Afaik, humans never tried to live according to shorter or longer days so the Martian astronauts might be the first humans from Earth that have to follow a different cycle. The Martian day (Sol) is about 40 minutes longer than the Terrestrial day. Martian astronauts might take particular clocks with them that follow the Sols instead of Earth days. Was it ever tested what effects the longer days would have on their health? Perhaps Mars' low gravity would have counter-effects since the astronauts wouldn't have so much effort in the low gravity environment. I think that astronauts in space don't sleep so long as on Earth because they don't get as exhausted in microgravity as in 1 g. So the longer day might not be much of an issue for Martian visitors if they don't get so exhausted in 0.38 g. Or would it be better to land near the Martian poles to follow the Earth's cycle while having permanent day on the Mars pole?

  • $\begingroup$ i dont think the change in time would have any effect at all, its so small its would be very noticeable, probably no more than jet lag after a flight $\endgroup$
    – Topcode
    Mar 31, 2020 at 15:56
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    $\begingroup$ @Topcode The jet lag of a flight is a MAJOR effect which is not healthy. On Mars, astronauts would have a permanent jet lag because they don't return to the standard (Earth's) cycle. The 40 additional min are actually very much. I think it is easier to get accustomed to an a bit shorter day than to a longer one. $\endgroup$
    – user35272
    Mar 31, 2020 at 16:41
  • $\begingroup$ hese the thing though, they would get used to it, 40 min isn't even a hour, like jet lag they would get accustomed to the new schedule . $\endgroup$
    – Topcode
    Mar 31, 2020 at 17:19
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    $\begingroup$ Quite a few experiments have been done on this. For most of human history people followed constantly varying sleep schedules tracking with the seasons, and people working with Mars probes sometimes switch to a daily schedule matching the location of the probe on Mars. The human circadian rhythm naturally settles to around 24 hours and 10 minutes without outside input, so the day/night cycle of Mars only mismatches by about 17 minutes more than Earth's. $\endgroup$ Mar 31, 2020 at 17:30
  • $\begingroup$ @ChristopherJamesHuff: Add some sources and you'll have a good answer. $\endgroup$
    – DrSheldon
    Mar 31, 2020 at 18:08

1 Answer 1


The short answer is: probably nothing significant.

There have been multiple investigations into human circadian rhythms and their variations, especially in the absence of external clues regarding diurnal cycles and timekeeping. The best appears to be the 1974 paper by Mills et al.,"The circadian rhythms of human subjects without timepieces or indication of the alternation of day and night" in the Journal of Physiology. The net result: it varies widely from person to person, but there appears to be a lot of resilience. Many subjects naturally adopted rhythms longer than 24 hours, some significantly so. Two actually would stay awake for 24 hours, then sleep for alternating 8 or 16 hour periods.

I know several of the people who worked with Mars rovers. They had watches built to run on a Martian day! They told me the problem with trying to adopt the Mars diurnal schedule was not that the length of the cycle was troublesome, but instead was the periodic disconnect from Earth's cycle, especially the day/night cycle. Grocery shopping wasn't a problem, but living at home with people on an Earth cycle, and trying to force yourself to sleep during bright daylight hours, made for net sleep deprivation.

  • $\begingroup$ It varies widely from person to person and I know it wouldn't have any good effects on me. :-) You say people had Martian-day-watches. How exact were they? Were they to the millisecond precise or where they more of an approximation? Also, do you think that Mars' low gravity has counter-effects because astronauts wouldn't get that exhausted on Mars? $\endgroup$
    – user35272
    Apr 1, 2020 at 5:27
  • $\begingroup$ @user30007 As I remember these were mechanical watches, and not particularly accurate. The owners would have to do what people 50 years ago did with their Timex watches: set it in the morning, maybe a correction of about a minute, and recognize that it's going to drift a bit during the day. With digital watches the drift is usually fairly uniform; with mechanical watches it varies with how you move, temperature, etc. $\endgroup$ Apr 1, 2020 at 20:00
  • $\begingroup$ Ah, I see, thank you. $\endgroup$
    – user35272
    Apr 2, 2020 at 4:57

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