Might be a stupid question but I think the length and measurement of time is probably important when dealing with the creation of imaginary creatures and their evolution.

On this particular planet a day cycle is 25 metric hours long, 1 metric hour equals 2.4 hours totalling 60 normal hours for a single day. The length of a night is 15 metric hours or 36 normal hours and the remaining 24 are day hours.

This planet is most of the time in the darkness and all the creatures receive 24 consective hours of radiation from the sun, this is twice as much as the earth's equator which has 12 consective hours.

So how different would creatures be from earth? is the sleep cycle of an animal based on actual needed sleep or based on the movement of the planet around the sun? I only know that larger animals need less sleep, which is kind of counter intuitive and I know that some scientists suggest the actual reason sleep exist is not known or certain. Still, how would this day and night cycle compare to Earth's?

I don't really know how to narrow it down anymore since I want to see the effects on an evolutive level thus the effects on all lifeforms.

Maybe, the only way I know about narrowing it down is to imagine that all life begins from one single lucky cell and splits from that cell. This is the most accepted theory of the beginning of life on earth (some suggest life started on earth and was erased completely multiple times from multiple cells formed by chance not just once)

Now knowing that all the life on this planet has one single lucky ancestor, then they have have something in common and should react to that day and night cycle on similar ways.... right?

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    $\begingroup$ I'm not sure if it's relevant for your question, but if half of the planet is illuminated at a given moment, it's pretty hard for all of the planet to have longer nights than days. $\endgroup$
    – Glorfindel
    May 24, 2021 at 10:08
  • $\begingroup$ +1 with Glorfindel; If you want to go on that assymetric day-night cycle path, you should edit your question to explicitly add the premise that nights are longer than days, whatever physics should be able to tell us. It'll help you avoid getting "challenge of the frame" answers, like LSerni did with good intentions :). $\endgroup$ May 24, 2021 at 10:29
  • $\begingroup$ The day-night cycle has nothing to do with sleep cycles, except for the convenience factor - it is easier to see during daylight. Just ask a cat. Many animals are nocturnal. $\endgroup$ May 24, 2021 at 14:40
  • $\begingroup$ in what way is the hour "metric"? $\endgroup$
    – ths
    May 24, 2021 at 17:19
  • $\begingroup$ @ths metric hours are made of 100 minutes, each minute is made of 100 seconds. 1 metric second = 0.864 normal seconds. I just don't like non-decimal systems. $\endgroup$
    – user85816
    May 24, 2021 at 17:20

2 Answers 2


This question very seductively tempts someone to answer how day-night cycles of a rotating planet would always be equal. With a tilt of an axis closer to the poles there would be periods of the year with longer night but only with other seasons of the year with more daylight. For your premise you need to have a reason why light shines on only 40% of the planet. However that is not what you asked.

No one, even biologists, know exactly why creatures sleep. I think this gives you freedom to choose whatever you want as the amount that creatures sleep.

One foremost consideration is that some creatures are nocturnal while only some are diurnal. You might be thinking only that creatures sleep at night but you should not. For some creatures the period of inactivity due to sleep might be shorter due to a lesser proportion of daylight.

Creatures evolving adaptation for better daylight or better nighttime environments may be a reason why we sleep. So creatures which became better hunters verses creatures which became stealthy evolved sleep to regulate their activities to the time of day where they had an advantage. This might mean that there would be a higher proportion of nocturnal creatures.


You cannot have this exact setup since the day/night cycle is symmetrical (due to the rotation of the planet on its axis and around its primary star).

You can have longer nights with the sun low on the horizon, but only for part of the year; then, the cycle will reverse, and you'll have longer days.

A simple way of getting rid of the problem is to change the ecosphere's biochemistry. Just imagine that biological processes are inhibited by the light, and faster at night; then the night will seem longer. Animals will drowse in the sunlight, accumulating energy, then be more active at night.

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    $\begingroup$ Note that with this biochemistry solution to make natives feel that night are longer and days are shorter, anything not ruled by this (aliens, cogs/clocks, free-fall etc.) will "move" quicker for natives during the day, and slower during the night. Also and on the reverse, anyone without this biology will see natives as sluggish during the day, and quick on their wits during the night. Still, it can lead to interesting things and stories! $\endgroup$ May 24, 2021 at 10:40
  • $\begingroup$ On the contrary, it could be technically explained by a VERY massive planet and a VERY diminutive sun, in the order of say the planet being several orders of magnitude larger than the sun, but relatively close to it. The rays from the sun would only subtend a small surface area of the planet. However, the poles would be in eternal darkness unless the axis of the rotation of the planet were tilted. In which case there would be seasonal fluctuation in daylight hours. $\endgroup$ May 24, 2021 at 14:48
  • $\begingroup$ "... all the creatures receive 24 consective hours of radiation...: does not imply that all of the PLANET receives 24 consecutive hours. Maybe the only habitable areas of the planet are around the equator, and the poles are uninhabited. $\endgroup$ May 24, 2021 at 14:51

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