Take the Jovian moon Ganymede as a real-world example. It has an orbital period of ~7 days around Jupiter, which is effectively its day and its year simultaneously.

My characters will need the words 'day' and 'night' to refer to the 7 day dark-light cycle. What words might they use to refer to the 24 hour wake-sleep cycle? All I could think of is 'cycle', which sounds a bit too cold and clinical for my fantasy setting.

"We had only two more BLANKS before the new day began and sunlight revealed our position."

EDIT: I think I might go with 'phases', as in phases of the day. Instead of 'days of the week' I'll have 'phases of the day', which will each be named after the light conditions. Dawnphase, Deepnight, etc.

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    $\begingroup$ Are these characters humans? You should specify that they are by editing in the humans tag, if that is appropriate. My first though was that any creature on a planet with a 7 day day/night cycle would have a 7 day 'circadian' rhythm. $\endgroup$ – kingledion Feb 7 '18 at 3:36
  • $\begingroup$ I know it's a little silly, but what about just calling it a nap? They are on a 168 hour day cycle and a siesta or two (or seven, but I wonder if you would get off the normal 24 hour cycle and do something a bit different) as a normal part of the day just makes sense. It would be interesting if everyone were on this 168 hour day cycle and just took naps at sort of irregular times throughout the day. $\endgroup$ – MParm Feb 7 '18 at 5:03
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    $\begingroup$ Keep in mind that due to the gas giant's size, you may have a more complex light-dark cycle depending on where you are situated on the moon - this is because the gas giant will eclipse the sun every "day". E.g. if your Ganymede base has Jupiter at zenith, each cycle will be: 1. Light (Sunrise to start of eclipse), 2. Really dark (sun eclipsed by Jupiter, you are seeing its dark side) 3. Light again (end of eclipse to sunset), 4. Sort of dark, becoming lighter and then darker again (no sun, but you are seeing varying degrees of the sun-facing side of Jupiter, so very strong "moon light"). $\endgroup$ – G0BLiN Feb 7 '18 at 7:30
  • $\begingroup$ Other position may be more or less complex - but it's worth remembering that if you see the gas giant, the reflected sunlight may be more than enough to feel like "daylight". I'd consider sticking with the familiar words ("day", "night", "good morning" "afternoon tea", "midnight snack" etc.) for the human-compatible cycles and adopting new words to describe the very unfamiliar varying illumination conditions faced "daily" by the inhabitants... $\endgroup$ – G0BLiN Feb 7 '18 at 7:34
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    $\begingroup$ a ganymede day is 7 earth-days? Then, for this special case, you could use day for the earth-day, and week for the ganymede day $\endgroup$ – Julian Egner Feb 7 '18 at 9:26

13 Answers 13


Something to do wth circadian rhythm. I'd use circads.

Alternatively, something to do with biological oscillations.

Alternatively, something to do with blood pressure (or other medical metric) that has a twenty four hour cycle.

But I'd personally recommend coining the term 'circad.'

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    $\begingroup$ The idea and name that you gave is just great! $\endgroup$ – skout Feb 7 '18 at 0:49
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    $\begingroup$ You can't recommend coining the term, when you just did. :) $\endgroup$ – Sled Feb 7 '18 at 17:21
  • $\begingroup$ @ArtB Mmm. But I don't lay any claim to it and this answer may well disappear some day. :) $\endgroup$ – DPT Feb 7 '18 at 19:02
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    $\begingroup$ @ArtB coining is a legal process. making up a term on a form is not by itself the act of coining. $\endgroup$ – user64742 Feb 8 '18 at 6:50
  • $\begingroup$ Note, "CIRCAD" is also a software for circuit design. Might be worth keeping in mind. $\endgroup$ – Nzall Feb 8 '18 at 9:30

NASA does already face this problem with its Mars rovers (since both visibility and power from solar panels require daylight): https://en.wikipedia.org/wiki/Timekeeping_on_Mars

Full sunrise-to-sunrise cycle is called Martian solar day, or "sol" for short. Since it is only 40 minutes longer than Earth day, they have sol divided into 24 Martian hours, and have people who live on Mars time.

In your Ganymede example with its much longer solar day, they can use "day" for 24 of earth hours, and call Ganymede's solar again sol, or a year, and divide it into seasons.

Edit: in US, people got used to naming timezones: e.g. "10pm Eastern". I bet NASA people say "10pm Martian", or "in 2 sols"

  • $\begingroup$ I worry that it would get confusing when talking about 'day' meaning the part of the Sol with light and 'day' meaning the 24-hour cycle. Sol is a good word though. It feels natural. $\endgroup$ – Pasqueflower Feb 7 '18 at 2:02
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    $\begingroup$ This may not be obvious in a first read, but this suggestion essentially sticks with "day" and "night" as the common parlance for waking and sleeping, and terms new words for the light and dark stages (Sols). $\endgroup$ – G0BLiN Feb 7 '18 at 7:19
  • $\begingroup$ I've used "photoperiod" to refer to the time when the sun's shining, such as when my sleep got out-of-sync over winter break in university. $\endgroup$ – Ethan Kaminski Feb 7 '18 at 10:03
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    $\begingroup$ It seems to be conventional in science and science fiction to use the term 'sol' for a local solar rotational period, when it needs to be distinguished from an Earth day (of 86,400 standard seconds), and 'anno' for a local circumsolar revolutional period, when it needs to be distinguished from an Earth year (of 365.25 standard days (of 86,400 standard seconds)). $\endgroup$ – Jeff Zeitlin Feb 7 '18 at 17:40
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    $\begingroup$ "have people who live on Mars time" How does someone work that into their schedule? You time would slowly desync from everyone else you know! $\endgroup$ – PyRulez Feb 8 '18 at 0:03

Considering that your questions implies military setting, I would suggest "circle".

Starting with "circadian rhythm" as basis (as suggested by DPT), no matter what official "circadian"-based word would be, I would expect common parlance to use shortened "circ"/"circs", which due to similarity directly leads to "circle" as slang version.

It sounds similar to cycle, circles are cyclic implying cycle, and word itself parallels "klick", existing US military slang for kilometre.

You could have potential way to distinguish civilian characters from military ones, by use of either circ or circle, with latter potentially seeping into civilian language in some areas or for some groups. Furthermore, statements in vein of "Remember to pack the blindfolds, Ganymedian day isn't very circular" become perfectly natural.


Possibly one would refer to the most common thing/action that happens during the 24h cycles. For instance,

  1. sleepies, from sleeping
  2. wakies, from waking up
  3. waits, from the fact that you need to wait for seven such cycles before a full day has passed.
  4. zeptees, from epta, or seven in Greek, and sieben, which is seven in German
  5. Dutes, based on chores, where the scheduling follows the 24h pattern, rather than the daily pattern. ( Suggested by can-ned-food in the comments )
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    $\begingroup$ You could add another scenario to this list: if these people perform watch or duty rotations during their wake–sleep cycles, then perhaps they could call one unit of cycle as a whole a dute or so. Of course, one problem with that is whether that word refers to a wake–sleep in which they stand watch, or whether it refers to the cycle whether or not they do so. Indeed, same confusion could happen if they call the periods ‘sleepies’. Of course, we do make mention of days passing even if there is no visible sun, and such … Pardon the rambling. $\endgroup$ – can-ned_food Feb 7 '18 at 6:27

The first part of your question (how to call one day-night cycle on a different planet) is pretty much settled like @Bald_Bear mentioned in his answer: Sols.

The second part of your question (how to call a sleep-wake cycle when it doesn't fit the above) is more interesting.

  • I do like @DPT answer of 'circads' derived from 'circadian rythm' which I think would be perfect for general biology-related situations.
  • An alternative for work or military related situations would be 'shifts'. As a bonus point this could give you potentially interesting ambiguity. Individuals would consider a sol to have 7 shifts (of 24 hours) or 7 shifts and 7 off-shifts. Where as the military would consider a sol to have 21 shifts (of 8 hours).
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    $\begingroup$ I like the term shift. I think it is not specifically a military term, though. Any job that has people working in turns or periods that include late night hours would use that term too. $\endgroup$ – Renan Feb 7 '18 at 12:52
  • $\begingroup$ @Renan true, I did write work or military... $\endgroup$ – Kempeth Feb 7 '18 at 13:16
  • $\begingroup$ My bad, I had misread it. $\endgroup$ – Renan Feb 7 '18 at 13:17
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    $\begingroup$ Stephen Baxter's Raft takes place in an environment with no natural day and night, and they exclusively use 8-hour "shifts" (and I believe a 'year' of 1000 shifts) for timekeeping. Even without military culture it's still a natural unit of timekeeping, since it's approximately the amount of sleep and work per 24-hour period. $\endgroup$ – Random832 Feb 7 '18 at 16:08

Even though we have a perfectly good word for years ("year"), sometimes you will see the old-fashioned/poetical use of a single season or other annual event to stand in for it. For example,

I've seen seventy-eight summers.

instead of "I'm seventy-eight years old," or

Five Christmases had passed since my father's death.

Similarly, we sometimes use "dawns" for "days".

You could adopt some version of this, using an every-twenty-four-hour event to signify the whole wake/sleep cycle.

  • "Sleeps" is an obvious option, since that is a distinguishing characteristic of the cycle you're describing. One problem with this is that "sleep" is not usually a countable noun, so sentences like "There are only two more sleeps before daybreak" sounds a little bit awkward or non-standard, which perhaps does not fit a fantasy setting so well as it would a more sci-fi setting.
  • "Breakfasts" or "suppers" (or whatever meal is most important in your world) would be another option, that sounds a bit more natural: Sentences like "There are only two more suppers before daybreak" or "It'll be dawn by next breakfast" are easily understandable without narrative explanation. Existing terms like "breakfast-time" and "lunch-time" could also be used to signify the particular time within a wake cycle, since terms like "morning" and "afternoon" may have different implications in your world.
  • "Matins" or an analogue would be another traditional option. If your world has a religion with regular observances of some sort, it would make sense to use the most prominent of those as a reference point. If there are several throughout the cycle, these could also be used for times within the cycle (as with meals). So instead of "from sun-up to sundown" your characters could say "from matins to even-song" (or "from hail-Alara to Bed-Sacrifice" or whatever fits your world).
  • "Tooth-flossings" or any other chore or activity, ceremonial or mundane, that pretty much everyone in your world does once per sleep cycle, would also be a possibility. This would work especially well if you want to emphasize the importance of some aspect of your society (for example, if they're a desert agricultural society, you could use "waterings").


The contextual outline suggests "revealed our position", which reads somewhat formal and possibly military in nature. Bearing in mind that sleeps is already British English slang (possibly elsewhere) for days, in a very similar sense, it might sound a bit offhand to use such a slang term. Rests sounds a little more formal, it's easy to understand what is meant, and could easily be imagined to be a military term. It could even be re-used in longer form as rest periods, perhaps in an excerpt of some military directive, or bureaucratic nonsense.


You say that ‘cycle’ seems too clinical, so that means that you'd like something slangish. At that, you don't give us enough information to devise something which would be consistent with the culture of your world.

However, I could walk through an example.

So, these people have cycles of sleep periods which differ enough from the cycles of daylight and darkness that they need their own metrics. Then begin by referring to them as just that — indeed, what we call them now: ‘sleep cycles’.
Of course, that's three syllables. In parlance, they prefer a short, brief single syllable that rolls out of the vocal apparatus. Some people began calling them ‘sleeps’, vis–à–vis

It's been three sleeps since then. Can't you let it go?

Indeed, some people already use it in that manner, more or less. However, it didn't really catch on in this fictional world: perhaps context wasn't enough to distinguish it from the homonym.
Then, someone noticed that another word for ‘sleep’ is ‘slumber’. Now, along it went, juggled about on the lingual pods of language; people couldn't exactly begin saying things like

Ten slums is more than enough time for you to reach a decision!

but they could, perhaps, say

Only nineteen bers ago, you told me that you couldn't stand the sight of me. Now, what — you want to follow me around like a anxious toddler?

Of course, the acceptance of that word depends on which vowel you use, but it seems as good a beginning as any to me.

  • $\begingroup$ Not keen on "slum/bers", but the concept is sound. Maybe lerts, for "periods of alertness". $\endgroup$ – Rich Feb 14 '18 at 19:03
  • $\begingroup$ @Rich Certainly; this was an example of one procedure to follow when inventing slang. $\endgroup$ – can-ned_food Feb 15 '18 at 19:12

Terran Day or Earth Day, both shorten to snappy a abbreviation TD or ED.

I'd go with Terran Day & TD, both that's just me.

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    $\begingroup$ This is close to what's used in the Honorverse. Each planet has its own day and year length, but they also refer to "T-days" (short for "terrestrial days") and "T-years" when discussing interplanetary scheduling. $\endgroup$ – Monty Harder Feb 7 '18 at 19:01
  • $\begingroup$ "Consult your doctor about ED now." $\endgroup$ – Rich Feb 14 '18 at 19:01

I think the most realistic scenario is that "day" continues to refer to the average person's cycle of routine, and that people would disambiguate it from a literal day by adding a qualifier to be more specific about literal solar days.

A few related considerations:

  • If you've ever done all-nighters, you might (at least I have) consider the whole span of time to be one "day". If you've done all-nighters consecutively, using literal solar days to measure time begins to become clunky and much harder to keep track of

  • In areas in the far polar regions of earth where days and nights are desynchronized from the rest of the Earth, people still maintain a roughly 24-hour schedule, which is surely the more common word for "day", or something like it. That could be a perfect inspiration, but I'm struggling to find linguistic research on it.

  • Most of the time someone talks about "their day", the reference to the solar cycle is an accidental part of the expression; "day" refers to routine more than it does to actual solar cycles. Going back to all-nighters, if at the end of an all-nighter, someone said that they had "a bad day", most people would never even consider that they're only talking about the second half of their two-day wakefulness.

  • If there's anything programming has taught me, it's that people prefer to overload words than to make new ones.

  • Following my supposition that "day" refers more to routine than solar cycle, the average reader would probably find this more intuitive than an invented word to describe this separately from the solar cycle.

Personally, if you used an answer like a "cycle" and a "day", I'd assume that "day" is about routine, and "cycle" is about natural light, rather than the other way around.

So, rounding it off: Routine cycle: "Day" Solar cycle: "Solar Day" or maybe "Literal Day" or maybe "Light cycle"

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    $\begingroup$ Hee hee. Props for casually inserting the homage to Light Cycle. $\endgroup$ – can-ned_food Feb 8 '18 at 5:56

There are two major scenarios:

Your society still maintains distinct calendars/clocks for Earth time vs. local time, and translates between them. For example, the Sabbath is every seven Earth days, Christmas is December 25th Earth time, et cetera.

In this scenario, you need distinct words for the local time periods. If (nearly) everybody has similar sleep cycles, can-ned_food's suggestion of "wakes"/"sleeps" instead of "days"/"nights" makes sense. If large fractions of the population have different ideas about when to be awake vs. asleep, then it makes sense to have a one-syllable local adjective attached to the one-syllable word for the closest Earth time period. On Ganymede, you might have seven "gandays" in a "ganweek". These words would be simplified over time, perhaps to seven "gandies" in a "nee".

Earth time has become a forgotten relic. This implies significant religious changes, and a loss of trade/contact with Earth.

The most likely outcome (in this scenario) is the continued use of the localized terms (from when Earth time was still distinct), such as seven "gandies" in a "nee". But if people rediscover older literature or religions, they might re-adopt Earth terms and give them local definitions. For example, seven days in a week.


If I understand correctly, one "cycle" is the rotation of the moon and there are 7 cycles between sunrises. In that case there are also 7 Planet rises.

Since your main planet is going to have it's own name and maybe mythology then you can use that to guide the wording around what people would call "The big thing in the sky is going to rise and set twice"

  • $\begingroup$ I think possibly you misread the question: it doesn't take place on Ganymede, and doesn't say whether a moon is available. I like the approach you took with the problem, but it might not work in this scenario. $\endgroup$ – can-ned_food Feb 8 '18 at 5:55
  • $\begingroup$ @can-ned_food I removed the word Jupiter from my answer. $\endgroup$ – Muuski Feb 8 '18 at 13:38

I think it would depend on what time is the important one for the locals. I assume the local time is the important (mostly used) one.

So the throughout the colonies day/night, week, year are all used for the respective local time.

When there is a need to refer to the earth time, one adds the prefix Standard-Earth- to the word. So Standard-Earth-Day (SED for short) refers to the 24h cycle.

In spoken language the abbreviations or the short term earth-day are common.

If it fits better, you can switch everything around and use Day for Earth time and Local- as prefix for the local time or even $Colonyname- if you need to compare a Ganymede-Day with a Mars-Day.


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