During the voyage of the Beagle, Charles Darwin was often overcome by seasickness and discovered that "nothing but lying in [his] hammock did any good."This remedy--lying down with eyes shut--is one of the most effective for treating motion sickness.
The idea behind this remedy is that motion is only one part of the problem. Lying down certainly does not negate the motion, but closing one's eyes does help the brain respond to it.
Motion sickness is caused by conflicting impulses that the brain receives from the eyes, the inner ear, and pressure detectors in the skin and joints. This explains why many people have difficulty reading in a moving car: the brain needs to respond to discordant messages from the senses. The eyes are focused on the steady page, while the body is jostled by the bumps and twists of the road. Likewise, many people experience motion sickness in the interior cabins of boats where the sensations of rising and falling contradict the stable visual cues.
When travelling, motion sickness can be reduced by strategically choosing seats where the least motion occurs. However, because it is difficult to minimize motion, many remedies try to reduce the other impulses that contradict it. When travelling in a car or boat, looking at the horizon often reduces motion sickness. Likewise, when travelling by air, watching the plane's wing can help orient the brain. Many travellers also benefit from wearing specially designed bracelets that stimulate pressure sensors in the wrists.