Space has no ‘down’ or ‘up’ and there is no gravity. This makes astronauts weightless and sleeping for them can be in any orientation. However, to avoid floating and bumping on things, they strap themselves inside the crew cabin to a seat, wall or a bunk bed.

The space crew avoids sleeping at the cockpit since the warmth and sunlight entering through the cockpit window disrupts a sleeper who has no sleeping mask on. Sometimes their sleep is disrupted by either the excitement of being in space or motion sickness. Sleeping in close quarters too can be disruptive since the crew members can hear each other easily. According to NASA, common drugs that astronauts take are painkillers and sleeping pills. The sleeping pills help combat sleep deprivation and insomnia that is common while on space.

To help avoid disruptions from sunlight, astronauts cover the windows and wear sleep masks to shut out any distracting light. To shut out constant whirring noises made by air filters, fans and other noisy equipment, they sleep with earplugs on. This helps dampen the sound but they get used to the noise after a while.

Though while strapped to a wall, your arms may float away like a zombie’s, “it’s really comfortable,” swears Mike Fincke, an astronaut. He says that at one time, he was so tired after a long day on the International Space Station, while accompanied by another astronaut and spacewalk. “We were sitting around the table drinking some tea, and I just fell asleep. I started floating away.”

The floating problem of sleeping astronauts is normally avoided by tucking themselves into sleeping bags strapped to the walls. They also have to cross their arms and legs to avoid having them hovering above them. The ventilation also has to be effective since according to Robert Frost, an astronaut trainer “the carbon dioxide they breathe out doesn’t float off — it just sits there in front of their mouth, waiting to be sucked back in.”

Most of them try to sleep as close to how they would while on earth- by strapping their sleeping bags to the floor, ceiling or walls. There is no such thing as ‘up’ on space which means that it’s as easy to sleep vertically as one would horizontally while on earth.
[source:www.theatlantic.com]