I have a world with days lasting 9 years. The inhabitants of this world have taken to migrating across the globe to avoid the deadly night, keeping themselves in a sweet spot. One of the biological quirks of the people living here is that sleep isn't a thing. Similar to dolphins, they sleep with only one hemisphere at a time, allowing them to maintain consciousness at all times. This allows them to get their required sleep while still being able to migrate throughout the 'day'.

As I began to develop the mythology and cultures of this world, a question popped into my head that would heavily affect everything this species does; how does their consciousness work. There are various signs that our right brain is really its own intelligent consciousness that works with our speaking left brain; But on a species where each half is only conscious for half the time, would they only be able to speak half the time? What about the other things?

The right half of your brain dominantly controls things such as spatial abilities, face recognition, visual imagery and music. While the left half deals with language, math and logic. Would a species that only uses half the brain at any moment only possess one half of these features at any given moment? Would it have two separate personalities? Or is there other things that I (admittedly not a neuroscientist) just am not seeing?

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    $\begingroup$ If their need to migrate is so pressing that they can't sleep, think about the fact that your people can't have any settlements. They have to be on the move, constantly. Any sick, wounded or too old needs to be left behind. They would not likely have enough time to get familiar with the territory they are moving through. A very stressful existence, in my opinion. $\endgroup$
    – Alexander
    Commented Jul 12, 2017 at 6:28
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    $\begingroup$ One thing to consider. They wouldn't be on a "half the brain all the time" schedule. Considering us a a species right here and now on earth. We sleep for 8 hours and are awake for 16. Now, if the brain sleep was horribly inefficient and each half of the brain needed 8 hours of sleep to function you'd still have 33% of the time using both halves. $\endgroup$
    – Doomfrost
    Commented Jul 12, 2017 at 8:24
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    $\begingroup$ Reminds me of the OnOff Star (Warning: link contains spoilers) $\endgroup$
    – gerrit
    Commented Jul 12, 2017 at 9:53
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    $\begingroup$ Biologically, I don't see why a species that evolved this way would ever lateralize to this extent (which is overplayed anyway), but it's an interesting story idea either way. $\endgroup$ Commented Jul 12, 2017 at 16:31
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    $\begingroup$ The brain may very well not specialize in a similar manner if the species requires to be constantly on the move. I wonder if sharks exhibit hemispheric specialization... $\endgroup$ Commented Jul 12, 2017 at 19:11

6 Answers 6


Firstly, if your only reason for the hemisphere-sleep adaptation is the inhabitants' constant migration, you might want to reconsider. A 9-earth-year day means it takes more than 78000 hours for a full rotation of the planet.
On an approximately earth-sized planet with a circumference of 25000 miles, the edge of the night is only advancing at a rate of about 1/3 a mile per hour, or less than 8 miles a day (not accounting for variance due to local geography like mountains). Human-like creatures could easily cover that on foot without needing special sleep adaptations.

I'll assume you want to keep the hemisphere-sleep idea anyway, as it is interesting.

Dolphins are less active while in their half-sleep, which lasts about 8 hours a day (in 2 hour cycles). They tend to either swim in place or slowly. Your inhabitants would likely be less physically active during half-sleep.

Studies of dolphins suggest that they remain fully alert during their half-sleep, and can recognize shapes with one hemisphere that were observed only by the other (meaning there is communication between hemispheres despite sleep). In humans, recent evidence shows that even people whose hemispheres have been physically severed experience slow communication between hemispheres. Your inhabitants would likely be fully aware of their surroundings and able to rouse themselves and others quickly when necessary.

Of course, popular opinion exaggerates the isolated roles of the brain hemispheres. If you are less concerned with scientific accuracy and more concerned with interesting story ideas, you can expand upon any of the traditional left-right brain ideas - after all, these are aliens whose biology is being defined by you.

Finally, while it may not be related to their half-sleep, dolphins also seem to have little to no REM sleep. Humans deprived of REM sleep face some consequences. Introducing this element could be an interesting twist.


Split brain hemisphere communication - http://www.uva.nl/en/content/news/press-releases/2017/01/split-brain-does-not-lead-to-split-consciousness.html

Dolphin sleep alertness - https://www.livescience.com/7763-dolphins-eye-sleeping.html

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    $\begingroup$ Excellent answer and welcome to Worldbuidling SE AjimOthy! Could you go a little more in depth about REM, please? $\endgroup$
    – TrEs-2b
    Commented Jul 12, 2017 at 8:31
  • $\begingroup$ Welcome to WorldBuilding.SE AjimOthy! Interesting first answer. If you have a moment please take the tour and visit the help center to learn more about the site. Have fun! $\endgroup$
    – Secespitus
    Commented Jul 12, 2017 at 8:50
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    $\begingroup$ @tres-2b I think a question on how humans would behave without REM sleep is a question worth asking on the site. go for it. $\endgroup$ Commented Jul 12, 2017 at 16:37
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    $\begingroup$ +1 Just for the back-of-the-envelope "you can outrun the night with a normal human sleep/wake cycle" calculation. $\endgroup$ Commented Jul 12, 2017 at 17:58
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    $\begingroup$ Also, most of the time the actual speed of night creeping will be less since you might not be running along the equator. It would be easier to do a full circle around the world if you are closer to a pole :P $\endgroup$
    – vero
    Commented Jul 13, 2017 at 2:23

Something to consider is also where your population migrates to on a continual basis. Assuming you have the two hemispheres sleeping in an alternating pattern like you are describing to allow for continual movement, a population could easily be moving 2/3 of the time or much more if they use boats, which should be feasible since a long day like this world has would generate consistent winds at the border region between night and day.

Assuming standard earth size you have a distance pole to pole of roughly 12500 miles. Even at a walking speed for 16 out of 24 hours, you can make the trip in 390 days. When at or near the poles, you would have a "goldilocks" zone where your migratory population could settle for several years before needing to move to the other pole due to seasonal axial tilt towards the parent star for your planet. If you don't have any axial tilt both poles would be permanent goldilocks zones.

You should also nail down just how large your buffer zone is for the border region. Earth rotates to cover over 1000 miles per hour at the equator. If your tolerable zone is even half that width (The equivalent of half an hour of twilight on earth), your migratory population could easily stop for over a month at a time before needing to move again.


The hemispheres have their own specializations, but there aren't any sophisticated activities that involve entirely one hemisphere or the other. Speaking, for instance, involves both hemispheres, both for understanding language and then creating it. Same for math, logic, creativity, etc.

Some quotes from the cognitive neuroscientist in this article:

The Truth About the Right Brain / Left Brain Relationship

"It takes two hemispheres to be logical – or to be creative."

"the abilities that make up math skills arise from processing that takes place in BOTH hemispheres"

"So, like other complex skills, the ability to understand what we read or what someone is saying to us requires both hemispheres, working together and separately."

If people were frequently "asleep" using only one hemisphere or the other, I suspect the obvious adaption would be for the hemispheres to become more generalist, less specialized. It would be an enormous survival advantage to able to perform all mental activities 24 hours a day, rather than certain activities really well 8 hours a day.

Interestingly, it might not even take any genetic changes. People who have had one hemisphere removed have adapted remarkably well:

When Half a Brain is Better than a Whole One


As you point out, dolphins already sleep using half brain at a time. Yet they are able to keep swimming and don't act weird when half of their brain is in sleep mode. Since dolphins and humans are mammals, I guess there is no big difference in the underlying brain processes, and the same can apply to your humans, too.

I remember reading somewhere that experiments on sleep deprived humans have shown that phases of microsleep (subject is awake but the brain shows sleeps fingerprints for short times) are a constant when no continuous sleep is possible. So, I assume that our brain would need "just" some updated "driver" to implement the same feature.


On a planet with 9 year days, it might be a serious problem for intelligent life to develop. If during that long day, the temperatures stay in the range people can tolerate, it would probably mean that in the dark regions it's really cold. I think the region where people should live would be very narrow.

Also, there would be little plant life as we know it on earth since no tree would resist freezing below -100C. I assume most land plants would be ferns and mushrooms who can produce frost resistant spores. The land animals would be small and would also take part in the great migration, like every mobile being on the planet.

The weather would be reasonably stable, and it would be mostly cloudy all the time, assuming you have enough water. Close to the interface regions between light and dark, you would have strong storms that would be a great motivator for someone not to fall too far behind with the migration.

Humans don't really belong to this environment, but if they existed, they would be faster and smaller than homo sapiens. They'd also have a lot of endurance. Maybe they'd weight about thirty to fifty kilograms, not more, because of the food scarcity. Other animals that could coexist with them would be birds, goats, or dogs. There could be also some sort of Sarlacc-like beings. Sarlacc would burrow into the ground, in the path of migration and wait for its prey. What would fall into its pit would be consumed over the nine-year night.

After years of migration, people will learn the pattern and the ones getting old and falling behind would start building settlements to be found by the next wave of migration. Enough generations would be able to learn how to build strong enough buildings to resist the cold and the storms at the interface. They could capture (tame) Sarlaccs and use them to preserve food for the settlements.

Once they start settling, people could grow slower and stronger to be able to keep up with the building. The settlements would look a lot like ant hills. This would especially be feasible if the planet has thermal waters.

For the migratory people, not having to sleep would be quite useful, as they would have to always be on guard for various dangers. Dolphin brains would help, and if they had those brains, the people would look more like the Ferengi in Star Trek. They would also have big ears, like elves. The eyes need to be like ours, only they don't need as good irises. They aren't adapted for night. They also have to be dark skinned to get more sunlight protection and they have to be able to sweat. Their kidneys have to work great to be able to drink whatever water they run into. I would also put a hump on their backs to store additional fat and water.


You don't need to go too far. The nomads could use polyphasic sleep techniques, like Uberman, and just take quick 20-minutes Uberman cycle naps. every 4 hours - ish. Different people could use different cycles.

This circunvents the lack of REM sleep, and allows them to be active most of the migration time.

And as for napping, they can always hit the hay and take a nap in one of the farm carts.


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