Living on Mars Time: How NASA Teams Wake Up on a 24.65-Hour Day

A Martian day, called a sol, lasts 24 hours, 39 minutes and about 35 seconds, and NASA teams that operate solar-powered rovers and landers have handled the extra 39.6 minutes by shifting their sleep later every day, using bright light while working, darkening their bedrooms during California or Arizona afternoons, and tracking their body clocks to see whether the schedule held. In the best-documented case, the 2008 Phoenix Mars Lander mission, 87 percent of studied staff showed a rhythm consistent with the 24.65-hour day.

In January 2004, when the Spirit and Opportunity rovers landed, the mission operations area at NASA’s Jet Propulsion Laboratory in Pasadena was reported to have no outside windows, which removed the one clock that could not be reprogrammed, the sun. Accounts from that time say the visiting scientists were housed in apartments fitted with pitch-black curtains. Inside, people looked down at their wrists. Two of the mission’s systems engineers, Julie Townsend and Scott Doudrick, wore automatic watches that a Los Angeles jeweler, Garo Anserlian, had loaded with tiny lead weights so that they ran slow by exactly the margin that separates a sol from an Earth day. Another team member, Jeff Moersch, carried a World War II Hamilton 4992B pocket watch that an Arkansas watchmaker, Steve Maddox, had slowed from 18,000 beats an hour to roughly 17,500. Those watches told the time on Mars, and for the people wearing them the time on Earth had stopped being relevant.

Why does a Mars day break a human sleep schedule?

Rovers and landers run on solar power. They work during the Martian daylight and sleep at night, and the data they send home arrives during the afternoon there. Engineers who want to review that data, plan the next day’s drive and upload commands before sunrise on Mars have a short window, and that window slides. Every sol arrives 39.6 minutes later on the Earth clock than the one before it.

A team that follows it lives in permanent jet lag without leaving Pasadena, one that never lets the body settle because the destination keeps moving. After a week the workday starts around four and a half hours later than it did on Monday. After two weeks the shift has crawled through dinner and into the small hours. The clocks on the wall keep the old day, the bodies inside the building are told another one, and the errands of an ordinary life (school pickups, bank hours, dentist appointments) stay pinned to Earth time.

The sleep science says the trouble is not only the tiredness of odd hours. Human circadian clocks are pulled toward the light-dark cycle and can be nudged, but their capacity to follow a longer day has limits. A rhythm that runs a little over 24 hours by itself, which is what free-running studies find when people are cut off from time cues, has to be stretched further to sit on a 24.65-hour cycle. Light is the main lever for that stretch, which is why the windowless rooms and blackout curtains in 2004 were not decoration. A room lit by afternoon sun tells the master clock in the brain that it is afternoon, whatever the watch says.

What did the Phoenix study measure?

The most detailed data come from Phoenix, which landed near the Martian north pole on May 25, 2008. Its science team worked from the University of Arizona in Tucson, and a group led by Laura Barger and Steven Lockley of Brigham and Women’s Hospital in Boston studied how the staff coped. Their paper, “Learning to Live on a Mars Day: Fatigue Countermeasures during the Phoenix Mars Lander Mission,” appeared in the journal Sleep in October 2012 (volume 35, issue 10, pages 1423 to 1435).

The numbers, as reported in the abstract: mission personnel were required to work on a Mars day for 78 days. The researchers followed 19 scientific and technical staff who were offered a short-wavelength (blue) light panel and education on how to shift their sleep. They used daily sleep and work diaries, wrist actigraphy, regular performance tests and biweekly 48-hour urine collections that tracked 6-sulphatoxymelatonin, the main metabolite of melatonin, as a marker of the internal clock. When the researchers plotted the data against the Mars day, they saw a strong rhythm at 24.65 hours. Most participants (87 percent) had a rhythm consistent with adaptation to the Mars day.

That did not make the team well rested. When work fell in the right place on a person’s body clock, their main sleep averaged 5.98 hours with a spread of 0.94. When work was scheduled at the wrong circadian phase, main sleep fell to 4.91 hours (spread 1.22), a difference the authors report as statistically significant (P < 0.001). Fatigue and sleepiness ratings rose when shifts landed at the wrong internal time, and staying awake for 21 hours or more was linked with poorer performance and alertness. The authors concluded that future missions should adopt a similar circadian and fatigue management program.

Read plainly, then, a rhythm can be pulled to 24.65 hours by a mix of light and scheduling, and the people pulled still sleep less than they would on Earth time. Adaptation is a matter of degree.

What does a 7 a.m. alarm do against a sol clock?

The drift is simple to state and surprising to see written out. Take a person who wakes at 7:00 a.m. on the first sol and keeps waking at the same point of each following sol. The sol is 24 hours, 39 minutes and 35.2 seconds long, so the wake time slides 39.59 minutes per sol against an Earth alarm. This is arithmetic done for this article from the standard sol length, not a figure from any of the studies above.

Sol	Wake time on the Earth clock	Total drift
1	7:39 a.m.	40 min
5	10:17 a.m.	3 h 18 min
10	1:35 p.m.	6 h 36 min
15	4:53 p.m.	9 h 54 min
20	8:11 p.m.	13 h 12 min
25	11:29 p.m.	16 h 30 min
30	2:47 a.m. (next day)	19 h 48 min
36	6:45 a.m.	23 h 45 min
37	7:24 a.m.	24 h 25 min

Over the first 30 sols, the sol-time wakeup moves 19.8 hours around the dial. It reaches an Earth-day lap at 36.4 sols, which is when the drift adds up to 1,440 minutes. On sol 36 a person waking on Mars time gets up 15 minutes before the Earth alarm would have gone off, and on sol 37 gets up 24 minutes after. By then 37.4 Earth days have passed and 36.4 sols have been lived, so the team has gained a full sunrise and a full sunset over the calendar.

A 7 a.m. alarm set on Earth time, in other words, would ring at the right moment for the sol wake-up exactly once, about five weeks in, and be wrong for the 35 or so sols in between. For the first five days the error is mild. By sol 10 it is a 1:35 p.m. wake-up for someone whose alarm says 7:00, and for the next three weeks it is an alarm that belongs to another person’s day.

That is the practical problem every Mars-time team faces. Their zeitgeber, the timing cue that fixes the clock, has to be supplied on purpose. The concept of a zeitgeber is usually described with daylight; here it becomes a light panel, a shade and a wristwatch.

What did families and colleagues make of the schedule?

The human side is documented mostly through journalism. Accounts from earlier Mars missions describe staff whose young children stayed on Earth’s day and could not see why a parent slept in the afternoon. School hours do not slide by 40 minutes a day, and that is a cost no study of melatonin picks up: the day drifts away from everyone who is not on the mission.

The workplaces were built to reduce the mismatch. Beyond the windowless rooms, the Phoenix personnel had access to light-tight shades in rented apartments and blue-light panels, according to accounts of the study. None of this repeals biology. It gives the clock a partial alternative to the sun, which is a different thing from moving to Mars.

For a comparison from another isolated workplace, Antarctic winter-over crews face the opposite problem: months without a sunrise or sunset, no natural cue at all, and a schedule that holds only because a station sets it. Mars teams have the sun available but deliberately ignore it, running a schedule that is 40 minutes off from it every day.

Did the teams stay on Mars time?

No, and the Curiosity story shows why. Curiosity landed in Gale Crater in August 2012, and its operations team worked on Mars time for the first 90 sols. In November 2012 JPL announced that the team was switching back to Earth time. Most work would now sit within 8 a.m. and 8 p.m. Pacific time. The shift was possible because the team had shortened its daily planning process, which took more than 16 hours in the weeks right after landing, to 12 hours. Richard Cook, the project manager of the Mars Science Laboratory at JPL, said at the time: “People are glad to be going off Mars time.”

The reason for the return is not exotic. A constantly shifting schedule with overnight hours is comparable to chronic jet lag, and once planning fits inside a normal working day, staying on Mars time buys nothing. A team that can plan a drive in 12 hours has no need to be awake at 3 a.m. for it.

That leaves a question the published record does not settle, and it is worth stating once. The Phoenix result shows that most of 19 people entrained to a 24.65-hour day while they were offered blue light and education. It does not show what would have happened to the same people without those measures, because the study describes no comparison group on the same mission. Nor does it show whether the 13 percent who did not adapt were the ones who used the light panels least. The claim that light and education did the work is plausible and consistent with what is known about light and the entrainment of the circadian clock. It is not a demonstrated causal finding, and the published Phoenix paper, as far as its abstract shows, does not test it.

Questions people ask about living on Mars time

Can a person permanently live on a 24.65-hour day? The 78-day Phoenix data show it is possible for most people over about eleven weeks, with light control and education. Nothing in these studies covers years, and the short sleep durations suggest a cost.

Why did teams use wristwatches instead of phone apps? In 2004 the point of a mechanical Mars watch was to make the local time visible at a glance and keep the team from mentally converting all day. Anserlian and Maddox both worked by adding mass to the balance wheel or changing the beat rate. The target was a daily lag of 39 minutes and 35.2 seconds, which works out to about 17,518 beats per hour instead of 18,000, a figure that follows directly from the sol length.

Is the Earth-time return a failure? It reads as a scheduling decision made once the work got faster. Curiosity’s team ran Mars time when planning took 16 hours and dropped it when planning took 12.