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Obviously there would be a lot of small (not to mention a couple large) changes humans would need to get used to on a planet that rotates much faster than Earth. The biggest one I’m concerned about right now though is how they’d measure a day.

Let’s say that, barring a few incidental adaptations or changes in sleep cycles, this planet’s inhabitants were humans as we know them. If a single 24-hour period on this planet sees three periods of “night” and three of “day”, then how might people start to think about and label this? Would they even attempt to frame it in terms of the 24-hour period Earth humans do, or would they simply accept the new day-night cycle and sleep multiple times in short bursts within 24-hour periods, coming to see a day not as a 24-hour period that’s split by multiple nights, but as a much shorter eight or six hour period of light followed by six to eight hours of darkness? How would they come to see their planet’s natural satellites, if it had any?

To clarify, this is a fantasy world. The humans living in it are, with the usual handwaving and suspension of disbelief, the same as humans in the real world. They’ve lived on this planet all their lives and have no preconceived notions about how time is measured on Earth or any other worlds.

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    $\begingroup$ Hours are a construct which came way after the basic pace sunset-sunrise-sunset-sunrise.... $\endgroup$ – L.Dutch Aug 19 '18 at 5:22
  • $\begingroup$ @Sarriesfan Weirdly enough without a day-night cycle the human circadian cycle is something like 25 hours. $\endgroup$ – Ash Aug 19 '18 at 11:42
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If the same sun rises and sets three times within 24 (Earth standard) hours, then one day is eight hours and not three sunrises per day. Nothing magical about those 86,400 seconds, except that homo sapiens comes from a world with this rotation period.

So you're asking, how would humans adapt to a shorter day?

Usually that question comes in the context of a science fiction setting, where artificial light sources are a given, along with species developed on a different planet. In your fantasy world, the humans are presumably from that world. My guess:

  • People would still try to work during all the daylight periods. Artificial light sources are too expensive and also unsuitable for fine work.
  • Only one meal per day.
  • Logistics might make it difficult to return home from the fields every night, to there might be a temptation to alternate one 'siesta' period (short nap between work) and one 'true rest' period. During 'true rest' one turns in before nightfall and wakes after dawn, during 'siesta' one sleeps through the darkest hours of night.
    • Villages become smaller to keep them closer to their fields. On Earth, it was possible to have a field one hour or more from the village.
  • Different villages might not be able to agree which are nights are which. One might be able to switch at the expense of some jetlag.
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They would definitely label each change between light and dark a "day". Like we label 7 days as a " week", they might find a similar label for a convenient number of days. Like a "month", that was originally one cycle through all moon phases, other celestial bodies visible to these people will likely influence how they structure their time.

They will also feel sleepy during dusk and probably sleep several hours of each night. This is caused by our bodies producing the sleep hormone melatonin when the light intensity decreases and the color of ambient light turns to red in the evening.

They would probanly eat only one meal per day, as this yields the best ratio between preparation time and calory intake.

Work might be structured as 3 or 4 days of work, followed by 2 days of free time. Due to the short days, going to work for half a day and then going home for free time is not practicable. You loose too much valuable time on your way to and from work. It's also very probable that most people either never leave the place where they live or that they have some sort of mobile homes for the purpose of travel. These could also be used to stay at your work site at night to eliminate the need to walk home, loosing valuable time.

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