What matters is not the darkness of the lenses but their ability to protect eyes against UV rays. Invest in a pair of good quality sunglasses, rather than cosmetic glasses that may offer no eye protection at all. Look for those carrying the CE mark and the British Standard BS EN 1836:2005. This ensures that they offer a good degree of UV protection. Ideally they should fit close to the face and wrap around.
It’s important because up to 1 in 10 skin cancers affect the eyelid area. Sun also increases the risk of cataracts, macular degeneration, sunburn of the cornea (keratitis), and eye cancers.