What would you do with an extra 40 minutes in a day?

Not 40 minutes shaved off some regular daily activity to devote to something else, but an extra 40 minutes actually tacked on to the 24 hours we all get. It would be your own personal overtime, a chance to stop the clock when there just didn’t seem to be enough time in the day.

“All the times you wish you had another hour of the day, you almost have that on Mars,” said Jennifer Trosper, deputy project manager for NASA’s Mars rover Curiosity. She has worked on Mars time with four Mars rovers. The key lesson she learned: When it comes to time, be careful what you wish for.

It turns out if you’re on Earth, that extra time wears thin pretty quickly. If you’re on Mars, or at least work by a Mars clock, you have to figure out how to put up with the exhausting challenge of those extra 40 minutes. To be exact, the Martian day is 24 hours, 39 minutes, and 35 seconds long, a length of day that doesn’t coincide with the human body’s natural rhythms. Scientists, Mars rover drivers, and everyone else in the space community call the Martian day a “sol” to differentiate it from an Earth day. While it doesn’t seem like a big difference, that extra time adds up pretty quickly. It’s like heading west by two time zones every three days. Call it "rocket lag."

Trosper first started working on Mars, if only virtually, in 1997 for NASA’s Mars Pathfinder mission and its small, hitchhiking rover, Sojourner. She has also worked with the rovers Spirit and Opportunity—and works now with the latest Mars rover, Curiosity. For each of those missions, controllers worked on Mars time to coincide with daylight operations on Mars, but they didn’t stay on Mars time for long.

“When we landed the Mars Pathfinder, we did Mars time,” Trosper said in a phone interview from the Jet Propulsion Laboratory. “That’s the first time we ever did Mars time.” They stayed on Mars time for only a month. Having an extra 40 minutes to the day every day quickly added up—the difference between 6 p.m. on Earth and 6:40 p.m. on Mars one week turned into the difference between 6 p.m. on Earth and 11:20 p.m. on Mars a week later. Unlike time-zone differences, where the time on the clock is different but days pass in the same 24-hour increments, the time differences between Earth and Mars compound quickly.

It proved exhausting for the controllers as they tried to keep the same time as Pathfinder for daylight operations on Mars, while outside JPL the sunrise, sunset, and the surrounding society stayed on Earth time. NASA officials had wanted to keep them on Mars time for three months, but the controllers had enough, and by some accounts rebelled.

“That mission was a little easier to operate,” Trosper said, “and easier to transition off Mars time.” The Pathfinder lander didn’t move at all, and Sojourner never traveled far from the landing site. When the twin rovers Spirit and Opportunity landed in 2004, Trosper and JPL controllers went on Mars time again and stayed on that schedule for three months. Trosper also led planning for Curiosity’s surface operations prior to its landing on Mars in August 2012, including figuring out how long to stay on Mars time to work with the newcomer to the Red Planet. Again, it was about three months. The controllers have been getting better at adjusting to Mars time even as the rest of California stubbornly stuck to Pacific Time. Some controllers even had family members adapt to the same Martian time zone, kept shades down during the Martian “night” even as the California sun shone brightly, and otherwise tried to follow a slightly longer daily routine. Yet by the end of 90 sols, they would find the rest of Earth had advanced 2.5 days ahead of them. (So far, scientists are leaving it to eventual explorers and emigrants to Mars to devise official names for “week-sols” and months, though you can find recommended names on the web and in fiction. JPL controllers for each mission simply number individual sols from the time a craft lands on Mars, starting with Sol 1.)

Trosper had it even rougher. While she was on Mars time with Curiosity, she was also on baby time with her youngest of three children, born shortly before Curiosity landed. Her baby and Curiosity were both very demanding of her time. Perhaps because of those dual demands while working on Mars time, “I can hardly remember it. It did go really fast. As much as you’re exhausted, it just flies by.”

As difficult as it has been for JPL controllers to follow Mars time while still on Earth, “I think it would be hard to be on Earth time if you’re on Mars,” Trosper said. She’s confident the human body would naturally adjust to the longer sols once explorers finally reach Mars. Except the body’s adjustment may not be that natural after all. The body runs on an internal circadian rhythm. That rhythm does adjust to its natural surroundings and responds to periods of sunlight and darkness, but only up to a point. Studies by the Max Planck Institute in Germany during the 1960s and early 1970s appeared to show that a human’s circadian rhythm was naturally set at 25 hours, and just adjusted to Earth’s 24-hour day.

So early in its exploration of Mars, NASA figured a person’s adjustment to the Martian sol would be a cinch. But Charles Czeisler, a professor of sleep medicine at Harvard and chief of the Division of Sleep Medicine at Harvard’s Brigham and Women’s Hospital, discovered that the 1970s finding of a 25-hour natural circadian rhythm for humans was wrong. The original study allowed test subjects to turn on artificial light whenever they wished, unintentionally resetting their bodies’ circadian rhythms. Czeisler went to the first meetings that NASA organized in the 1990s to bring together scientists to discuss issues arising from going to Mars. He said NASA officials “sort of had a little check mark that adjusting the circadian rhythm was going to be no problem on Mars.” Czeisler had to tell NASA officials that they did have a problem, perhaps a big one.

A computer-generated image of Olympus Mons, one of the massive shield volcanoes on Mars (NASA)

A person’s natural circadian rhythm averages about 24 hours and six minutes for women, and 24 hours and 12 minutes for men. It varies for each individual, but doesn’t stray very far from 24 hours. At about the time Pathfinder landed, Czeisler and his team began conducting studies at the hospital’s special laboratory that shielded study subjects from all outside influences. With their test subjects in isolation, they simulated the Martian sol to see how the test subjects adjusted to the longer day. “What we learned was none of the people adapted their circadian rhythms to the Martian day,” Czeisler said.

This wasn’t good. If Martian explorers were trapped in Earth time, they would never last the months- or year-long stay expected of them without severe exhaustion. That exhaustion could pose grave safety risks as the explorers attempted to carry out their duties, yet there would be no way to cut their time on Mars short. NASA was at that point working on a tight schedule with a goal of landing explorers on Mars around 2014. “So we had a relatively short time frame in 1997 to develop countermeasures” to a person’s natural clock, Czeisler said. “The first thing we tried was increasing the light intensity, but that was not sufficient.”

Eventually Czeisler and his team discovered that for Martian explorers to adjust to the Martian sol, they needed light therapy. “We found that one of the most effective ways of achieving entrainment (to adjust circadian rhythms) was to enhance light exposure in the evening.” The researchers discovered that it wasn’t necessarily the light intensity but the wavelength that made a significant impact. Czeisler said his team has yet to test specific wavelengths to extend a person’s body clock to sync with a Martian sol, but what he believes the new Martians will need is evening doses of light at short wavelengths—blue light not unlike the glow of a smartphone in a darkened room.

The daylight a Martian explorer is exposed to, Czeisler explained, determines the wavelength and amount of exposure needs in the evening to reset the individual’s circadian rhythm. “In terms of circadian theory, we change daylight all the time. We just proportionately change it.” Our internal clocks may also be impacted by the blue sky we see when we start the day, and the longer wavelengths of red sunsets we see in the evening.

But Martian daylight is red, and sunsets on Mars are blue. Might that have an impact?

Czeisler insisted the evening exposures to bright light or blue light will still do the trick. His research team has already found that blue-light exposure from computer tablets reduces the level of melatonin, a hormone that normally increases in the body during the evening to induce sleepiness. For people on Earth time, the tablet’s blue light means you fall asleep later than you should and wake up feeling groggy. On Mars, the proper light therapy will extend a person’s circadian rhythm for those extra 40 minutes to minimize rocket lag.

In an actual adventure in space and time, Czeisler and his research team will be able to take their theories on resetting circadian rhythms and test them off Earth next year. A set of lamps his team developed with more than 100 LED bulbs that can be set to project a range of wavelengths will be heading to the International Space Station. Besides shining white light when all bulbs are lit, the lamps can be adjusted to give a dose of blue-toned light to help astronauts stay alert, and the longer waves of red-toned light that increases melatonin and promotes sleep. NASA will use the off-world laboratory for more studies on keeping astronauts alert when they need to be, and adjusting their circadian rhythms to test if explorers will stay awake when they reach Mars.

So what will the Martian explorers and eventual settlers do with those extra 40 minutes in their sol? Or because of the longer sol, a week that’s always out of sync with the week on Earth? Or a year that’s not quite the length of two Earth years? All of that will be up to the immigrant Martians to decide.

During the workweek, their daily extra 40 minutes will likely be parceled out throughout their daily routine. Come Saturday night, they may think up something special, something unique to the Martian sol. Trosper imagines that for the settlers of any society that could eventually establish itself on Mars, staying a half-step out of time with Earth could set the Martians even farther apart than the distance that separates them from their home world. In adapting to the “physical realities of the universe,” the future Martians “may engage with those aspects that are more cultural, more societal. How they count years and things like days will make them a very unique society.”

“I would love to go there,” Trosper said. Though she experiences Mars more intensely than most people, “Nothing compares to standing there at the edge of Olympus Mons.”

Of her children, Trosper said her 6-year-old son shows the most interest in Mars. And who’s to say? He may end up standing on Mars by the time he’s 18–counting in Mars years, anyway.