Question: Why does it take our eyes longer to adjust to the the dark (like when we turn the lights out at night) than it does to light (like opening our eyes in the morning and how do our eyes do this?

Michael Kelly answered on 13 Jun 2014:

This is a good question and gets to the fundamental biology of how we see. As you may know, in the eye we have two types of light sensing cells or photoreceptors called rods and cones. Rods are responsible for our vision in dark conditions and cones in bright light. These photoreceptors contain different pigments. These pigments absorb light and trigger an electrical signal that is sent onwards to the brain. Once a pigment molecule has absorbed light it must be replaced.

In the light the pigment in both cell types is used up and must be replaced. The rod pigment (rhodopsin) is very good at capturing light and so is quickly exhausted, while the pigment in cones (iodopsin) is less efficient and is used up more slowly. As well as this the mechanisms to restore the pigments are different too. The used rod pigment is replaced very slowly, but the cone pigment is very quickly restored.

So we have two processes to signal light being absorbed; one (rod-based) that is very sensitive but quickly exhausted and slow to recover, and another (cone-based) that not so sensitive, slow to use up but fast to recover.

When we are in an area with constant light these two processes reach a steady level of activity, or equilibrium, and we are unaware of any changes if they occur slowly.

In a light room, the rod pigment is quickly used up and constantly needs replacing and there is no signal from the rods. The cone pigments are used up much more slowly and they are easily replaced by the surrounding mechanisms, so cones can continue to send signals. When we suddenly change the lighting, by turning the light off, these steady states of use and recovery alter, and so the rate of signals to the brain changes too.

Turning the light off means that the levels of pigment increase and the sensitivity of both types of photoreceptor increases. The cones rapidly recover their maximum sensitivity, but because they aren’t very good at capturing light there is no signal generated and no sensation of light created. The rods are slower, much better at capturing light and as they recover we slowly begin to see in the dark.

When we enter the light after being in the dark for a while, both systems are very sensitive. At first we have a massive signal, and perhaps feel dazzled. But the cones very rapidly reach their equilibrium and begin to send useful signals to the brain, and the rods are quickly exhausted and say nothing.