Cycling sunglasses are as important for mountain biking as they are for road cycling, if not more so. MTB sunglasses offer protection from debris that gets flicked up from the trails or falls from trees above. Mud, water, stones and twigs all pose a threat to your vision when riding off-road, especially if you're following a friend. A robust pair of sunglasses will ensure your eyesight is protected at all times.
When riding through forests you'll often find yourself cycling in and out of tree-cover. This causes a big change in lighting which your eyes can struggle adjusting to. The best cycling sunglasses come with lenses that control light transmission as well as offering UV protection.