The sun will come out tomorrow, the saying reassures us. Our rotating planet promises that half the time we are bathed in sunlight, and the other half we are exposed to the great enveloping cosmic dark. This has been the case for all of Earth’s history. Every form of life that evolved on this planet did so with an everlasting, guaranteed cycle between day and night.

Mars rotates, too, but just a tad more slowly. A Mars rotation, the “sol” to our “day,” takes 39 minutes and 35 seconds longer than the 24-hour day we experience on Earth. For future colonists living on Mars, this added time would be akin to flying two time zones westward every three days. You would be constantly jet-lagged.

We mark our lives according to the passage of time, but on Mars, this will be different in every way. Requiring a new vocabulary, a new circadian rhythm and a thousand other adjustments, a sol will be nothing like a day.

The most noticeable and most frustrating change will be the length of the sol. Tacking on 40 minutes every night adds up quickly; after a short while, the Mars clock would be so far offset from the 24-hour clock that noon would be the Earth equivalent of midnight, and you might start to feel the consequences. You might experience a persistent cognitive fuzz, causing you to forget mundane things and take longer to learn new ones. You might start making mistakes.

In mice, a misalignment between internal timekeeping cues and external cues creates a host of problems, says Erik Musiek, a neurologist at Washington University School of Medicine in St. Louis. To study jet lag in mice, Musiek changes their wake-up time by six hours every week, which keeps them out of sync.

“It’s creating a conflict in the mouse between the external environment and the internal environment. The internal clock is telling the mouse, ‘Oh, it’s this time of day,’ but the light is saying it’s a different time of day. There is something about it that seems to be particularly bad for your physiology,” Musiek said. In humans, we don’t really know what would happen in the brain of someone experiencing a protracted circadian disruption. It’s never been studied, Musiek said.

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Although humans have never been to Mars, plenty of people in Europe and the U.S. have lived on Mars time. When a new Mars robot arrives at the red planet, scientists and engineers will work on a Mars schedule for a while, keeping them in sync with their scientific instruments — and out of sync with their families. In 2012, David Oh and his family were a notable exception. The Curiosity rover’s lead flight director, his wife, Bryn, and their three kids all lived on Mars time for the first month of his three-month stint (before the kids’ summer vacation ended). In news coverage at the time, the Oh family said the shared experience made it more bearable.

But most people who work on Mars time hate it, and it takes a toll, according to the scientists who do it and the sleep scientists who study them. When the Sojourner rover and Pathfinder lander arrived in July 1997, controllers at NASA’s Jet Propulsion Laboratory worked on Mars time for a month before abandoning the schedule, although the mission was not yet over. In January 2004, during the Spirit and Opportunity rover arrivals, 250 engineers and controllers at NASA’s Jet Propulsion Laboratory started working on Mars time; later, 82 percent of them complained of sleepiness, irritability and decreased concentration and energy, according to Laura Barger, a sleep and circadian biologist at Brigham and Women’s Hospital in Boston. So when the Phoenix lander arrived in May 2008, Barger and NASA were prepared. They offered sleep hygiene training to anyone who wanted it, and people who agreed to be part of Barger’s study were given a portable light box that bathed their desks in a bright, alertness-promoting hue of blue light. Workers used it during their Mars-time shifts, even in the middle of the night on Earth. Blackout curtains in the control room also helped prevent confusion.

Eventually, 87 percent of the people in the study were able to adapt to the Mars sol, though results were hardly ideal. Participants slept less than six hours a night half the time, and people generally got less shuteye when they tried to sleep during the Earth day than when their Mars-dictated bedtime coincided with the Earth night. Future Mars missions should use a circadian rhythm management system to help people adapt more fully, Barger said.

Sleeping pills are some of the most commonly used drugs on space missions. A seminal study Barger published in the Lancet Neurology in 2014 states that three-fourths of crew members on the International Space Station reported taking sleep medication during their stay. Crew members’ schedules leave them eight and a half hours of sleep time a night, but they generally only manage to get about six hours, the study found. It may be that sleeping pills aren’t working as effectively in microgravity, and it may be that a chilly, noisy, unforgiving spacecraft is just a really lousy place to sleep.

“You’re missing circadian cues that we all evolved as signals of when to be alert and when to be sleepy,” said Virginia Wotring, a pharmacologist who studies space medicine at Baylor College of Medicine in Houston.

Mars will not be much better for astronauts trying to get some shuteye. The planet’s gravity is just one-third as strong as Earth’s, so colonists are likely to experience some of the more unpleasant side effects of life in space, including stuffy noses, blurry vision and headaches, which result from fluid buildup in the head that occurs when there’s not as much gravity to assist with drainage. This won’t make sleep any easier.

When your circadian rhythm is disrupted, your metabolism will slow, so you might start gaining weight, even if you eat typical amounts of food at typical mealtimes. You might become less sensitive to insulin, which, combined with the extra weight, could increase your risk of developing Type 2 diabetes. You might feel tired all the time, even if you get several hours of sleep. You might be at greater risk for cancer. And your brain may feel trapped in a fog, which could cause you to make mistakes.

There is some good news, though: With the right lighting and sleep strategies, we might be able to train our circadian rhythms to match the Mars sol, rather than the Earth day our bodies are used to.

In just the past few years, scientists have learned that a molecular timekeeping system resides in almost all the cells of the body, in almost all forms of life. Experts think the genetic underpinnings for this clock arose about 2.5 billion years ago. Light is the most important mechanism for setting this clock. That means we can use light like a drug to promote sleep and wakefulness at appropriate times, just as Mars shift workers did in Barger’s study.

But studies of mice confined to perpetual darkness show that the cycle persists, even without external cues. “It’s hard to believe that living things can have a clock,” said Fred Turek, a sleep and circadian biologist at Northwestern University in Chicago. “When you see it, you’re blown away at the precision of it.”

In a few limited studies on humans, researchers have found that without light, the natural clock runs about 25 hours rather than 24; we live according to the 24-hour cycle because the sun resets our clocks every morning. That could be good news for Mars colonists, because it suggests that humans could settle into a natural Mars sleep cadence if they’re exposed to sunlight at the right times.

This will not happen, however, without help in the form of carefully calibrated sunlight-mimicking lamps. Mars is, on average, about 50 million miles farther from the sun than Earth is, so it gets much less light. When the sun comes up in the Martian morning, it illuminates a blood-red sky, which may gradually turn bright orange or a shade of salmon as the day wears on. There are no swathes of cerulean and ultramarine at noon. The dust in the Martian atmosphere absorbs more blue-light wavelengths, leaving less available to reach the blue-light-sensitive cells in our retinas that are mostly responsible for setting our clocks.

Apart from these dramatic physical changes in our bodies and in the world around us, time on Mars will feel unfamiliar. We’ll need an entirely new lingo, for starters: Yesterday, today and tomorrow will need replacing, so we might choose new words like yestersol, tosol and solmorrow, the way the Oh family did when they lived on Mars time. Months will no longer follow the moon, because Mars’s two dinky satellites, Phobos and Deimos, zoom around the planet far too quickly to divide the calendar according to their phases. Instead, scientists mark the calendar using the longitude of the sun, said Michael Allison at NASA’s Goddard Institute for Space Studies, who built a Mars clock. The Martian New Year is when the sun stands directly above the Martian equator, moving north as viewed from Mars — the start of spring.

Even if we split up Mars time into Earth months for tradition’s sake, we would need to add 10 extra ones, because Mars takes 1.88 Earth years to complete one orbit around the sun. Mars is tilted on its axis the way Earth is, so it has seasons like Earth does, but its orbit is deeply eccentric, meaning Martian seasons last much longer and are much more extreme than anything on Earth. And of course, there are no blooming flowers, leafing plants, or mass animal migrations to mark those changing seasons.

Indeed, just about the only familiar aspect of life on Mars will be the tilted spin that Mars shares with Earth. As it does on our planet, each nightfall will be accompanied by the reassurance that at the very least, the sun will come out solmorrow.