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As pilots, our biggest oxygen consumer is our brain. Night vision is one of the first things to go. The retinas rod cells, which provide night vision, are heavy oxygen consumers. Retinal function begins to deteriorate at altitudes as low as 5000 feet, and so will your night vision. Low perfusion of oxygen also reduces visual acuity. Blurred vision and tunnel vision are both common symptoms of hypoxia. If you experience these symptoms before reducing altitude or starting supplemental oxygen, they may linger.