Near the front we don't see so well at night, but if we look a bit to the side objects ahead of us can pop into view. The effect is that at night we can detect motion at the edge of our view. Since threats tend to sneak up from the side or behind, the rods are placed at the periphery of our eye while the cones occupy the central part of our vision. The rods work best at detecting motion, for night survival. In daytime we need detail to live our lives, but at night our primary need is threat detection.
To do so, the eye has two types of cells cones are used mainly for color vision, rods for black and white in low light levels. We tend to live a diurnal life, so we need eyes that can fill our needs both in light and dark environments. Humans only need eight hours of sleep, maybe less, each night but there are more hours of night in most latitudes for much of the year.
By Jim O'Connor of the Tucson Amateur Astronomy Association
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