Quick Question: How Often Should I Reapply Sunscreen If I Don't Go Outside?

Apply your sunscreen every two hours, the experts say. Regardless of the ingredients (like extra antioxidants), SPF value, or thickness of the formula, you need to slather on additional layers to sun-exposed skin, full stop. 

"A lot of people think you have to reapply after being in the sun for two hours, when really it's just two hours after you've put it on, whether you've been exposed to the sun or not," says board-certified dermatologist Angelo Landriscina, M.D., in a recent TikTok video. That's because it's not the sun itself that degrades the SPF overtime—it's your sebum and sweat that breaks down the formula.

"Sunscreens need to form a film over the skin in order to work," says Landriscina. (Physical options manually block UV rays, while chemical formulas absorb the rays and transfer them into heat, but both provide some sort of barrier.) "That film is disrupted by the oils your skin makes, and just by normal wear and tear and moving around." So even if you aren't noticeably sweating your sunscreen off, your natural sebum levels do break down the formula throughout the day—and, thus, an extra layer becomes necessary. 

You may be thinking: But if I'm indoors all day, why do I still need to reapply? Theoretically, if you remain inside, blinds drawn, with zero exposure to sunlight, you don't. (But this, for what it's worth, isn't something we'd recommend. Spending time outdoors in natural sunlight has tons of health benefits, namely some much-needed vitamin D, stress management1, and stabilized melatonin levels. Of course, we do understand there may be days when you don't step out the door, but the thought of reapplying sunscreen shouldn't keep you cooped up inside.)

And remember: Even if you don't actually venture outside, most of us are still exposed to the sun's rays through bright, sunny windows—and even on overcast days. So when your skin's oils disrupt your sunscreen as the day rolls on, you don't want to leave it unprotected from any light filtering in.