Why do astronauts in space sleep while standing?

Actually, this is only slightly related, the thread being unusual manifestations of near zero g. read more

Robert answered most of the part of the reason why astronauts can sleep while standing. But the true reason is that they sleep in standing position because of not to harm to themselves, and the instruments onboard. They actually sleep in sleeping bags that are fixed to the inner wall of the spacecraft in vertical position. read more

In space there is no up or down, and you do not feel the pull of gravity. As a result, astronauts are weightless and can sleep anywhere. Astronauts can attach themselves to a wall, a seat or a bunk bed inside the crew cabin so they don't float around and bump into something. Space Station crews usually sleep in sleeping bags. On the Space Station there are two small crew cabins. read more

Here, Marsha Ivins, an astronaut on the ISS, describes it perfectly:"One of the strangest experiences in space is one of the simplest on Earth: sleeping. On the shuttle, you strap your sleeping bag to the wall or the ceiling or the floor, wherever you want, and you get in. read more

But it's a little different in space. In space there is no up or down, and you do not feel the pull of gravity. As a result, astronauts are weightless and can sleep anywhere. Astronauts can attach themselves to a wall, a seat or a bunk bed inside the crew cabin so they don't float around and bump into something. read more