We know that big screens have a big impact on sleep. But what about smaller screens like smartphones?

Screen size, brightness, and distance from your eye all matter, just like color. So a small screen should look a lot dimmer to your body. But a lot of people, especially kids, like to use phones really close.

So we started our first informal survey to figure this out. And we got about 3,000 responses over two days. We ran the survey using the wonderful, free typeform.

We asked people about the screen they use before sleeping, how big it is, and how close they hold it. We found out that people hold small screens closer. A lot closer.

Light follows the “inverse-square law”, so if you hold your phone twice as far away, it will appear 75% less bright to your eyes. But our median phone user was holding their screen just 8 inches away.

Our analysis suggests that people use screens at a distance that normalizes both screen size and brightness. Our median phone or tablet user is achieving a 35 degree field of view for each device. With larger, fixed desktop displays, that number goes as high as 60 degrees. For comparison, a normal movie theater is a 54 degree field of view, and an IMAX theater is an average of 70 degrees (source: imax.com).

Considering the inverse-square law, that sounds bad enough on its own. But most 6-10” devices are about twice as bright (500 cd/m²) as the big desktop displays (250 cd/m²), so to a first approximation we can say users are cancelling out the beneficial effects of a smaller screen size by holding smaller screens closer.

We don’t really know if it’s the ergonomics of these displays (no keyboard, no desk) that makes people hold them closer, or if it’s just how people like it. But our results show that a lot of people may be getting just as much bright blue light from small screens as they do from large ones.

Please read our tips for using screens more safely at night, and hold those phones a little further away.