What causes Motion Sickness?

The exact causes of motion sickness (also known medically as kinetosis) are not fully understood. However, experts broadly agree that motion sickness occurs when the brain receives conflicting messages from the various parts of the body that sense and respond to movement.

For example, below decks on a ship the balance receptors in your inner ear may sense a lot of movement, while your eyes don't register any movement at all. Similarly, while reading in a car or bus, your eyes are focused on the unmoving page, while your body registers the movements of the vehicle.

Conversely, motion sickness can also be triggered by the eyes seeing movements, but the body feeling nothing (for example with simulators, 3D movies and IMAX shows).

This sensory confusion disorientates the brain, which doesn't know how to respond, and triggers dizziness. This in turn activates the brain's 'vomiting control center', resulting in nausea and - ultimately - active vomiting.

Why does motion sickness trigger vomiting?

Why sensory disorientation should cause vomiting is not really understood. But I recently came across the fascinating suggestion that it may be because the brain interprets the sensory confusion as hallucinations resulting from poisonous food. The brain then causes the body to vomit the toxins out.

If true, this would be an example of a mechanism that was probably crucial to our distant hunter-gatherer ancestors, but is rather less useful to today's travelers!

Some researchers suggest that low frequency sounds may also play a part in causing motion sickness. They argue that the sound, as well as the vibrations, of vehicle engines and the swell of the sea may disorientate our bodies and trigger nausea. It seems to be a controversial theory, however, as other experts discount it completely.


Other motion sickness triggers

A number of other factors are likely to trigger motion sickness, or make the symptoms worse:
  • Traveling when tired. Do try to get a good night's sleep before traveling!
  • Eating a heavy meal, or greasy or spicy food, before or during a journey.
  • Dehydration. This is a major trigger for many people. Make sure that you are well hydrated before you start your journey, and keep a bottle of water with you and sip from it regularly.
  • Alcohol, tobacco and drugs. Despite old sailing tales, none of these are your friend if motion sickness is likely. It's also wise to abstain the evening before a trip - suffering from motion sickness and a hangover simultaneously is probably one of the worst things you can experience!
  • Feeling cold. Always keep a jacket or travel blanket with you.
  • Strong scents and smells. Obviously vehicle fumes and strongly unpleasant smells are going to make you feel worse, but even pleasant smells, such as perfume, can make you feel ill while traveling.
  • Being near someone who is already feeling motion sick. This can cause a 'chain reaction', with everyone around the sick person starting to feel queasy too.
For more on avoiding these triggers, and what else you can do to prevent motion sickness in all its forms, see Behavior.


The psychological aspect

Motion sickness is undoubtedly a real physical condition (as anyone who has ever suffered from it will confirm!), however it also has a strong psychological aspect. Anxiety, fear and negative associations can make people travel sick while a plane is still on the runway, or the boat still in port. In extreme cases, people have been known to get sick at just the thought of a journey, or from looking at a photo of a ship in rough seas. On the other hand, you may find that positive thinking goes a long way towards preventing severe motion sickness.




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