Sunglasses stores and fashion spreads may have us thinking differently, but sunglasses aren't just a stylish accessory. They can be a stylish accessory as well – and there are some incredible designs out there – but first and foremost they need to protect your eyes. That means they should be tested and certified. Look for this product information if you're buying online, or look for the sticker or tag if you're buying in a shop.
In Australia, sunglasses are broken down into five categories based on levels of protection – make sure you choose category two or higher. Wearing sunglasses with inadequate protection can be really damaging to your eyes as a dark tint can actually open up the pupils to the glare, without protecting them. To answer your question about when to wear them, it's whenever you feel your eyes are straining in the light. That could be on a sunny or overcast day – if in doubt, pop them on.