Jokingly referred to as the “sixth sense,” the vestibular system may be the most underappreciated part of the human body. For thousands of years, we’ve known that we can see, hear, touch, smell, and taste the world around us. But it wasn’t until the latter half of the nineteenth century that scientists discovered something surprising: The ear does more than hear. It also allows us to balance ourselves by sensing movement and gravity.

Most of the time, the vestibular system works so seamlessly that we’re completely unaware of it. It’s only when something goes wrong—causing vertigo, dizziness, imbalance, or falls—that we begin to appreciate just how essential this system really is. Dr. Jeff Sharon, author of The Great Balancing Act: An Insider’s Guide to the Human Vestibular System and a professor in the Department of Otolaryngology—Head and Neck Surgery at the University of California, San Francisco, highlights the importance of this forgotten sense.

1. Almost Every Animal on Earth Has a Vestibular System

The vestibular system performs two critical tasks: It senses gravity, and it senses movement. Knowing which way is “up” is an ancient ability found among nearly all lifeforms on Earth. Even plants rely on gravity sensing to grow upright.

For animals that move, the physical demands are even greater, so this system becomes even more important. To run, fly, or swim without constantly falling over, animals must sense not only gravity but also how their heads are moving and oriented in space. That’s why all vertebrates—from toads and clownfish to bats and humans—have inner ears that are remarkably similar. Even extinct animals like dinosaurs had three semicircular canals, just as we do. Some invertebrates like jellyfish also have the ability to sense gravity using small organs called rhopalia, despite not having brains.

The vestibular system is so widespread because, in the arms race of natural selection, animals that couldn’t sense gravity or their own movement were unlikely to survive. 

2. Your Smartphone Has a Vestibular System

Have you ever noticed that your phone knows when to switch from portrait to landscape mode? Or that it can tell whether you’re walking, running, or climbing stairs—and even count how many steps you’ve taken?

That’s because smartphones contain sensors that function much like a vestibular system. By sensing gravity, the phone figures out its orientation. By detecting patterns of acceleration, it can infer movement, including steps.

Biologically, the vestibular system does the same job as an accelerometer and a gyroscope, which measure gravity, turns, tilts, and acceleration. Many human-made devices rely on these same principles, including airplanes, camera gimbals, rocket ships, video game controllers, and humanoid robots.

In fact, an airplane cockpit is filled with instruments that display vestibular information. The attitude indicator shows pitch, while the turn coordinator shows yaw and roll—information that your own vestibular system senses as well.

3. The Vestibular System Doesn’t Work in Space

More accurately, part of the vestibular system stops working in space. The system has two main components: the semicircular canals, which sense rotation, and the otolith organs, which sense gravity.

The semicircular canals work just fine in space because they rely on inertia, i.e., the tendency of moving objects to keep moving in the same direction. Inertia is why you need a seatbelt: When a car suddenly stops, your body wants to keep going. It’s also why hard science fiction often invokes “inertial dampeners” to explain why spaceship crews aren’t smashed against the walls when coming out of hyperspace.

The otolith organs, however, depend on gravity. They contain tiny, heavy crystals that rest on sensitive hair cells. On Earth, gravity pulls these crystals downward, allowing the brain to sense head tilt and linear acceleration. In space, without gravity, the crystals float and thus the otolith organs can no longer function properly.

This mismatch is why astronauts often become disoriented shortly after entering space and may experience odd sensations like being in freefall. Many also develop severe nausea and vomiting. When the brain receives vestibular signals that don’t make sense, it interprets them as a sign of poisoning—a response that triggers nausea as a protective reflex.

4. Vestibular Disorders Are More Common Than You Think

Problems with dizziness and balance account for about 5 percent of visits to emergency departments and general medical clinics. In national surveys, roughly 12 percent of people report experiencing dizziness or balance problems in any given year. Almost everyone knows someone who has had vertigo, which is a vestibular illusion that occurs when the inner ear signals movement even though the body is still. Like other sensory systems, vestibular function also declines with age. One commonly used test of vestibular function, called the cVEMP, is more likely to yield an absent response than a present one by around age 70.

Despite how common these problems are, there are relatively few doctors with specialized training in diagnosing and treating vestibular disorders. This is a huge problem for patients, who can struggle to find the expertise they need.

5. People Without a Vestibular System Don’t Get Seasick

In the late nineteenth century, the psychologist William James—often called the “Father of American Psychology”—made a curious observation: Deaf individuals with balance loss didn’t seem to get seasick. He followed this insight with experiments involving intense spinning, confirming that something about the inner ear was essential for motion sickness.

Because the inner ear is responsible for both hearing and balance, it wasn’t initially clear which sense mattered. Later studies showed that people with complete loss of vestibular function are not susceptible to seasickness at all.

This finding supports the “sensory mismatch” theory of motion sickness. According to this theory, motion sickness occurs when different senses provide conflicting information. On a boat, for example, the vestibular system senses the rocking motion of the waves while the visual world appears relatively stable as you move with the boat. The brain struggles to reconcile this conflict.

People without vestibular input don’t experience this mismatch. Without conflicting signals, there’s no motion sickness—making them immune.