Adjusting to natural light
In a sense, web design can be seen as a light acupuncture against the rods and cones of the user's eyes. (Just a weird opinion.) Every web designer that spends a lot of time in front of a monitor is effectively in a relationship with the light it is emitting. It can affect our mood, it can cause headaches in case of overdose, it can prevent us from easily falling asleep at night. We seem to be more sensitive to brightness than to contrast. In low-light surroundings, the brightness of a monitor suddenly starts to feel much stronger and stimulating. Light intensity affects how we perceive colors too, which is why in such conditions they will feel more saturated. Even if we carefully plan the look of a website, we can't be certain that the user will be exposed to the same lightning conditions. It's more likely that he/she will perceive things slightly differently.