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Setting: We have settled a planet with days and years not even approximately like earth. The settlers must decide what their workday is and their week, year, etc.

Premise: Colonists will establish some routine calendar to do their business. The planet has a natural orbit and rotation, completely unlike 24 hours and 1 year. Several multiples of them (exact number is irrelevant)

Other factors:

  • Interaction with earth is trivial and takes years anyway
  • Terrestrial plants and animals are in the colony, with their own rhythms.
  • Nights are pitch black: No moon

Which rhythm will dominate the decision to set an extra-terrestrial routine? The endogenous 24-hour rhythm or the alien environmental rhythm?

I’m not sure if it’s realistic to have the colony keep earth time when the daylight completely ignores their clock, or if they will develop a calendar and days that “make sense” for their environment. These competing rhythms will tip one way or another. Can the endogenous rhythms be effectively subdued? Or will they be forced to have arbitrary daylight times and stick with their biology?

  • NOTE: This does not ask for any calculation.
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Alien environmental rhythm
Humans largely rely on photic input (light) for their Circadian rhythm. People not stimulated by sunlight, the blind, therefore often suffer from non-24(This is a chronic deviation from the normal 24-hour cycle). In one experiment, Stefania Follini lived in a cave for a long time. She did not have sunlight, but controlled the light herself, and ended up with a 48-hour day schedule.

Now depending exactly on how long your days take, it is very well possible to adapt to this new day cycle. A 48-hour schedule is already proven, but more might require the population to take a mid-day nap, to wake from their sleep in the middle of the night to exercise before sleeping some more, or any other solution to neither sleep too long, nor be awake for too long. Other than that, it is very likely people will adapt to local factors (even first arrivals). The process of adapting could be smoothed out on the space journey, by gradually shifting the 'simulated daylight cycle' on your spaceship from that on Earth to that of planet X.

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  • $\begingroup$ For days longer than 48 hours, colonists could split them up into several sleep cycles. A 60-hour day could for instance be split in two by sleeping 8-9 hours after midnight (when it might be too cold to be out) and in the early afternoon (when it might be too hot). $\endgroup$ Mar 1, 2022 at 10:54
  • $\begingroup$ But how well did Stefania really adapt? From the same article: "She ate less frequently and lost 17 lbs. Her menstrual period stopped. In short, her internal biological clocks had gone out of whack." $\endgroup$
    – Daron
    Mar 1, 2022 at 15:44
  • $\begingroup$ 17 lulubs is nearly eight kilos. $\endgroup$
    – Daron
    Mar 1, 2022 at 15:44
  • $\begingroup$ @Daron While that is a lot, you need to keep in mind that it wasn't just the sleep schedule that did that to her. She was almost completely isolated for the whole test. (having a few test animals and a computer to keep her company). She had a deficiency of vitamin D because of the lack of sunlight, and she stayed cooped up in not a lot of space for the duration of the whole test. She also ate fewer meals a '24-hour day' because her days became longer. None of this seems to help her stay healthy. An extra meal a day would solve the weight-loss issue. $\endgroup$
    – vinzzz001
    Mar 2, 2022 at 10:12
  • $\begingroup$ @vinzzz001 That is a fair point about the extra conditions in Stefania's bunker, my friend! Though there are also people on the surface with weight problems. So "eat an extra meal" might be more complicated than it seems! $\endgroup$
    – Daron
    Mar 2, 2022 at 12:20
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Millions of years old habits on earth

Living things on earth are habitual of 24 hour rhythm since millions of years. That is why a small disturbance (jet lag or shift duty) is very annoying. Somebody traveling from Los Angeles (GMT-8) to Sydney (GMT+11) needs some days to adjust his sleeping or working hours.

Adjustment on the Other planet

Colonists traveling from earth to the other planet will initially keep the 24-hour earth time. After a certain time (small if differences are small, large if differences are large) their later generations will adjust to the alien environmental rhythm.

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If the days and years on this alien world are 'several multiples' of those on Earth, attempting to match the local day and year lengths might end badly.

Humans are highly adaptable beings. It has been shown experimentally that humans can adapt to day lengths significantly different to the 24 hour cycle of Earth. However, a significantly longer day length has also been shown to negatively affect mental performance, similar to the effects of sleep deprivation.

However, other animals and plants are not necessarily as resilient. Humans can continue to thrive with stress levels that would literally kill other animal species. Many species of plant have been experimentally shown to die or fail to reproduce if they are not exposed to the correct variations of day length for a couple of years.

So, with this world's significantly longer days, it would be necessary to impose artificial days and years (complete with varying day lengths) in order to keep the colonists and their livestock and plants healthy. Some fraction of the world's natural cycles would probably work best, if a suitable fraction with integer divisors could be found.

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  • $\begingroup$ The more I think the more broken it sounds. Many plants would just die if you put them in a dark closet for three days. Pollinators would probably all die, and there goes the colony. Three days of sunlight? I guess some succulents could do it? Seems like an endogenous calendar would win out, and deal with midnight beach outing or headlights at noon. $\endgroup$
    – Vogon Poet
    Mar 2, 2022 at 0:59

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