Q&A: How Astronauts Sleep

Question: How do astronauts sleep without floating around and banging into stuff? Do they have like seat belts on their beds, or what? — WW, Portland, OR

[If the reader’s name looks familiar, this is the same student who asked about astronauts exercising last week. Each question required a lengthy response so I split them into two posts. This practice also discourages students looking for quick help with homework. 🙂  And ChatGPT agrees.]

Answer: To really understand this we first need to recognize an important fact: When you are in orbit you are in free fall. That is where the weightlessness (officially called microgravity) comes from. And when you are weightless there is no up or down.

Sure, astronauts may talk about Earth being “down there” and themselves as being “up here” but they know it isn’t a real “down” or “up” — for that you need to feel gravity. That’s why, on the ISS, they don’t really have a “ceiling” or “floor” and have instruments and controls mounted all around them. This makes best use of the expensive square-footage available. Astronauts just float over to what they need to work on, and have at it.

So for sleeping there’s no need for beds. That would also be a waste of precious space. Instead, they just zip themselves into what are basically “sleeping bags” tethered to the hull. In the video the lights are on just so you can see what that looks like, but when they sleep the lighting is significantly dimmed.

Another good reason to have the bags tethered is in case the thrusters need to be fired. This video shows what happens to un-tethered astronauts when that happens:

Note: The astronauts move at the same instant the thruster is fired. The delay in the video is because of the scene change.

The astronauts “move” because the thrusters push on the ship, not the astronauts. So when the ship starts to move it “runs into” the free-floating astronauts inside. Once the wall runs into them and starts pushing on them, then their speed will also change to keep up with the ship. These two astronauts appear surprised. Not sure why they weren’t given advance notice of the burn.

There’s one other important factor to deal with when astronauts sleep: carbon dioxide bubbles. When you sleep in your bed at home, you inhale oxygen (which you need to stay alive) and exhale carbon dioxide (which is a waste product). Carbon dioxide has a density of 1.87 kg/m3, and air has a density of 1.29 kg/m3. So on Earth the heavier carbon dioxide flows off your bed and sinks to the floor. From there it’s recirculated with the rest of the air in the house and gets mixed in well enough not to be dangerous.

That would not happen where astronauts sleep. Every time they exhale they add to a bubble of carbon dioxide that slowly grows around them. There is no “down” for the carbon dioxide to sink to. Eventually, they would wake up gasping for air. So to prevent this, there are fans all around the sleeping area to keep the air circulating. The same is true for areas where they work. Good air circulation is crucial in space.

Next Week in Sky Lights ⇒ Nyctinasty


Q&A: How Astronauts Exercise
Q&A: Nyctinasty