethargic. It functions as a time for reflection and emotional processing, often used for family bonding as well as physical healing.

Right

At this time, activity resumes at full speed, but the spontaneous/creative part of the brain rests, while the rational planning side runs at peak performance. This part of the cycle is used for planning the day's work or longer term strategies. It is the preferred time for business and administration activity.

Up

This is the rest phase for the introspective and regenerative part. The effect is a hyperactive phase where previously planned work is executed and problems can be solved in creative ways, but the aliens are distinctly less social and emphatic. Physical work, sports and fighting are done in this phase when possible.

Left

Riding the last of the frantic energy, the alien now focuses their attention on creative and emotional work, while the rational side rests. Art is created, music and theater performed and attended, problems are solved and ideas generated, but for obvious reasons, the custom is for them not to be executed until the next cycle, after vetting by the rational side.

Turbo

The aliens can pull the resting part out of its sleep cycle and go into a well-balanced state that allows superior intelligence and thinking, at the cost of lapsing into a catatonic state afterwards, as the delicate cycle has been disrupted and needs to recover.

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I would suggest using two brains or splitting our brain in half, but I see that's already been suggested above. (Rest one brain and then use the other). Maybe utilize some sort of expedited hibernation that could potentially be triggered within minutes? Also, if your species is some kind of plant, you could logically use photosynthesis, chemosynthesis, or some other sort of bioenergy to refuel your organism.

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