Using your phone in the dark? Study reveals what a too-bright screen could do to your eyes

The last message of the day usually gets read with the lights off. Then one more article, a few more messages, maybe a video. The phone is inches from your face, the room is dark, and the screen is the only light available there.

A study has examined how different lighting conditions and screen brightness levels influence the eyes during smartphone reading, finding the most noticeable changes when participants used bright screens in dark surroundings.

Published online in October 2025 in Contact Lens and Anterior Eye, the study examined tear film stability, visual fatigue and blinking patterns during mobile phone use. Its findings focus on a specific viewing habit rather than suggesting that all smartphone use is harmful.

Researchers tested four combinations of light

The study was led by Meng Lin and colleagues affiliated with Wenzhou Medical University in China. A total of 30 young adults took part in the experiment. Each participant was asked to read on a mobile phone under four conditions: a bright room with a bright screen, a dark room with a dark screen, a bright room with a dark screen, and a dark room with a bright screen.

Each session involved a 30-minute mobile phone reading task. Researchers measured tear film indicators before and after reading, recorded blinking patterns using an infrared camera, and assessed participants’ reported discomfort and visual fatigue.

According to the paper, tear film stability changed significantly after reading under all four conditions. The most pronounced changes in the measured indicators occurred when participants used a bright screen in a dark room. This setup was also associated with a higher rate of incomplete blinking and greater reported visual fatigue than reading on a bright screen in a bright room.

However, this does not mean everyone’s eyes will react in the same way. The findings reflect how this small group of participants responded under controlled conditions.

Why blinking matters when you stare at a screen

A blink is more than just a reflex. Each complete blink spreads tears across the surface of the eye, keeping it lubricated and helping maintain its optical quality.

When the eyelids do not close fully, the tear film may not spread evenly across the eye’s surface. Changes in tear film stability can be associated with discomfort and symptoms such as dryness and tired eyes.

The researchers recorded incomplete blinks alongside participants’ reported symptoms. The rate of incomplete blinking increased over time. The dark-room, bright-screen condition also produced a higher rate of incomplete blinking and a higher partial blink ratio than the bright-room, bright-screen condition.

The researchers also found a positive correlation between the frequency of incomplete blinking and the level of visual fatigue reported by participants. However, this is an association observed within the study and does not prove that incomplete blinking alone caused the discomfort.

What the study does not prove

Thirty people cannot tell us how others will respond, whether they are older, have different eye health or use screens differently. The experiment also looked at changes around one short reading session, not months or years of nighttime phone use.

So it can’t show that reading on a phone in the dark causes permanent eye damage or chronic dry eye disease. Nor does it show that any brightness setting is safe for everyone.

What it offers is narrower. Room light and screen brightness together can affect short-term eye surface measurements and comfort. That’s a more precise takeaway than blaming screen time for every eye complaint.

A small change to a familiar routine

If you read on your phone before bed, there is no need to panic over an occasional late-night scroll. The key is to avoid using your phone in conditions that may cause unnecessary eye discomfort.

Avoid reading in a completely dark room with your screen brightness set too high, and take regular breaks during prolonged screen use. If you experience persistent dryness, eye pain or blurred vision, consult an eye care professional rather than assuming your phone is the only cause.

A 30-minute experiment cannot tell us what years of late-night scrolling might do to the eyes. Still, if your phone is the last thing you look at before sleep, it is worth paying attention to both your screen brightness and the lighting around you.

Disclaimer: This article is based on published research and is intended for informational purposes only. The findings do not replace professional medical advice. Anyone experiencing persistent eye discomfort, dryness, pain or blurred vision should consult a qualified eye care professional.