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.
Frequently asked questions
Do you need sunscreen indoors?
If you sit near windows during daylight — yes. Standard window glass blocks UVB (the burning rays) but lets most UVA through, and UVA is the wavelength that drives photoaging and pigment. Deep in a windowless office, sunscreen is optional.
Does UVA really pass through windows?
Yes — ordinary glass transmits the majority of UVA. It is why long-term drivers show more aging on their window side, a pattern documented in dermatology case studies.
Do phone and laptop screens damage skin?
Screens emit visible blue light at intensities far below sunlight. Current evidence suggests minimal skin impact from screens themselves — the sun remains the overwhelming source of skin-damaging light. Prioritize window-side UVA, not your laptop.
How much sunscreen should I use on my face?
About a quarter teaspoon for face alone (two finger-lengths), or half a teaspoon including neck and ears. Most people apply a third of the tested dose, which slashes the real SPF dramatically.