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This question already has an answer here:

The astronauts on the International Space Station have a daily schedule, particularly when they are asleep versus awake. Which Earth time zone is closest to their daily schedule?

What time zone is used on ISS to define a day? asks a similar question. Three answers state that time is referenced by UTC. That's not what I'm asking. There is one answer relevant to my question, but it is not the accepted answer, so I'm not sure whether it is an accurate answer. Also, @PearsonArtPhoto made a comment there that this issue should be asked as a separate question.

Also somewhat related, but not directly answering the question: What kind of time regime/schedule do ISS astronauts have?

It is my understanding that the Canadarm2 can be remotely operated from Earth, and that it is often done so while the astronauts are sleeping. In that case, it would make sense to have the astronauts awake when it is daytime in Moscow, so the robotics can be done when it is daytime in the U.S./Canada.

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marked as duplicate by Erik, Steve Linton, peterh, Mark Omo, Rory Alsop Oct 14 '18 at 19:52

This question has been asked before and already has an answer. If those answers do not fully address your question, please ask a new question.

  • $\begingroup$ The second question linked shows a picture of the astronauts actual schedule.. You can see that they wake up at 0600 GMT and go to bed at 2130 GMT. $\endgroup$ – Organic Marble Oct 13 '18 at 22:38
  • $\begingroup$ I'm not sure that an earth time zone is enough to define a sleep cycle. I'm currently still awake at 3:12 in the morning my time zone, others here usually go to bed at 22:00 or even earlier. $\endgroup$ – Paŭlo Ebermann Oct 14 '18 at 1:13
  • $\begingroup$ UTC ±0h (same as GMT) would be simplest. $\endgroup$ – jtheletter Oct 14 '18 at 1:19
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The wake/sleep cycle actually does use GMT. It was chosen as a compromise between Moscow time and Houston time. That permits times for meetings between the ISS crew and personnel in either city during some of their normal working hours.

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    $\begingroup$ I can't find it right now, but it's a video, possibly Scott Manley's or Curious Droid's or someone like that, that points out that the supposed "compromise" (actually GMT is fairly close to Moscow time) is related to the subway schedule in Moscow. While the US is car-based (you'd know a little something about that in SoCal), much of the world goes back and forth to work via public transportation. $\endgroup$ – uhoh Oct 13 '18 at 18:48
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I called a retired NASA employee to check on this question. And the answer is GMT+0. I also found this answer:

https://www.astronautabby.com/the-international-space-station-time/

Here is the answer from the above link incase it becomes inactive GMT +0 (Greenwich Mean Time) or UTC +0 (Universal Time) which would be the same time that it is in Iceland. Why is it that this was the selected time for the ISS? The time was originally set to be GMT -5 to match the time zone in Houston, Texas since this is where the main command center for the ISS is located. But the controllers in Moscow also need to be at the console while the astronauts and cosmonauts are working from about 7 a.m. – 7 p.m. The two countries agreed to GMT +0 as a way to accommodate the controllers in both parts of the world. Once again, international cooperation at it’s best.

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    $\begingroup$ Chances are that whatever is in that link will be gone eventually. Could you summarize the relevant information and put it into the actual answer? $\endgroup$ – pipe Oct 14 '18 at 0:28

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