How do we get used to the dark?

Anoushka Sharp

First thing in the morning, it’s nice and dark in your bedroom and you suddenly turn on the lights. It is so bright that it hurts your eyes and you can’t see anything for a short while.

16 hours later, you turn off the light in your room. It’s literally pitch black, but after a few minutes of lying awake in bed, you can see better and it’s not so dark.

Fascinatingly, both events are caused by exactly the same thing and in 500 words time, you will know what it is.


How does vision work?

Light enters the pupil and travels to the retina, which is covered in cells called photoreceptors connected to branches of the optic nerve. When light hits a photoreceptor, a signal is sent down the optic nerve, and the brain senses light. The brain is very clever because it can work out what the world looks like based on which parts of the retina got hit by light rays.

Did you know you’re colour blind in the dark?
Rods and Cones are the two types of photoreceptors. Cones see colour but they only respond to bright light. Rods do not see colour, but they send signals down the optic nerve even if the light shining on them is weak. This means that in dim light, you can see shapes but not colours. Turn off the light and see for yourself!


What do you mean, “cones see colour”?
There are three different types of cone called Red, Green and Blue. They are very fussy. The Red cone only sends a signal down its optic nerve branch if it gets hit by red light. Same idea for Green and Blue. We can see zillions of colours – each a different combination of red, green and blue light. Unfortunately, some people have cones that either don’t work properly or aren’t there at all, so they are colour blind.


How do photoreceptors create a signal?
There are two proteins inside photoreceptor cells called Opsin and Chromophore. In the dark, these proteins are tightly attached to each other. When light shines on the photoreceptor, Opsin and Chromophore move apart – they have just seen just how repulsively ugly each other are in the light and want to get away ASAP! This moving apart process makes a signal in the optic nerve.

If Opsin and Chromophore have only just detached, they can’t detach again!

Once they have detached, Opsin and Chromophore take about a minute to re-attach again. This happens naturally, regardless of the light level (the couple make up…. they loved each other really!). Once Opsin and Chromophore are back together, light can make them move apart again. Therefore, Opsins and Chromophores in photoreceptors will continuously be attaching and detaching if it’s bright. A larger percentage of the Rods will have their Opsins and Chromophores detached if the environment is brighter.


So why do we need to ‘get used to the dark’?

When it was bright, the Opsins and Chromophores detached in most of your rods. Therefore, when it suddenly gets dark, these rods can’t send signals. The brain receives minimal optic nerve signal and perceives the environment as pitch black. Eventually, once enough Chromophores and Opsins have re-attached, there is more opportunity for them to detach to create more optic nerve signals. Thank goodness, we can see again – even if the darkness means it’s only shades of grey!


And why do we need to ‘get used to the light’?

When it was dark, most of your rods had their Opsins and Chromophores tightly attached. When a bright light is turned on, ALL of the rods will suddenly get their Opsin and Chromophore to detach. This overwhelms the brain with a HUGE load of signals from the optic nerve. WAY TOO MUCH to process! So the brain just shuts down for a bit and does not see anything. Eventually, once enough Chromophores and Opsins have re-attached, there is more of a balance between how many couples are detaching (telling the brain stuff) and re-attaching (being nice and quiet).


In conclusion

It’s time for a really cheesy pun about how I hope I’ve ‘shined some light’ on getting used to the dark. Thank you for reading!