Why does it take a few minutes for your eyes to adjust to darkness?


Why does it take a few minutes for your eyes to adjust to darkness?

In: Biology

[Rhodopsin](https://en.wikipedia.org/wiki/Rhodopsin) is a protein photopigment used by the rod cells of the retina to detect gray-scale brightness and contrast, and is the key to adaptive night vision. Intense light causes the protein to decompose reducing its sensitivity in bright light (when we rely more heavily on our color-sensing cone cells anyway). Darkness allows the protein to regenerate in a process called “dark adaptation”, allowing the eye to see in low light conditions, but with reduced color discernment.

This process allows us to be able to see fairly effectively in both bright and dim light, but it takes time for our vision to adjust to sudden shifts in brightness in either direction.

Fun fact: That’s why you’ll see pirates depicted as wearing an eye patch. Most would assume it’s due to a battle wound (ala the “peg leg”) but, in fact, it’s to quickly go between the brightness of the deck to the darkness of below decks. Keep one eye patched when in brightness, switch the patch to the other eye when you go below and the previously patched pupil is already dilated.

It’s because of a process called phototransduction cascade.

To make it short, a photon (basically, light) causes a reaction, that causes another one, that causes another one etc etc until the point where a your photosensitive cell sends a message to the brain that he’ll use among all the others signalshe receives at the same time to make a picture of what you’re seeing.

Among thoses reactions, at some point, one of them will also start a little loop whose goal is to cause everything to be turned back like it was before the photon came in. But to be slightly slower so that your cell have the time to send a signal.

This last feature, combines with the fact photons dont hit every single of your cells at the exam same moment, make all the signals very slightly de-synchronized.

Now let’s say you’re outside in a summer day, there’s a lot of light, many photons, all your cells are working at full speed, and boom you enter one of thoses dark old school houses, it’s very dark, your eye is already using muscles to reduce the light entrance trough the pupil (because it was bright) and therefore suddenly, a very little amount of photons comes in.

As your photosensitive cells are slightly desync, some of them are still processing the signals from the last moment of huge light, but thoses who are done have, for a big part, not received a new photon. No photon, no signal, no signal, no image. And that’s why your vision quality is hindered, until the flow of signals evens up and the pupil adjust the photon entry.

SImilarly, the same system is at fault for when oyu’ll go outside again that day: multiple cells were resting due to having no photons and your brain was processing images with the other ones, and all of a sudden, BOOM, photons everywhere, all your cells go crazy and send a signal, but because most of them were at rest before, they’re in the first loop of the cascade, so they’re way more in sync. Your brain get’s an absurdly high amount of signals and you see nothing but brightness until your eye muscles and the sync fixed it.