Most sunscreen bottles will tell you to reapply your sunscreen every two hours. These instructions are generally correct, but they lack some nuance. With modern sunscreens, the SPF remains quite stable for a very long time. The reason you need to reapply is simply that the sunscreen wears away throughout the day, especially if you’re out and about being active. In day-to-day life, most people can apply their sunscreen just once in the morning, and it’ll keep them protected when they take their lunch break or drive home from the office. However, if you’re on the beach, hiking, or participating in a marathon, sweat, water exposure, and constant wiping remove the sunscreen from the skin. In that case, it’s important to reapply sunscreen more often – every two hours if you’re just participating in light activity, or every hour if you’re constantly in and out of the water or sweating profusely.