When you walk into a dark movie theater, you cannot see anything at first. But slowly, you start to see. Our eyes have two kinds of light cells. Cones work in bright light and see colors.

Rods work in the dark. Cones get used to the dark in a few minutes. Rods need about 30 minutes, because bright light breaks a special chemical in them, and it takes time to rebuild it.

When we enter a dark room, we see almost nothing at first, but our vision slowly improves. This process is called dark adaptation. The retina has two types of light-sensitive cells.

Cones work in bright light and detect color, while rods are far more sensitive and work in the dark. About 95 percent of these cells are rods. In the 1930s, American researchers measured how quickly people could see faint lights after leaving bright light.

Sensitivity rose in two stages. Cones adapted within a few minutes, and rods took about 30 minutes. Rods are slow because their pigment, rhodopsin, is broken down by bright light and must be rebuilt using a part made from vitamin A.

Entering a dark cinema, we are initially almost blind, yet vision gradually returns. This dark adaptation depends on two classes of photoreceptors. Cones, which mediate color vision in daylight, account for only about 5 percent of the retina's photoreceptors; the remaining 95 percent are rods, sensitive enough to signal the arrival of a single photon.

In experiments published in 1937, Hecht and colleagues tracked the faintest light observers could detect after exposure to very bright light. Sensitivity improved along a two-branched curve: a rapid rise over the first few minutes that leveled off as cones adapted, followed by a slower rod-driven rise approaching its maximum after roughly 30 minutes. The brighter the preceding light, the longer recovery took.

Why are rods so slow? Light converts their pigment, rhodopsin, into a bleached form, and a vitamin A-derived component must be regenerated before it can function again. A 2004 review concluded that recovery after intense light is limited by the rate at which this component is delivered to the rods. Because rods respond poorly to red light, stargazers use red lamps to preserve adaptation. The popular claim that pirates wore eye patches to keep one eye dark-adapted, however, lacks historical evidence.