Why does it take eyes so long to adjust to the dark?

[uglybeech]

I know the pupil has to dilate in order to let more light in, but that takes less second. So why is it when I switch off the light at night it takes at least a minute or more before I can make out shapes in the bedroom?

Anybody know what explains the lag?

[The Tim]

Part of the answer is that the cells responsible for night vision are fatigued by exposure to normal light conditions. Certain frequencies of light (those that are seen as red) don't provoke much of a reaction from rods and as a result going from red light to low light involves faster acclimation than going from normal (white) light to low light.

[Joe Random]

Originally Posted by uglybeech

I know the pupil has to dilate in order to let more light in, but that takes less second. So why is it when I switch off the light at night it takes at least a minute or more before I can make out shapes in the bedroom?

Because dilated pupils are only part of the equation. The rod cells in the retina (the ones that are sensitive to low-light conditions) sense light because of a pigment called rhodopsin (a.k.a. visual purple). Rhodopsin is broken down by exposure to light (which is how it lets us detect light). However, when the light level is very high, all of the rhodopsin is bleached out of the rods, making them useless. The time needed to adapt to night vision is generally the same as the time the retina needs to synthesize enough rhodopsin for the rods to start functioning.

Here's a useful link to the appropriate section of Howstuffworks.com.

[DocCathode]

IIRC From a few documentaries, the opposite is true when it comes to sharks' eyes. They adapt very quickly to a darker environment but very slowly when moving to a brighter one.

[CC]

And, when your pupils dilate, they don't dilate 100% of the way. There's more accomodation to be done. Takes time.

[astro]

Also evolution. I would imagine there are fairly few pre-fire, pre-historical circumstances where things just went dark immediately, so there was not need to adjust to an "instant off" situation.

[Excalibre]

Does that mean the cones don't use rhodopsin? What do they use? And if the rhodopsin is normally in its 'used' form during daytime vision (I forget what it breaks into) and takes time to recombine, what's with the pain of stepping from dark areas into sunlight? Why is our eyes' return to their normal state so unpleasant?

[Shalmanese]

Originally Posted by astro

Also evolution. I would imagine there are fairly few pre-fire, pre-historical circumstances where things just went dark immediately, so there was not need to adjust to an "instant off" situation.

I disagree, walking into a cave or even moving into the shade of a tree can significantly impair your vision for up to 20 seconds.

[DocCathode]

Originally Posted by astro

Also evolution. I would imagine there are fairly few pre-fire, pre-historical circumstances where things just went dark immediately, so there was not need to adjust to an "instant off" situation.

I agree with Shalmaneese. Pre-fire, what were the instances of sudden light?

BTW- I wasn't trying to just throw in something tangenitally related to the subject with the shark comment. I'm genuinely curious as to our eyes evolved one way and shark eyes respond in the opposite manner. I'm also curious as to the differences that enable both mechanisms to work.

[Joe Random]

Originally Posted by Excalibre

Does that mean the cones don't use rhodopsin? What do they use?

From Howstuffworks.com:

Rhodopsin is a mixture of a protein called scotopsin and 11-cis-retinal -- the latter is derived from vitamin A (which is why a lack of vitamin A causes vision problems).

From another page at the same site:

The color-responsive chemicals in the cones are called cone pigments and are very similar to the chemicals in the rods. The retinal portion of the chemical is the same, however the scotopsin is replaced with photopsins. Therefore, the color-responsive pigments are made of retinal and photopsins. There are three kinds of color-sensitive pigments:

• Red-sensitive pigment
• Green-sensitive pigment
• Blue-sensitive pigment

Originally Posted by Excalibre

And if the rhodopsin is normally in its 'used' form during daytime vision (I forget what it breaks into) and takes time to recombine, what's with the pain of stepping from dark areas into sunlight?

My semi-informed guess would be that the pain caused by going from relative darkness into sudden light is caused, in part, by the sudden, simultaneous breakdown of large amounts of rhodopsin in your retina causing over-stimulation of the rods.

In the case of going from a relatively well-lit area into a very bright area, I'd imagine it would be the same situation, just with color pigment breakdown causing over-stimulation of the cones.

[herownself]

One of the few medical fact I have retained from my job almost 15 years ago working in a famous hospital in the opthalmology research labs is how much variation there is in how much time it takes for people to "dark-adapt."

I, myself, dark-adapt very quickly. For most people, full dark-adaptation occurs within (IIRC) about 20 minutes of sitting in full darkness. I remember one unusual patient who took more than 24 hours to dark-adapt, poor kid.

For what it's worth, certain visual exams (of the retina) can only be performed when the eye is fully dialated, which is why I know about the variation in the human population in time to full pupil dilation.

Why does this stuff never come up at trivia night?

[dtilque]

I understand that the speed of dark adaption is a function of age. That is, as you grow older, it takes longer and longer for the eyes to adapt to darkness. Also, the eyes don't expand as much when you're old as when you were young. I believe this is mostly due to the iris losing flexibility as you age.