Question: why do we struggle to see in the dark?

Pete Etchells answered on 20 Jun 2012:

Great question, katybrightwell! There are two different types of light receptors in our eyes; one set (called cones, because of their shape) work in relatively bright light (like daylight) and are responsible for colour vision. The other type, called rods (again because of their shape) are much more sensitive to light, and so are responsible for our night vision. However, while we have three different types of cone, we have only one type of rod. This basically means that at night, we can’t see in colour (as you would need to compare the responses of two different types of rod cell in order to tell a colour difference, as you do with cone cells in daytime). Moreover, while cones are densely clustered in the central region of our vision and so give us a high resolution image there, rods are located further out in the periphery of our vision. All of these things together mean that our night vision, while alright, is not good compared to other animals, particularly nocturnal ones.