Does hanging upside down make you feel a little panicky? Like it might explode your head? Like all your spit might pool in your nose and drown you? Or like your eyes might pop out of their sockets and plop on the floor?
Don't worry — none of those things would happen. But hanging out in a head-down position isn't completely harmless, either. In fact, being suspended upside down for too long might not eject your eyeballs (although it can occasionally lead to temporary loss of vision in some people), but it would eventually kill you.
In 2009, a Utah man named John Jones died after spending 28 hours stuck upside down in a cave. Rescue workers tried to work fast, but the walls of the passage were so narrow, they weren't able to get him out before he died — most likely of asphyxiation. Turns out, your lungs evolved to sit atop of all the other organs for a reason. Delicate organs that they are, it doesn't take them long to get squished by the bigger, heavier organs like the liver and intestines that usually sit below them. This isn't as much of a problem when you're lying on an incline with your feet slightly elevated above your head, but when your head is directly underneath your feet, your lungs simply can't absorb enough oxygen given the available space they have to work with.
But it's not just the lungs you have to worry about during extended periods of reverse suspension. Our bodies are well set up to move blood around when we're standing upright, and our blood vessels are customized to make sure blood doesn't pool up in our feet. But that system is a one-way street — our bodies didn't evolve to keep blood from pooling in the brain. When this happens, all sort of things could go wrong, including ruptured blood vessels, which can lead to brain hemorrhage.
And don't forget about your heart! Medical professionals think heart failure is the cause of death in most upside-down fatalities, for much the same reason our brains can suffer: when you're head down, your heart slows down its pumping and starts receiving more blood than it has the capacity to manage at one time. It begins to have a hard time maintaining blood pressure, and eventually loses its ability to move enough blood around to maintain all the body's essential functions.
But here's the thing: Although hanging upside down will occasionally kill people, if you take it in shorter stints, it can confer some pretty compelling health benefits. Inverted poses encourage blood to flow from the feet, legs and pelvis back to the heart and lungs, where it picks up fresh oxygen. Studies have shown that, over time, brief inversions can lower the resting heart rate, increase overall endurance and help your body use oxygen more efficiently during exercise.
"In yoga, inversions are considered beneficial in a number of ways; headstand and shoulderstand are among the most important of all the asanas, cleansing the lymphatic system and stimulating all of the systems in general," says Julie Phillips, an aerial yoga and dance instructor at Canopy Studio in Athens, Georgia. "Yogis are known to hold inversions for long periods of time — 12 minutes, and I've even read there are some who hold them for hours. On the personal level, while I don't hold inversions anywhere near that long, I find them to be very therapeutic."
So although inverse suspension can be dangerous, it's actually pretty rare that people die from it. There doesn't even seem to be a hard-and-fast rule about how long it's safe to hang out upside down. For instance, people with heart conditions die a lot faster in situations like these, and young, healthy people can survive longer than the old or sick. Afraid of getting stuck upside down on a roller coaster? Well, it's not impossible. In 1997, a group of people found themselves in exactly that position for an hour and a half, and everyone survived the ordeal.
So the rule of thumb seems to be, if you're upside down and start to feel like you should stop, then stop.
"In my aerial yoga classes, we take five minutes — sometimes longer — with students hanging upside down," says Phillips, "but I make it clear they can come into and out of their inversion throughout that time, and emphasize they should come out at any point that it feels like too much."