Ageing is the most common reason, but diabetes, long‑term steroid treatment and previous eye injury can speed things up. Ultraviolet light exposure and smoking may also contribute, which is why good UV protection and healthy habits are helpful.

For the technically minded: Reactive oxygen species drive protein denaturation and pigment deposition within the lens, leading to yellow–brown discolouration (brunescent change). Disruption of lens epithelial cell metabolism impairs antioxidant defences such as glutathione, accelerating protein cross‑linking. These biochemical changes underpin nuclear sclerosis, cortical opacities and posterior subcapsular cataract patterns.