Why Your Eyes Need Time in the Dark

When you walk from a sunny street into a dark cinema, you may not see the seats. After some time, you can see more. The room is not brighter. Your eyes can now work better with less light.

One change happens in the pupil, the dark opening at the front of each eye. It gets smaller in bright light and wider in the dark. A wider pupil lets more light into your eye. This change is quite quick, but other changes take longer.

At the back of your eye are cells, very small living parts of your body. Some help you see colours and small details in bright light. Others work much better in low light, but they cannot help you see colours. This is why a colourful garden can look grey at night.

After bright light, the cells that work in low light need time to work well again. This can take many minutes. It can take longer after strong sunlight than after time in a room with low light. A bright light can also stop your eyes from getting used to the dark for a while.

Even when your eyes are ready, they still need some light. You cannot see in complete darkness.