Sickness caused by contradiction between external data from the eyes and internal cues from the balance centre in the inner ear. For example, in seasickness the inner ear senses the ship's motion, but the eyes see the still cabin. This stimulates stress hormones and accelerates stomach muscle contraction, leading to dizziness, pallor, cold sweat, and nausea and vomiting. Minimizing changes of speed and direction may help, as may reclining, not turning the head, closing the eyes, or focusing on distant objects. Drugs can prevent or relieve motion sickness but may have side effects. Pressing an acupuncture point on the wrist helps some people.
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Seasickness is a form of motion sickness characterized by a feeling of nausea and, in extreme cases, vertigo experienced after spending time on a craft on water. It is typically brought on by the rocking motion of the craft. Some people are particularly vulnerable to the condition with minor stimulus and will feel seasick simply by setting foot on a boat, even if the vessel is in dry dock, while others are relatively immune, or become immune through exposure.Some people have even claimed experiencing sea sickness while watching nautical themed television programs.
Seasickness can be a debilitating condition and can be dangerous if the sufferer has an important role to carry out, such as steering a yacht through stormy seas while avoiding rocks and other hazards. It is also particularly hazardous for scuba divers who, through dehydration following vomiting, are at increased risk of decompression illness.
Sea-sickness has such a remarkable effect because both the sense of sight and touch are disturbed by the motion of a craft on water. The severity of seasickness is also influenced by the irregular pressure of the bowels against the diaphragm as they shift with the rising and falling of the ship.
In his book The Human Body, Isaac Asimov related the anecdote about a seasick passenger whom a steward cheerfully assured that nobody ever died from seasickness. The passenger muttered, "Please--it's only the hope of dying that's keeping me alive."
Many experience similar effects while not at sea:
Those suffering from seasickness who are unaccustomed to the motion of a ship often find relief by: