These are the hardest things to get used to when living in space, according to astronauts

Above a door on a platform standing 200 feet in the air that connects to the NASA space shuttle that ferries astronauts to the International Space Station (ISS), there's a sign that holds some serious significance for astronauts. It reads: "Last bathroom on Earth."

Living on the $1 billion space station that floats 260 miles above Earth is clearly a once-in-a-lifetime experience, but it takes a lot of getting used to.

Astronaut Ron Garan, who spent 6 months on board the ISS in 2011, wrote a book called "The Orbital Perspective" describing the time he spent living in space and how the experience changed his outlook on life. But in the seconds before his first launch, Garan caught himself wondering what he was getting himself into.

According to Garan's book, and the experiences of other astronauts, here are some of the hardest things to get used to when living in space:

1. Weightlessness

After taking advantage of the last bathroom on Earth, Garan blasted into space for the first time ever.

Unfortunately his first few hours in space were full of nausea. Astronauts train extensively for zero-g, but training for a few hours is very different than living in it 24/7.

Garan described the initial moments of weightlessness as pleasant, but it didn't take long for the nausea to kick in. Garan said it was as if his body was saying "'Hey, zero-g isn't suppose to last this long. Something must be wrong.'"

By the end of the first day his body had adjusted enough for the nausea to go away.

But dealing with zero-g on a day to day basis is an adjustment too. In June 2014, astronaut Reid Wiseman chronicled his first experiences in the weightless environment on Twitter:

"Still adjusting to zero g. Just flipped a bag upside down to dump out its contents. #doesntworkhere."

2. Sleeping

You can't actually lay your head down to sleep in space, so it makes sense that it's a challenge to get some solid shut eye.

The first night Garan spent in zero-g, he said "We staked out spots on the floor, walls and ceiling, attached our sleeping bags, and called it a night."

But there was a problem. What are you suppose to do with your head? You can't lay it down on a pillow, it just drifts there awkwardly, Garan said.

It took several weeks before the muscles in his neck adjusted to the bizarre sleeping position and before he could finally get a decent night's sleep.

Canadian astronaut Chris Hadfield also recently gave a description of life in space and the trouble with sleeping. One strange thing about living on the ISS is that the cycles of day and night are different. The station orbits the Earth about every 90 minutes, so every 45 minutes astronauts see a sunrise or a sunset. Hadfield says they still turn out the station lights before sleeping.

"We shut off most lights at bedtime - it feels right to do it," Hadfield said.

3. Keeping track of time

In space you don't keep track of the days of the week. Instead, you keep track of flight days (FD). The first day that Garan stepped on board the ISS was referred to as FD1, the day after that FD2, and so on.

American astronauts Barry Wilmore and Terry Virts who are currently living on board the ISS said that on New Year's Eve they counted down to midnight and rang in the New Year 16 different times. That's because the space station passed over a part of Earth during midnight 16 different times that night as it orbited the planet at about 17,400 miles per hour.

4. Dealing with body fluids

In space, your nose doesn't run and you can't cry. Tears will still form, but they don't fall. Instead, they congeal into sticky balls, Hadfield said.

Like the ominous "Last bathroom on Earth" sign suggests, using the bathroom in space is an ordeal. When liquid or solid waste comes out of the body in space, it has to get sucked into the toilet for fear it would escape and float around the chamber. Urine gets purified into drinking water. Poop gets collected onto an unmanned space ship that jettisons from the space station and burns up as it returns to Earth. So yes, that shooting star may have been burning poop.

There are no showers since all the water would end up floating around the station in clumps and destroying the equipment that's all over the station walls. Instead, astronauts take sponge baths.

5. The view

There's a windowed dome on the ISS that many astronauts have said is their favorite spot on the station.

It's called the Cupola, and the view from this window is unparalleled. Looking down at the Earth astronauts can watch breathtaking aurora displays, lightning, and cities sparkling at night.

This view from the ISS completely changed Garan's outlook on life, and turned him into an activist after he returned to Earth.

"Seeing Earth from this vantage point gave me a unique perspective - something I've come to call the orbital perspective," Garan writes. "Part of this is the realization that we are all traveling together on the planet and that if we all looked at the world from that perspective we would see that nothing is impossible."