Okay, so I have this binary planet system. The main planet a bit bigger than earth with a size appropriate moon-planet. Both are habitable and with a humanoid species that evolved there.

But, I know that the rotations of the planet are tidally locked as a result and a 'day' on the moon planet is much longer than the primary planet, about 27-29 days in earth days and a much heavier tide on both sides

On both planets, the natives are humanoid enough; requiring almost all basic things we do; oxygen, food, water, sleep etc... and a lot of vegetation— and accompanying wildlife (predator & prey).


Since the days are vastly longer on the moon-planet, how would that effect the wildlife and natives?

I'd assume 2 weeks in day and 2 weeks in night; if that's the case then It could make things easier to see.

Vegetation life would be good, the 2 weeks or so, the plant life would develop bio-luminescence (using the energy absorbed in the 2 weeks with sun) to keep itself alive.

But my primary focus is on the humanoids.

Would hibernation for 2 weeks be the more obvious choice for their physical wellbeing to deal with the long days. (I am aware humans cannot hibernate but other mammals do)

Or would they have to live 2 weeks in darkness and 2 weeks in light and use our sleeping pattern of 8 hours a sleep in a designated amount of time?

  • $\begingroup$ I suggest you use "earth day" and the like to refer to time unit. Something like "a day last like 14 earth days". Else your question is pretty confusing to follow. $\endgroup$
    – L.Dutch
    Commented Jan 17, 2019 at 14:17
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    $\begingroup$ It would depend on how they evolved. Hibernation is also a time when they're vulnerable to predators. 14 days' risk vs. 8 hours. If they evolve a social structure that can protect them, and sufficient food to not starve when hibernating, 14 days of sleep can be viable $\endgroup$
    – nzaman
    Commented Jan 17, 2019 at 14:22
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    $\begingroup$ Just boiling this down to 'would humans on this planet hibernate or sleep 8h' is - in my opinion - the wrong approach. You need to consider that us earth-humans have evolved from animals that adapted to an 24h day-night-cycle in which it was advantageous to be inactive for some part of that cycle to conserve energy. With a 28x longer cycle it is MUCH harder to build up large enough energy reserves for a full sleep/hibernation cycle, but at the same time the temperature drop you get from a 28h night makes it VERY advantageous to actually do that. $\endgroup$
    – Nicolai
    Commented Jan 17, 2019 at 14:57
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    $\begingroup$ [cont.] Additionally you need to consider the evolution of your humanoids - their adaption to the day/night cycle will largely be determined by what was advantageous for the species during evolution and not by what could be advantageous for an highly evolved intelligent species. $\endgroup$
    – Nicolai
    Commented Jan 17, 2019 at 15:00
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    $\begingroup$ Slightly off topic, but it sprang to mind: Be sure to look into meteorological/climatological consequences of your proposed day-night-rhythm. Most likely your moon will have a rather harsh climate with strong winds, sturdy, low growing plants adapted to cold, long nights aka mini-winters every two weeks and rather strong thermical erosion effects. $\endgroup$
    – Erik
    Commented Jan 17, 2019 at 15:15

2 Answers 2


Sleep need is relative. Many animals don't need sleep at all, even more complex ones such as whales and dolphins.

Cetaceans can't really sleep or they will drown. So they just "shutdown" (actually, reduce or change activity) one hemisphere of their brain for some time everyday. Then they swap the "sleeping" hemisphere. During this time they just float around doing nothing intensive, but they are still awake. After that "whale rest" they'll have both hemispheres in gear again and will do whatever it is that they need to do.

Humanoids in your world that don't hibernate could do it like whales do, even if they are land dwellers.

They may also have eyes like the ogre spider:

Its eyes are able to gather available light more efficiently than the eyes of cats and owls, and are able to do this despite the lack of a tapetum lucidum; instead, each night a large area of light sensitive membrane is manufactured within the eyes, and since arachnid eyes do not have irises, it is rapidly destroyed again at dawn.

With such prolonged days and nights, this mechanism to change the way the eye works from day ti night could evolve to be even more efricient than the ogre spider's.


Perhaps the humanoid species has diverged into two subspecies (or modes!): one is nocturnal, and the other is diurnal. The two have a symbiotic (and perhaps interfertile) relationship. During the day-month, the diurnals manage things. During the night-month, the nocturnals take over.

This dual-benefit-ism could form a very strong part of their psyche. It would also form a rather important kind of trust between the two groups, since a group that acted selfishly against the other would essentially destroy the whole. I can hear fables or parables being told about this world-wide.

So as not to be boring about it, I would assume that each species or order (or phyla...) would have solved that problem in DIFFERENT ways. So if you don't like the sophonts being dualist, perhaps an order of "regular" animal does it this way.


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