Does sunlight through a window count?

How much light does window glass let through?

Ordinary clear glass holds back only a little light. In studies collected in a 2023 review, clear glass let through about 91% of visible light and a window with two clear panes about 80%. Triple glazing with coatings let through 46%, and one tinted double pane only 25% (Ghaeili Ardabili et al., 2023).

So with a clear window, the glass explains only a small part of the gap between indoors and outdoors.

Why is sunlight through a window weaker than outdoors?

Indoors, daylight has to come in through the window opening. How much of it reaches your eyes drops the further you sit from the window, and you get the most when you look toward it (Ghaeili Ardabili et al., 2023).

A 2026 study measured 671 indoor and outdoor scenes. Melanopic light, the part of light weighted for the eye’s main input to the body clock, averaged 280 lux in rooms without a window view, 2,386 lux in rooms with one and 12,142 lux outdoors. Measured as ordinary brightness, outdoor scenes were about five times brighter than indoor scenes with a window and 34 times brighter than rooms without one. Scenes with a window varied a lot (Tabandeh & Spitschan, 2026).

Indoor lighting guidelines also tend to keep rooms at least ten times below the natural light level outdoors (Brown et al., 2022).

How much daylight is in your day? Take the 1-minute light check.

Does window glass block UV light?

Window glass filters out UVB and lets UVA and visible light through (Tuchinda et al., 2006). How much UVA gets through depends on the type of glass (Almutawa et al., 2013).

For your daily rhythm, UV is not what you are after. A 2026 policy brief puts it plainly: the goal is visible light at the eye during the day, not unsafe UV exposure (Piper et al., 2026).

How to get more light in the morning

Indoors during the day, experts recommend at least 250 lux of melanopic light at eye level and suggest using daylight first where it is available (Brown et al., 2022). A seat by the window is a simple place to start. In the morning you have two easy options:

• Step outside, even briefly. A 2026 policy brief names morning time outdoors or a lunchtime walk as practical examples for people who work indoors (Piper et al., 2026). LightOtter counts a day from 2 minutes outside.
• If you stay in, open the window and sit right next to it, facing out. Light at the eye drops with distance from the window, and looking toward it gives you the most (Ghaeili Ardabili et al., 2023).

See how many minutes outside fit today’s weather in your city.

Check the light instead of guessing

The LightOtter app has a brightness check that uses your phone’s back camera to estimate brightness. It is an estimate, good enough to tell indoor light from outdoor daylight, not a calibrated meter. Point the camera at the sky, never at the sun.

More on lux and how bright daylight gets.

What this is not

This guide is about light, not a medical assessment. Research gives general guidance rather than one dose for everyone (Piper et al., 2026), so there is no figure for how long at a window equals a few minutes outside. LightOtter’s minutes are a rule of thumb for time outdoors, not a measurement. If you feel tired for a long time, talk to a doctor. Never look directly at the sun.