When I was growing up in the cloudy north of England, we didn't often need sunglasses. But now I live in the UAE, a land of almost perennial brightness where sunglasses are really a necessity.
But they are also more than a practical accessory: perhaps more than any other item of apparel, sunglasses have a psychological story to tell.
Part fashion accessory, part eye protectors, part disguise - sunglasses are ubiquitous. Worn equally by men and women, shades can simultaneously say we are cool, mysterious, intimidating, anonymous.
It is no coincidence that designer sunglasses have one of the highest profit margins of any product. Luxottica, the world's largest eyewear company, offers Burberry, Oakley, Ray-Ban and many more sunglass brands; the firm's global sales were over $7 billion (Dh25.7bn) last year.
Our earliest accounts of sunglasses come from 12th century China, where some individuals from among the aristocratic classes reportedly wore glasses with smoky quartz lenses to protect their eyes from the bright sunlight. Such shades were also popular with China's judges, because they hid any emotional response the judge might exhibit while listening to testimony. Justice is blind, but the dark glasses help all the same.
Psychologist Paul Ekman, an authority on facial expressions, says that of the 23 facial expressions relating to human emotion, about one-third involve the eyes. Shades may slightly obscure your view of the world, but they also hide the world's view of how you are feeling.
Not being able to see a person's eyes greatly reduces our ability to infer his or her emotions. For example, telling a fake smile from a real one can be difficult, but if the smiler is wearing shades then detecting the sham smile is almost impossible. In a real smile, a muscle called the orbicularis oculi is activated, creating a hard-to-fake crinkling around the eyes. The eyes truly are the windows to the soul.
Another psychological dimension to sunglasses is their ability to promote the illusion of having a baby-face. Developmental biologists use the word "neoteny" for the retention of juvenile physical features in mature adults. In females this includes: large widely spaced eyes, small chins and small noses. Such neotenous (babyish) features appear to be particularly important in attractiveness ratings. Across cultures, neotenous female faces are judged to be more physically attractive, regardless of actual age.
And large eyes are the most effective cues of neoteny. The outsize sunglasses that are a fashion favourite for women arguably derive their popularity from their ability to signal neoteny, however artificially.
Hiding the face, or part of it, also has a psychological aspect. The psychiatric condition known as body dysmorphic disorder (BDD) is characterised by severe appearance anxiety. This anxiety is always disproportionate, and often the sufferer's appearance has in truth no discernible flaw or defect. For this reason, BDD is often referred to as "imagined ugliness".
A classic BDD behaviour is to attempt to camouflage an imagined physical defect. Anyone suffering BDD in regard to his or her eyes or face will see big sunglasses as a great hiding place. Even those without BDD may use shades to conceal something, be it a black eye or the absence of make-up.
And then some people take semi-delusional comfort from wearing shades. We feel invisible, or less visible, behind our sunglasses. This is of course flawed logic - I can't see you very well, therefore you can't see me clearly either.
On one level we know this is egocentric nonsense, but a small naive part of us still believes in this imagined invisibility.
Finally, there are also a small number of individuals who actually need to wear sunglasses for physical or psychological health reasons. Photophobia is a condition where an individual is particularly negatively affected by bright light.
Such individuals may develop streaming eyes, migraines, or even begin sneezing in response to too much bright light.
Bono, the U2 frontman and celebrity philanthropist, falls into this category. But even if he didn't, we could easily accept his perennial sunglass-wearing as just a standard aspect of the rock-star persona. Sunglasses are so much more than just sunglasses.
Justin Thomas is an associate psychology professor at Zayed University in Abu Dhabi