How would humans be affected by a 12-hour day?

Question

In Can I significantly shorten the days on a planet that can support human life? I asked about changes needed to a planet to support human life with a much-shorter day (12 hours or so instead of 24ish), and learned that speeding up the rotation of an earth-like planet would be feasible. (Answers are divided on whether such a planet should have a moon.) In this question I want to ask about the physical and psychological effects on the humans living on such a planet.

Assume that these are human colonists from earth; they did not "grow up" on this planet but found it in their explorations and settled. They live in structures above ground and spend time outdoors, same as most of us. They're affected by the natural light and the (apparently?) moderated temperatures.

How would this shortened day affect humans over time? Would their circadian rhythm change drastically? Would there be psychological effects, or would it seem normal that you sleep "every other night, plus a bit"? How would their outdoor activities be affected by the shortened day?

On earth we can look at prolonged sunlight and prolonged darkness at high latitudes, but I'm not sure how to research this faster oscillation.

Answer 1

6 hours of daylight, 6 hours of night, twice a 'day'.

Mapped to our 24 hour days from midnight to 6am, it's darkness. From 6am to noon it's daylight. From noon to 6pm it's dark. Daylight from 6pm to midnight. With this schedule, for an office worker in a First World country, one could easily argue that there's no difference.

Waking up for the day happens at approximately same time as on Earth. The morning hours are the same as on earth. There's plenty of light to wake up to. Those people who need lots of light in the morning to get started will be pretty happy. A local adaptation might be to have lunch at "first dusk". Nap time for kids after lunch should be really easy with this light schedule. Perhaps an after-lunch siesta as specified by NASA experience? Office workers who work inside will know that it's time to quit when "second dawn" happens. They then have six hours of daylight to work with for extra-curricular activities. This graph seems to indicate that at least within the US, most people go to bed around midnight. Madrid seems to be able to make the party last till much later.

This fits well with the slightly modified work/play cycle. Morning people still get their quiet hours of solitude. Night owls get more daylight to work with. Some people are really going to have a hard time with this change because for some reason they are very sensitive to dawn and dusk.

A discussion about whether getting only 6 hours of sleep a 'day' is healthy is a different question.

Based on this scenario, I don't think this 'day' schedule will have a huge impact on human circadian rhythms. Artificial lighting would mitigate the effects of 'second night'.

Answer 2

I think a conflict of circadian rhythms alone would pose significant social and economic difficulties for a human population settling there.

Since the day-night cycle is only 12 hours, by day 2 every human on that planet who followed a 24 hour circadian rhythm (read: everyone) would find themselves developing shift work syndrome.

While over time the population could get used to the cycle, they still would have to get into the habit of getting less than the 'optimal' 8 hour sleep session per night. That means less time to enter and exit REM, significantly impacting the restfulness of sleep. The consequences could be many, but the biggest I'd foresee is weakened immune systems. On a new planet, unusual microbes are a certainty, and at the best allergies would run rampant while at the worst a plague would kill even more effectively.

The social effects beyond just sleep could be interesting. To allow at least a consistent day-night sleep cycle, work shifts would be shorter. Add in commutes and the pace of daily life could be hectic. Work productivity might increase, however. Don't they say people only really do 4 hours of work in an 8 hour day anyways?

If an 8 hour workday is followed, it might restore some social balance but the shift work syndrome problem remains. And that could hurt productivity long term.

On the other hand, companies selling coffee or sleeping aids could make a KILLING in this society.

Over time, an indigenous population could of course adapt, but the first two generations might be grumpy and stressed for their entire lives, chugging java and popping pills habitually.

Answer 3

What you describe is very similar to what people go after when they try polyphasic sleeping, which is having more than one sleep phase per day.

The general opinion I have seen on it is that people get away with sleeping less per 24 hour period but every now and then end up sleeping for multiple days in a row.

Answer 4

Others have suggested a sleep schedule of sleeping every alternate night which is not too different from our current 24h cycle however an alternative (and perhaps more natural to these people who have never lived on Earth) schedule would be to sleep every 6h night (so twice every 24h).

For issues with lack of REM sleep I believe the human body would adapt over time to compensate for this and this natural compensation would be aided by the shorter day/night cycle. (See REM Rebound).

In any case, I think a society capable of interplanetary travel and colonization would have the sufficient technology to adopt appropriate solutions through artificial lighting etc. (Especially lighting more advanced than ours, such that it can accurately replicate natural sunlight)

So in short, I believe human individuals wouldn't really be all that affected by this change especially if they were raised in this environment (it wouldn't really be a change to them at all). Human society might be affected in some significant ways such as relatively more significant commute times for a shorter working day.