No one's landed on Mars yet and when they do it's hoped they'll be able to tell us if Martians really exist or not.
But scientists have discovered that the "clock" in the brain which controls the body's sleep cycle can only keep it's 24-hour rhythm for 90 days after leaving Earth.
When the clock doesn't work properly, it can cause problems in how much sleep an astronaut gets and how good that sleep is.
It's because the clock evolved on Earth - which spins in blocks of 24 hours.
It could affect longer mission like those to Mars unless research finds a way to trick the clock into thinking it's still on earth.