Your pupils clamp down quickly because very bright light can damage your eyes. Kinda an "emergency lockdown" situation.

They can expand more slowly to be cautious of overdoing it.

Your eyes have to adjust quickly to bright light to prevent damaging them. That’s why going from a dark room into bright sunlight hurts your eyes. Adjusting to darkness takes longer because there’s no I’ll effects to it being slow to your vision.

Your eyes contain 4 different types of light sensors. The first 3 see color and the 4th sees shades of gray.

In order to "respond" to light, the sensors use chemicals that interact with the light color they see. So chemical X responds to Green colored light, chemical y to Red and chemical Z to any light, but only responds in shades of gray.

Long story short, the gray chemical, chemical Z gets destroyed by bright light, any kind of bright light melts it away and your eye needs to build more up before you can see gray-scale.

So if you're in bright light and then suddenly go into the dark, you have no chemical Z and it needs to restore over a short few minutes and then you can see in the dark at least a little bit.

Fun Fact: Red light does NOT destroy chemical Z. This is why military aircraft and vehicles that operate at night use Red lights in the cabins before dumping the soldiers off into a battlefield. You can leave the vehicle with full night vision intact.

Adjusting to bright light is different, a muscle in your eye just squeezes the eye-hole tighter reducing the amount of light that gets through into your eye in the first place. It happens pretty dang quick.

Although there is some truth to what you're saying, the adaptation period is due to the retina rather than the pupil

There are molecules in your eyes that undergo chemical reactions when light hits them. Those chemicals are responsible for the eye's sensitivity to light. It takes a while for them to transform back to their original state. The reason for the difference in speed is because it takes a while to go from daylight to darkness so during that time your eyes would slowly be adjusting to the change in the amount of light until it was completely dark and you went to bed. At which point you'd wake up and it would be mostly light out again and you'd need to be able to see right away.

Other fun military fact, they tell you to decrease the time to gain "unaided night vision" you should close your eyes and shut out as much light as possible. This way they adjust faster and better. Learned it in the Marines and I still do it to this day to adjust to the dark better.

A third fun fact is that the Mythbusters found it plausible that pirate captains could've used eye patches to preserve some darkness sensitivity in one eye, so that they'd be able to see well both above and below deck without having to adjust to the gloom below.

This error be evolutionary good. It’s much more dangerous leaving a cave than entering it (assuming you know there’s nothing dangerous in the cave).

Bomber pilots would close one eye during the explosion so that they could maintain "night vision". I use the same strategy if i need to pee at 3am

After learning this, I tried the trick myself a few times on late night trips to the bathroom. I can navigate my bedroom fine in the dark, and I'd close one eye before turning on the light.

It worked surprisingly great, though it gave me an oddly unbalanced feeling. I'd compare it to vertigo maybe? The eye that I kept closed/covered could still see perfectly fine when I turned off the lights and opened it again. The other eye couldn't see squat. I think it messed with my depth perception a bunch, because I was basically blind in one eye.

Some headlamps have red and green lights — why also green??

I just have a night-light in my bathroom, aimed towards the toilet. That way I never have to turn on the main bathroom light at night.

I do the same thing for the same reason as you. Only difference is that when I turn off the light I close the bright-adjusted eye and rely solely on the dark-adjusted eye. Keeping them both open after turning the lights off messes with my head and lack of depth perception isn’t a huge trouble for a quick trip to the bathroom.

On the handful of occasions where I am in the dark for longer, I will open both eyes after a minute or so.

crazy i found out this by myself, figured out closing my eyes before entering a dark room and opening them a bit was easier to adjust, finally found the reason thanks to reddit randomness 😎

If it ends up being more than a pee, do you also say "bombs away"?

If Gronk go in dark cave not see big teeth of lion.

I don't think I have seen one with green and red, only white (regular) and red.

Not an expert here. I believe certain colors of other lights can also preserve night vision. Low intensity green light does it to I believe.

Most military night vision goggles have a green filter on them so you can use a green light and not have your goggles shut down. Particularly helpful in the cockpit if you're flying with goggles on and need to look at a map or something inside

But why use a green light instead of red?

I'm not entirely sure, but I do know from experience that red light will blow out your goggles (like if Joe infantryman in the back wants to try and read his map in flight). I assume it's something to do with wavelengths but I'm no scientist.

I mean, I am a scientist and I don't know. ¯_(ツ)_/¯ But like you said, maybe it has to do with the goggles being sensitive to infrared and red is close enough in wavelength to infrared that it can overload the gogs.

As long as we're speculating - here's a substantially unsubstantiated web page about it. It says green light at low levels also doesn't mess up night vision. And green can be used at lower brightness, has better contrast and color discrimination, and is better for common tasks like reading maps.