Jim O'Connor of the Tucson Amateur Astronomy Association,
on dark adaptation of the eye and the value of red flashlights
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. We tend to live a diurnal life, so we need eyes that can fill our needs both in light and dark environments. 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. In daytime we need detail to live our lives, but at night our primary need is threat detection. The rods work best at detecting motion, for night survival. 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 effect is that at night we can detect motion at the edge of our view. 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. Astronomers call this averted vision, and it is used to find faint objects in an eyepiece.
Rods don't work on their own; they are inert. Their type of nerve cell need a chemical to enable their function. The body does not produce this chemical in daytime. It takes a very low light level sensed by the eyes to produce this chemical called rhodopsin, or visual purple. When the light is detected at a low level for 20 minutes or so, the body starts producing rhodopsin and night vision starts setting in. The other contributor to night vision is the pupil opening, but that goes to maximum within a few minutes of dark exposure. The big player in night vision is rhodopsin, and that takes from 20 to 40 minutes for humans to start benefiting from it. A key trait of this feature is that rhodopsin is photoreactive. It only takes a few seconds of bright light to cause the rhodopsin to decay into two parts with a photosensitive reaction, and the rods stop working. Then the cycle starts again. It is an interesting trait that deep red lights do not trigger the neutralization of the rhodopsin, so astronomers and safety officials use red lights for night lighting to allow night vision to continue. Since, unless the light is monochromatic like a laser, even red light has elements from other colors, even a bright red light can reduce the rhodopsin so a dim red light is best for maximizing after-dark eye behavior.