You're inside all day — so skipping sunscreen feels logical. But "indoors" isn't one place: a desk by the window and a basement office are different planets, UV-wise. Here's the honest physics of glass, screens, and SPF.
Near windows in daylight → yes, wear SPF. Glass blocks UVB (burning) but passes most UVA — the aging/pigment wavelength. Windowless interior all day → genuinely optional. Screens? Their blue light is a rounding error next to the sun. Dose matters: ¼ teaspoon for the face, or your SPF 50 performs like a 15.
What glass actually blocks
Ordinary window glass stops UVB almost completely — you won't sunburn at your desk. But the majority of UVA passes straight through, and UVA is the deep-penetrating wavelength behind collagen breakdown, pigment, and photoaging. Dermatology's famous evidence: long-haul drivers and office workers with measurably more aging on their window side. Your commute counts too.
Screens: the over-hyped villain
Phone and laptop screens emit visible blue light at a small fraction of the sun's intensity — current research finds minimal skin effect from screens themselves (the sun delivers more blue light through a window in minutes than your monitor does all day). Worry about the window, not the laptop.
The practical rulebook
- Desk near a window / lots of daylight rooms / driving: SPF every morning, as the last skincare step (order guide).
- True interior, no daylight: skipping is defensible — re-apply before going out.
- Dose: ¼ tsp face, ½ tsp with neck/ears. Under-dosing is the #1 silent SPF failure.
- Cloudy days: up to 80% of UV gets through cloud — the sky doesn't need to look sunny.
- Reapply if you're window-side all afternoon or heading out at lunch.
Know your real exposure
The hard part isn't knowing the rules — it's knowing your pattern. Rosee Skin lets you log daily sun exposure (indoor / mild / heavy) in one tap and folds it into your weekly sun report alongside your scan trends, with weather-aware tips for your city. Skin that's photo-protected this decade thanks you next decade — and the data makes the habit stick.