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This is the same as my previous question Sleep Cycles on very fast and very slow rotating worlds Part 1 but this part is for how humans settling these world would alter their sleep cycles!

I would think for the slower worlds, humans could keep their old 24-hour cycles and just act as though they are naps throughout the days and night.

For the faster ones, humans might have to take shorter naps and act like they are full nights of rest.

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  • $\begingroup$ Vernor Vinge's Grimm's World is set on a 44-hour planet whose humans have forgotten that they came from elsewhere. Most people have a short sleep at midday and a short waking period near midnight; but one religious sect insists on staying awake all day, arguing that if God meant us to sleep more often He would have made the day shorter. $\endgroup$ – Anton Sherwood Oct 25 at 5:18
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There have been a few experiments into the natural circadian rhythms of humans and it seems that we seem to be have an inbuilt clock which is just slightly over 24 hours.See experiments here.

I imagine that, like with the experiments, that the new colonists could just use lighting to keep themselves in a 24 hour(ish) rhythm in their sleeping quarters. The only issue would be with with outside work, but they could just use portable lighting during dark periods. Humans survive harsh conditions like working night shifts, living at research stations near the North Pole (where days and nights can be long), and even on the International Space Station, and just adjust to it thanks to electric lights.

A bigger issue would be growing plants outside, which also seem to have a similar 24 hour rhythm. They may adapt to a planet with a short day/night cycle, but may be killed off if the planet has an extremely long night period, as they would not be able to photosynthesize until it is day again. While you could again get round this with hydroponics and UV lights, it may not be practical for a large scale colony.

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If the day length was equal to Earths day length or fairly close to it – perhaps within 10-20% humans could probably adapt to the difference some more easily than others. If the day length was around half of an Earth day adaption might occur by sleeping every other night.

In other short or long day length scenarios I imagine that humans would simply use a 24 hour clock and sleep as we do not paying any attention to the planets day or night cycle.

With short days people would adapt to do things that required day light at different times after waking up. Sometime it would be dark and they would do indoor activities until it got light and at other times they would be able to go straight out into day light and do indoor tasks later.

With long days the effect would be a little bit like summer/winter seasonality. During the long daylight hours people would schedule most of their outside tasks in between their standard human day night cycle sleep periods. During the long night time hours people would schedule most of their indoors activities in between their standard human day night cycle sleep periods.

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  • $\begingroup$ Mars may become the only place in the Solar System where humans use a different clock. Every other body has a cycle either too long or too short for our comfort, or else is so small that it's all one city with one time zone. $\endgroup$ – Anton Sherwood Oct 25 at 5:23

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