Astronauts have been adjusting to the challenges of sleeping in space for years — and the lessons learned from their zero gravity slumbers will ensure that one day the first crewed missions to Mars will have gotten enough rest before exploring the red planet.

Rotating crews have spent an average of six months living and working aboard the International Space Station for nearly 23 years, and they struggle with sleep issues just like people on Earth. Some of the challenges are similar to those of shift workers or people with abnormal schedules, but others are more unique to the space environment.

For example, most people don't have to worry about floating away from their beds due to zero gravity. Don't worry — astronauts use special restraints to keep them from floating through the space station while asleep.

Two of the biggest challenges for astronauts include their sleep environment and the establishment of a natural sleep cycle.

Sleeping in space

Astronauts have dark, quiet and private crew quarters on the space station conducive to good sleep — but that won't always be the case on other space missions, said Dr. Erin Flynn-Evans, director of the Fatigue Countermeasures Laboratory at NASA's Ames Research Center in Mountain View, California.

Like their historic Apollo predecessors, the Orion capsules that will be used during future Artemis missions to the moon are small vehicles with limited space for crews and sleeping bags for rest periods.

"I think of it like camping," Flynn-Evans said. "If it's for a couple days, probably no big deal. But the longer you're in close quarters with someone, the more disruptive that can be."

While the space station affords incredible views of Earth, the 16 sunrises an astronaut witnesses a day can wreak havoc on circadian rhythm, the body's natural clock for sleeping and waking.

On Earth, disruptions to circadian rhythm occur for people who work overnight shifts or experience jet lag while traveling across time zones.

"Light is what resets our circadian rhythm and keeps us organized to that day-night cycle, but in space we have several challenges," Flynn-Evans said.

The space station orbits around Earth every 90 minutes, creating alternating cycles of darkness and light. Rather than force the astronauts to adapt to such a strange cycle, experts at NASA have added lighting to the interior of the space station that mimics what people experience during a normal day on Earth.

"We have to try to block out the light from windows during the night," she said, "and we have to really try to maximize the light either through windows or with internal lighting to make sure the crew are getting that synchronizing stimulus so that they're able to stay awake and asleep at the right time."

Jet lag begins before astronauts ever arrive at the space station, and their sleep schedules are shifted for days before liftoff based on the time of day and time zone from which they will launch. Once they reach the space station, each astronaut is shifted to Greenwich Mean Time, "a nice middle ground between all of the countries that participate," Flynn-Evans said.

At the Fatigue Countermeasures Laboratory, Flynn-Evans and her colleagues develop tools to help astronauts overcome sleep challenges. Some of the strategies involve managing when the astronauts are exposed to blue light, the primary synchronizing wavelength for the circadian system, and when to reduce blue light to help them sleep.

Astronauts have regimented schedules, but the arrival of resupply missions or new crews sometimes interrupt those. Flynn-Evans and other researchers develop approaches to shifting sleep safely for the astronauts, such as determining when to take naps or stay up later to accommodate schedule changes.

The same tips that help astronauts sleep also apply on Earth, including following a regular schedule with waking and falling asleep at the same time as much as possible and limiting exposure to blue light before going to bed, which is emitted by LED TVs, smartphones, computers and tablets.

'Fake space missions'

Although scientists have sleep data from years of spaceflight, conducting simulated missions on Earth allow for more control.

"We do fake space missions all the time," Flynn-Evans said. "We have what we call an analog space environment at Johnson Space Center called the Human Exploration Research Analog or HERA, and that's basically a small habitat."

The habitat mimics the size of a lunar base or small spacecraft and can house crews of four people for long periods of time. Flynn-Evans was involved in a study in which crews spent 45 days in the habitat and were restricted to five hours of sleep on weeknights and eight hours on weekends. The participants were tested for alertness and performance.

Findings from the experiment showed that if crew members only got five hours of sleep one night, they needed more opportunities to catch up on sleep on subsequent nights to prevent the ill effects of sleep deprivation. The current requirement is that crew members get 8½ hours of sleep per night on missions to avoid long-term sleep loss, fatigue-induced errors and health complications, according to NASA.

In June, NASA will begin the first experiment in a new 3D-printed Martian habitat at Johnson Space Center called the Crew Health and Performance Exploration Analog, or CHAPEA.

Over the course of one year, a four-person crew will live and work inside a 1,700-square-foot (158-square-meter) space to simulate living on Mars. The focus for the first experiment is nutrition, but Flynn-Evans and her fellow researchers will also monitor how well the crew sleeps.

Habitats such as HERA and CHAPEA allow scientists to simulate surprises that may happen on a real mission to the moon or Mars, such as limited resources, failing equipment, communication issues and other stressors of small habitats.

Preparing for Mars

An unexpectedly rich source of sleep data has proven to be studying the Earth-bound scientists and engineers who work on Mars missions such as the Perseverance rover.

A day on Mars lasts about 39 minutes longer than one on Earth, but it's just enough that the members of Mars mission control have to adjust their schedules constantly to stay on Perseverance's timetable.

"If you're shifting 39 minutes a day, that means that you're basically going to bed 39 minutes later every day," Flynn-Evans said. "It doesn't seem that bad on a single night. But after five days, it's like you've crossed like six time zones. It's a real stressor on the body."

Many unknowns still exist about being on "Mars time," such as how the time shift affects the human body's metabolism.

Understanding how people on Earth adapt to live on Mars time is one way of preparing for future missions to the red planet. Flynn-Evans and her team are working closely with those planning the Artemis lunar missions to optimize the astronauts' schedules and ensure that the lighting is sufficient and the noise is dampened inside Orion when they need to sleep.

Researchers also want to study how much caffeine astronauts require for alertness to make sure crews don't run out of coffee in a spacecraft with limited storage.

"Sleep is intimately tied with performance, alertness, interpersonal communication and relationships," Flynn-Evans said, "so we want to make sure that the crews are set up for success and getting that sleep they need."