Your turn
Each lab is one step from the video. Change things and see what happens.
Each lab isolates one step. Facts from NASA, CSA, JAXA and a 2014 sleep study; pictures are not to scale.
Tap the buttons.
Is there gravity on the space station? Why does the sleeper float?
Gravity at station height is about 90% of surface gravity. So why is nothing pressing the sleeper down?
Tap the buttons. Does the sleeper float or fall?
What happens if the straps come undone? Where do the arms go?
Undo the restraints, then compare the neutral body posture with arms tucked in.
Tap to undo the straps. Tap to tuck the arms in.
Turn the fan off. What happens to the breathed-out air?
Without buoyancy-driven convection, what removes exhaled CO₂? Compare Earth, orbit with the fan, and orbit without it.
Turn the fan off and on. Where does the breath go?
Illustration only: the dots are not real measurements, and real CO₂ is invisible.
Drag the station around Earth. How many sunrises fit in one day?
At about 90 minutes per orbit, how many sunrises per 24 hours? Then compare daytime and pre-sleep lighting.
Drag the station around. Is it day or night?
Not to scale: the real station orbits much closer to Earth.
Which tools help most? What happens with no fan?
Toggle the countermeasures. Why does the model cap the night when there is no ventilation?
The sleep meter is a made-up guide, not real data.
Send the astronaut to space and back. How much taller do they get?
Unloaded intervertebral discs expand. Compare heights in orbit and back on Earth.
Tap "In space". Does the back stretch?
The stretch is drawn bigger than life so you can see it.
Astronauts sleep in sleeping bags tied to a wall! In space, everything floats, so there is no up and no down.
The space station is always falling around the Earth, like a ball that never lands. Everything inside falls too, so it all floats. Gravity is still there. It just can't press you onto a bed.
Each astronaut has a tiny bedroom, about as small as a closet. The sleeping bag is tied to the wall, so they don't float away and bump into things. Their arms float up, so they tuck them in.
In space, your warm breath doesn't float up and away. Without moving air, some of it can stay by your face, like a little cloud. So a fan blows fresh air at every sleeping astronaut. It's noisy, so they wear earplugs.
The station zooms around the Earth so fast that astronauts see the sun come up about 16 times every day! They use a clock, an eye mask and soft, warm lights at bedtime to know when to sleep.
So tonight, when you snuggle into bed, say thank you, gravity!
Want more? See how rockets fly to space and how gravity works. ← More explainers
On the International Space Station, astronauts sleep in sleeping bags fixed to the wall of a tiny private cabin about the size of a phone booth.1 They loosely strap themselves in so they don't drift around and bump into things during the night.313 An air vent blows fresh air past their face, and most wear an eye mask and earplugs to block out light and the noise of fans and machines.13
Why all this fuss? Because in orbit nothing holds you down on a mattress, your own breath can hang around your face, and the Sun rises about 16 times a day.12 Each of these problems has its own fix, and the toys in the player above let you try every one.
A common idea is that there's no gravity in space. But the space station orbits only about 250 miles (400 km) above Earth,2 and at that height gravity is still about 90 percent as strong as on the ground.5
Astronauts float because the station and everyone in it are in free fall. They are always falling toward Earth, but they are also moving sideways so fast (about 5 miles, or 8 km, every second) that they keep falling around Earth instead of into it.45 Because the astronaut, the bed and the walls all fall together, nothing presses on anything. That's why scientists call it microgravity or weightlessness, not zero gravity. Read more in how gravity works, and see how crews get up there in how rockets work.
Earth: gravity presses you into the mattress vs orbit: you and the mattress fall together → you float
Each astronaut has a personal sleep station (also called crew quarters), a compartment the size of a telephone booth. According to the Canadian Space Agency, it holds:1
The sleep stations are designed to block light and keep sound below about 50 decibels.6 Private quarters also mean one crew member's alarm or late-night work doesn't wake the others.2 Anyone who wants to sleep elsewhere can clip their sleeping bag to a floor, ceiling or wall anywhere in the station.13
In photos they seem to sleep standing up against a wall, but to the astronaut it doesn't feel like standing at all. With no up or down, sleeping on the floor is just as comfortable as sleeping on the wall.1 Astronauts can sleep facing any direction.13 Because they are used to a mattress pushing on their back, the sleeping bag has a firm cushion that presses gently against them.1
One odd thing happens to your arms. When a body relaxes in weightlessness, it settles into a neutral body posture: arms float up and forward, shoulders move outward, and knees and hips bend a little. NASA first noticed this in photographs taken on the Skylab space station.9 If floating arms feel strange, a sleeper can simply keep them inside the bag.
This is the hidden danger of sleeping in space. On Earth, the air you breathe out is warm, and warm air is lighter than the cooler air around it, so it rises and gets stirred away. This gentle stirring is called convection, and gravity drives it.
In free fall, that gravity-driven stirring almost disappears.10 So the carbon dioxide (CO₂) an astronaut breathes out can form a bubble around their head.1 NASA scientists warn that pockets of CO₂ can build up wherever no fan pushes fresh air through, so the crew ends up breathing their own exhaled air again, which can cause headaches.11 The fix is simple: sleep near an air vent that keeps fresh air moving past your face.1
The downside is noise. Fans, pumps and air conditioners hum all night, which is one reason astronauts are given earplugs.113
The space station circles Earth about every 90 minutes. In 24 hours it makes 16 orbits and passes through 16 sunrises and 16 sunsets.42 That sounds amazing, but it's a problem for sleep. Your circadian rhythm (your body clock) uses light to decide when you should feel alert and when you should feel sleepy, so a sunrise every hour and a half can badly confuse it.2
Crews fight back in three ways:
| Light mode | Colour | Used for |
|---|---|---|
| General | Neutral white (4500 K) | Everyday work |
| Pre-sleep | Warm, less blue (2700 K) | End of the work day |
| Phase-shift | Cool, bright, bluer (6500 K) | Shifting a sleep schedule |
Source for the table: NASA Johnson Space Center.8
Crews are usually given about 8 hours of sleep time after a 16-hour work day.3 But they often don't get it. The biggest study so far, published in The Lancet Neurology in 2014 by Laura Barger and colleagues, tracked astronauts with wrist monitors. The 21 space station astronauts in the study slept about 6 hours a night on average (6.09 hours), and space shuttle crews slept even a little less.6 About three out of four of the station astronauts reported using sleep medicine at some point.6
Why so little? Excitement, motion sickness, noise from the station or other crew members, light and long work days can all cut into sleep.313 Some astronauts simply stay up to enjoy the view.13 This matters, because tired people make more mistakes. To see why, read why sleep is important.
It can be. Nothing is pulling you down, so there are no sore spots where a mattress presses on your hips or shoulders. But it takes getting used to, and not everyone sleeps well. The Canadian Space Agency notes that some specialists think astronauts may need a little less sleep in space, because their muscles don't work as hard as on Earth, though noise still makes a good night's rest hard.1 Astronauts even dream and snore in space.3
Yes, for a while. Your spine is a stack of bones with squishy discs between them. On Earth, gravity squashes that stack a little all day. In weightlessness the spine can stretch out, and astronauts can grow up to 3 percent taller. For someone 6 feet (1.8 m) tall, that's as much as 2 inches (5 cm).129 Back on Earth, gravity squashes them back to their normal height.12
Coming home can feel strange in other ways too. After long stays in space, some astronauts say that for a few days they feel as if they are floating above their mattress.1
People have slept in space almost as long as people have flown there. In August 1961, Soviet cosmonaut Gherman Titov spent more than 25 hours in orbit aboard Vostok 2, and so became the first person to sleep in space.1415
Everything learned along the way, from straps to fans to lighting, will matter even more on long trips to the Moon and Mars. Animals have their own clever sleep tricks too: see how whales sleep without drowning.
| On Earth | On the space station | |
|---|---|---|
| Bed | Mattress, lying down | Sleeping bag strapped to a wall |
| What holds you | Gravity | Straps (you're in free fall) |
| Your breath | Rises and drifts away | Needs a fan to blow it away |
| Sunrises per day | 1 | About 16 |
| Typical sleep | Depends on you | About 6 h (8 h scheduled) |
| Your height | Normal | Up to 3% taller |
Often, yes. Crews are scheduled about 8 hours of sleep, but a 2014 study that tracked 21 space station astronauts with wrist monitors found they slept about 6 hours a night on average. Excitement, noise from fans and machines, light, busy schedules and a body clock confused by 16 sunrises a day all get in the way, so astronauts use eye masks, earplugs, planned bedtimes and special lighting.
Each astronaut has a small private sleep station about the size of a phone booth. It holds a sleeping bag fixed to the wall, a pillow, a lamp, an air vent, a laptop and a spot for personal things. Crew members can also clip their sleeping bag to a wall, floor or ceiling somewhere else in the station.
It only looks that way in photos. In free fall there is no up or down, so a sleeping bag on the wall feels exactly the same as one on the floor. Nothing presses the astronaut down, so 'standing' and 'lying' mean the same thing.
On Earth, the warm air you breathe out rises and gets mixed away. In free fall, warm air doesn't rise, so the carbon dioxide you breathe out can collect around your face and cause headaches. That's why every sleep station has an air vent, and why astronauts are told to sleep where air is moving.
About 16. The space station circles Earth roughly every 90 minutes, so it passes from night into day about 16 times every 24 hours. To keep a steady routine, crews follow Greenwich Mean Time (GMT) and a planned bedtime.
Yes. NASA says astronauts can dream and snore in space, just like on Earth.
They tried. After landing in 1969, Neil Armstrong reportedly couldn't sleep at all and Buzz Aldrin managed only a couple of hours, because of light, noise and cold in their small lunar module cabin.