Vision is the first of the senses to be affected by a lack of oxygen. This is especially true of night vision. Even at a relatively low altitude of approximately 5000 feet, where no other effect of hypoxia can be detected, night vision is appreciable reduced. Color vision starts to deteriorate between 1500 - 3000 m (5000 – 10 000 ft). Therefore, at night, because vision is particularly sensitive to diminished oxygen, a prudent rule is to use supplemental oxygen when flying above 6000 feet MSL. For smokers, this altitude may be even lower.