I'm thinking on the colonizing of another planet by humans. The conditions of this planet are quite different from Earth, and particularly the day-night cycle is much larger.

In my fictional planet, the day will last 48 hours. The day-time and the night-time is duplicated, and that means than the proportion between the hours of sunlight and the hours of night is the same than in the Earth.

Would the humans living on this planet adapt their sleeping to the new day-nigth cycle, i.e., will they sleep during all night-time and be awaken during day-time? (This is after some years or even, if needed, a couple of generations.)

  • 1
    $\begingroup$ possible duplicate of Sleeping on planets with very long days $\endgroup$
    – James
    Commented Feb 11, 2015 at 21:23
  • 2
    $\begingroup$ @James this question is much older than the other. $\endgroup$
    – Vincent
    Commented Feb 11, 2015 at 22:06
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    $\begingroup$ also, the question is about human adaptation (short term ) and the other is about evolution (more long term) $\endgroup$
    – Vincent
    Commented Feb 11, 2015 at 22:08
  • $\begingroup$ It is worth noting that the 8 hour sleep thing is a relative recent introduction and not really mandatory. There are plenty of people who sleep less than 8 hours a night, as well as people who go by a system of taking shorter powernaps. The main reason it is so "popular" these days is because people have 9 to 5 jobs that require them to be up at certain times, forcing them into a rhythm. People in medieval times did not have this problem as much, people in the future might not either. $\endgroup$
    – Theik
    Commented Jul 24, 2015 at 11:33
  • $\begingroup$ Possible duplicate of Effect of longer day/night cycle on human sleeping patterns $\endgroup$
    – Mołot
    Commented Feb 25, 2017 at 9:06

5 Answers 5


Actually, an 48 hour cycle is great, because it's exactly twice the normal 24 hour cycle. Thus adaption would be easy; people would just sleep twice for each planetary day. There might, however, be the need to have bright enough illumination during the "night-day" time, in order to stay healthy for a prolonged time (I guess they are on that planet for the whole rest of their life, not just for a year or so).

However, also something else might happen: In experiments to the circadian rhythm where people were living in bunkers without any indication of current time, it turned out that some people actually switched to a ca. 48-hour rhythm, and when the time was over, they didn't believe that much timme had passed because for their own feeling, only half the number of days had passed.

So maybe with an external 48h stimulation, most people would automatically switch to that cycle.

Also note that this cycle is special exactly because it's twice the normal time. On a planet with e.g. 30 hours per day, people would likely experience a permanent jetlag.

Here is an article mentioning the time experiments and the 48h rhythm.

  • $\begingroup$ A quite complete answer. In fact, I chose the 48 hours cycle because it may make it easier the change. BTW, do you have by chance a link to those experiments you talk about? $\endgroup$
    – Garoal
    Commented Sep 17, 2014 at 13:13
  • $\begingroup$ Actually I don't know if it still would but when I had a chance my body clock defaulted to a 36 hour cycle even in the presence of 24 hour day-night cycles. As a student I was regularly awake for 24 hours then asleep for 12 and just defaulted to that whenever I didn't have regular days to keep. $\endgroup$
    – Tim B
    Commented Sep 17, 2014 at 13:47
  • $\begingroup$ If you have access, I guess this link should provide you with everything about the experiments. Otherwise I've now found this PDF with a bit of additional information. $\endgroup$
    – celtschk
    Commented Sep 17, 2014 at 13:50
  • $\begingroup$ @TimB: Interesting. I note that again, the cycle is in a low-integer ratio to the normal day cycle (24:36 = 2:3). $\endgroup$
    – celtschk
    Commented Sep 17, 2014 at 13:52
  • $\begingroup$ @Garoal: I just notice that I forgot the at-attribution in my answer to your comment, so I add it with this, so you get notified. $\endgroup$
    – celtschk
    Commented Sep 17, 2014 at 15:47

Yes people would adapt and live.

People need approximately 8-9 hours a day of sleep. So people would get that much sleep during a 24 hour period. If people go 24 hours without rest on Earth, they have problems. People on the other planet would too. According to this article, people in Space (which has weird day night cycles) sleep pretty much like they do on earth.

Generally, astronauts are scheduled for eight hours of sleep at the end of each mission day. Like on Earth, though, they may wake up in the middle of their sleep period to use the toilet, or stay up late and look out the window. During their sleep period, astronauts have reported having dreams and nightmares. Some have even reported snoring in space!

  • $\begingroup$ So they will still maintain the Earth sleep cycles, instead of adopting new cycles adapted to the day-night cycle? $\endgroup$
    – Garoal
    Commented Sep 17, 2014 at 11:59
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    $\begingroup$ Yes, we have to. We're built for this planet. We'd have to completely change to not follow this day-night cycle. $\endgroup$
    – DonyorM
    Commented Sep 17, 2014 at 12:00

I suspect that the early settlers would adapt by taking a relatively short nap(maybe 4 hours or so to start) during the day cycle and a longer sleep period during the night cycle. As the generations grow up used to the cycle I would expect that the population would adjust to extended cycle times without the need for the naps.

Some things that are going to affect this:

If it is signfigantly more friendly of an environment in the day than it is at night, there is more pressure to get things done during the favorable cycle, this is going to push for the longer cycle times. For instance in a desert world the temperature is likely to cycle greatly making the nights extremely cold and the days extremely warm. In this the cycles are going to be optimized for the dawn and dusk cycles where the temperature is the most friendly.

On a planet where the local Fauna come out and pose a threat during one cycle or the other are going to push for the opposite cycle to be the primary operations cycle. Many people probably think that predators are most active at night on earth but the truth is most predators are Crepuscular meaning they operate most at dusk and dawn. So in a world with large predators to contend with the population is probable going to adjust to a dusk and dawn sleep cycle with waking periods during the day and middle of the night.

A civilization forced to rely almost exclusively on solar power is certianly going to want to get the most out of their daylight hours pressuring people to be active for the daylight period and resting(and thus using less power) during the night cycle.

So there is more to think about than just what is the cycle of the planet. You need to consider those factors that are going to have a huge impact on when humans operate. Humans are adaptable and will adjust to their environment in the way that they need to.


There are 2 main issues that come to mind:

  1. Sleep is strongly affected by light. The presence of light inhibits the production of melatonin, darkness encourages it. Sleeping during the day light hours would require a lot of shade. Staying awake during the dark hours would require good lighting and an extra pot of coffee.

  2. Human beings do not NEED to have all of their sleep in one stretch. Thomas Edison famously power napped instead of sleeping all at once. Ultimately REM sleep is cumulative, and if you get enough of it, it doesn't matter how you do it.

That said, here is how I think humans would adapt:

Daytime would be mostly work time, with catnaps interspersed through at the light hours. I would tend to believe that there would be easily accessible bunks in work places for people to go at naptime.

Nighttime would most likely be the opposite. If we assume there would be 16 hours of darkness, I would guess that at least half of it would be spent sleeping, probably in 2 chunks. Maybe 4 hours of sleep, 8 hours of fun-time followed by another 4 hours of sleep before getting back to the daily grind.


It would be most likely very easy to change daily activities and sleep schedules, the amount of time it would take for someone to change may depend

some people are very heavy sleepers but other are very light and wake up in the middle of the night periodically to use the restroom or other utilities thus it may be easy for other not so easy for others


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