Submitted via Google Form:
How many generations would it take the population to change their circadian rhythm when moving to a new planet? Their original planet had 22 hour rotations but their new planet has 38 hour rotations. Also, how would the generations in between adapt to life/arrange schedules when their physical cycle doesn't match the planet's day/night? If someone say ate breakfast at 9AM daily according to their own cycle, their breakfast time will vary at different times of the local day. Sounds rather confusing.
Tex: Chronobiology is a bit of a specialized field, and because of that, there’s still a lot of research being established about the subject. I haven’t found a lot of research on the presumed epigenetic component of altering the CLOCK gene (and and assorted related genes) that govern the circadian rhythm, but I’m going to guess that an individual could slowly adapt to a new planet, and that those adaptations would be reflected genetically in their lineage. How these changes show up - and correspond to the original expression of the gene(s) - is currently unknown, but since this is your world, you can play around with it.
Feral: Tex has already talked a bit about the biological aspect, so let’s talk about the environmental - specifically the built and social environmental aspects - which play more into your second question. How much control will settlers have over their built environment and how it is lit? What type of built environment will they have? In 2014 Alexander Gerst provided a look at his daily schedule on the ISS, showing a wake-up call for the astronauts at 6am and bedtime at 9:30pm (GMT). Among the other rows showing how their day is segmented into tasks, one row shows Day/Night, in other words when the station experiences daylight and when it experiences darkness. Remember that clocks are made up, and the clock you use to determine when breakfast is is not actually coordinating with the sun. Ptolemy defined a second somewhere around 150CE, but astronomically, not only was that not as precise as what we can measure now, the Earth is actually spinning several milliseconds faster than it was in the second century. Our concept of how many seconds there are in a day hasn’t changed; the length of a second (and therefore a minute and therefore an hour and therefore a day) has. Further, although we have a 24-hour cycle, day and night are not equal lengths throughout the year, yet jobs and schools do not shift timing to match. That’s not how it was a couple centuries ago, before electric lights became the norm, but social time has been quite static since - at least in monochronic cultures. A polychronic culture may adapt very differently to a different daily rhythm.
One note on scientific accuracy (if you care, but you are absolutely not required to care): the rotation of your planet taking 38 Earth hours may be too slow for comfortable habitability far beyond circadian rhythm. It’s not an outlandishly long time, but it’s worth considering what other environmental adaptations your people will need to make. Daily scheduling might be the easiest thing they’ll have to overcome.
Synth: Over the decades there have been several studies on how human circadian rhythms shift when deprived of external cues. There are many body processes that exhibit cyclical patterns (core body temp, various endocrine functions and levels, etc.). A lot of them maintain their patterns with very little fluctuation from baseline even under extended periods of altered or completely absent light cues, or erratic sleep patterns, which indicate that they are governed primarily by internal genetic factors, not external environmental ones. There is of course some variety between individuals, but it all averages out to a cycle just a bit over 24h in length [Stability, Precision, and Near-24-Hour Period of the Human Circadian Pacemaker; Czeisler et al. 1999 → a very summarized overview in this article.]
Now sleep, specifically, can vary a lot more, especially when there are no external signals (called “zeitgeber”, German for “time giver” or “synchronizer”) providing any hints as to time of day. During some “free-running” experiments, where there is a complete lack of zeitgeber and the study participants are left to sleep and wake as they feel like it, sleep patterns would drift gradually forward by around an hour each day, and the subjects’ “days” lengthen toward a period of 25-26 hours, which later studies revealed was due to the influence of artificial lighting [Entrainment of the Human Circadian System by Light; Jeanne F. Duffy and Kenneth P. Wright, Jr.] A study in 1973 by Mills, Minors and Waterhouse had subjects who adopted “days” spanning from 24 to 30 hours, though this study was relatively short, lasting not even two weeks total, so who knows if the patterns would have changed more over a longer timespan.
All the way back in 1938, Nathaniel Kleitman and Bruce Richardson spent just over a month in Mammoth Cave, isolated from the normal light and temperature zeitgeber, with the goal of seeing if they could shift to a 28-hour “day”. Richardson did adapt to it; Kleitman struggled. In 1965, spelunkers Josie Laures and Antoine Senni spent 88 and 126 days, respectively, in caves in the French Alps for a similar experiment, though theirs were free-running instead of strictly regimented. There was a lot more variation in the lengths of their “days”, with extremely erratic sleep cycles. A similar experiment was carried out in 2021, with fifteen participants and lasting forty days (I have not found any research papers related to it, but it was pretty recent and there might just not be that much published yet).
Here’s another paper talking about studies in chronobiology that has more links to other research papers in it that I wanted to include but couldn’t find a good in-line place to put it. Reading up on sleep disorders like delayed sleep phase disorder, advanced sleep phase disorder, and non-24-hour sleep-wake disorder might help with brainstorming too, since they all deal with sleep patterns that don’t fully mesh with the planet’s day-night cycle.
Maybe your characters adapted within a few generations, or considerably more than a few generations. Maybe there are some family lines that never adapt and others that manage it really quickly. Since they have the tech to migrate to a whole other planet, they presumably have artificial lighting, so perhaps they adapt the environment to themselves instead of the other way around, and maintain a semi-artificial “day”. Or everybody gradually time-shifts their way through all the days, going to sleep later and later until they loop back around, or their society splits into the night-owls and the early-birds as people settle into the rhythms that best fit their own natural tendencies and their new society is structured around that.
Tex is right in saying it’ll ultimately be up to you to decide how you want things to unfold in your story. The sample sizes and time spans of the experiments that have been carried out so far are not large, so really the only way we’d ever know for sure how any species would adapt to a planet with a day almost twice as long as they’re used to is to actually go out and settle such a world and see what happens, and we are quite a ways off from being able to do that. It might be more to think about when building your own world, but also opens up narrative flexibility since you can go with whichever options best support the story you want to tell.
