When you're floating around space for an extended period of time, your body undergoes some interesting changes. Just ask NASA astronaut Scott Kelly, who returned from a 340-day trip aboard the International Space Station on Tuesday.

NASA plans to use Kelly as a galactic lab rat by comparing him with his identical twin brother, Mark Kelly, who stayed gravity-bound on Earth for the past year. But previous research offers some insights into the effects of space-travel on the human body, Tech Insider reports.

"Because life on Earth has evolved to function best under Earth’s gravity, arguably all human organ systems are affected by gravity’s absence," Mark Springel, a research assistant in the Department of Pathology at Boston Children’s Hospital, wrote. "The body is highly adaptive and can acclimatize to a change in gravitational environment, but these physiological adaptations may have pathological consequences or lead to a reduction in fitness that challenges a space-traveler’s ability to function normally upon return to Earth."

In particular, spending a year in space can be hard on your bones, muscles, vision, sleep patterns, and immune system.

Muscles and bones

Without gravity, your body doesn't have to rely upon bones and muscles — so you have to make a concerted effort to stay active. To avoid excess muscle loss, astronauts on the ISS allot an average of two hours for exercise each day. Without regular exercise, humans lose about 1 percent of their bone density per month in space.

Vision

A 2012 study also showed that astronauts who spent an average of 108 days aboard the ISS had suffered vision problems. Researchers at the University of Texas Medical School discovered "optical abnormalities similar to those that can occur in patients with intracranial hypertension, a potentially serious condition in which pressure builds up inside the skull" in the 27 astronauts who participated in the study.

Sleeping patterns

Space is also bad for sleep, apparently. Astronauts report having difficulty acclimating to sleep schedules while working on the ISS because it's hard to get used to sleeping while strapped inside a bag and random flashes of bright, cosmic light are not all too uncommon. A 2014 study published in the journal The Lancet Neurology found that NASA astronauts were only getting about six hours of sleep per night, despite the fact that they're scheduled to sleep for eight and a half hours.

Immune system

On the subject of comic lights, another troubling effect of space travel on the human body relates to the impact of cosmic radiation on astronauts' immune system. Being in space for a long time appears to increase your chances of contracting bacterial or viral infections because "radiation, microbes, stress, microgravity, altered sleep cycles and isolation" all compromise the immune system," Brian Crucian, a biological studies and immunology expert at NASA, said in a press release.

There are a range of studies that have explored the various ways that time in space affects the human body, but the identical twin study with the Kelly brothers promises to yield some of the most accurate results.

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