A new study puts a number on how much screen time before bed is too much
You probably already know that scrolling through your phone at night isn’t great for sleep. But now researchers have a clearer picture of exactly how much is too much, and what it costs you the next day.
A 2026 study from Qazvin University of Medical Sciences followed 187 university students and tracked how long they used their phones before bed, both with the room lights on and with the lights off. Then the researchers measured the students’ sleep quality, their ability to concentrate and focus, and their reaction speed.
The results were striking. Seven out of 10 students had poor sleep quality. Nearly 40% had below-average reaction times. And the more time students spent on their phones before bed, the worse things got.
What the Study Found
Students used their phones for about 75 minutes before bed with the lights on, and another 38 minutes with the lights off.
That’s nearly two hours of screen time right before sleep.
The researchers found two key thresholds:
Using your phone for more than 60 minutes before bed with the lights on raises your odds of poor sleep by 2.4 times.
Using your phone for more than 30 minutes in the dark raises those odds even further, up to 3.7 times in some analyses.
Students with more screen time before bed also scored lower on concentration tests the next day and had slower reaction times.
Why Your Phone Keeps You Awake
Your brain runs on a roughly 24-hour internal clock called the circadian rhythm. One of its most important jobs is to release melatonin, the hormone that makes you feel sleepy, when it gets dark.
The problem is that your phone emits blue light, and your eyes are extremely sensitive to it. Even low-level blue light signals to your brain that it’s still daytime, which delays melatonin release and pushes your sleep later. This doesn’t just mean you fall asleep later. It means less total sleep, a harder time staying asleep, and more fatigue the next day.
The effect is stronger in the dark. When you use your phone in a dark room, there’s no competing light source, so your eyes absorb the full impact of the screen. That’s why the 30-minute threshold in a dark room is half the limit of using a phone with the lights on.
Beyond the light, there’s a second problem: your phone is stimulating. Social media, messages, videos, and news keep your brain aroused and alert right when it needs to wind down. That mental activation makes it harder to fall asleep even after you put the phone down.
What It Does to Your Brain the Next Day
Sleep isn’t just rest. Your brain uses it to consolidate memories, restore attention, and reset your reflexes.
When you sleep poorly, the effects show up fast. This study found that students who used their phones more before bed performed worse on concentration tasks and had slower reaction times the next morning. Those aren’t abstract findings. Slower reaction time affects driving, sports performance, and any situation where a quick response matters.
Research also shows that poor sleep blunts the prefrontal cortex, the part of your brain that handles focus, decision-making, and impulse control. That’s the part you most need when you’re studying, working, or managing a stressful situation.
A Few Things That Help
You don’t have to throw your phone across the room. Small changes add up.
Set a screen cutoff. Aim to put your phone down at least 60 minutes before bed with the lights on, and 30 minutes before bed if you’re already in the dark. This study suggests those two windows are where the risk of poor sleep rises sharply.
Wear blue-light blocking glasses at night. Alternatively, your phone probably has a night mode or a blue-light filter, but the glasses are much more effective for sleep.
Turn off non-urgent notifications. A buzzing phone doesn’t just interrupt your sleep. It keeps part of your brain on alert even when you’re trying to wind down.
Keep your phone out of the bedroom. If your phone is your alarm clock, a cheap clock radio solves that problem without temptation.
Wind down with something else. Reading, light stretching, or a short conversation are gentler on the brain than a screen.
A Note on the Research
This study was observational, meaning it shows a connection between phone use and poor sleep, but it can’t prove that the phone use caused the problem. Stress, anxiety, and other habits could all play a role. The sample was also drawn from one university in Iran, so the specific thresholds may shift somewhat when tested in other populations.
That said, the findings line up with a large body of research from other countries, including Turkey, South Korea, and China, all pointing in the same direction: more screen time before bed means worse sleep.
The researchers put it plainly. These thresholds are worth taking seriously, even if future studies refine them. For most people, two hours of phone use right before bed is too much, and the dark-room effect makes even 30 minutes a meaningful risk.
Your phone will be there in the morning. Your sleep won’t wait.
—Chris Aiken, MD
Director, Psych Partners
Editor in Chief, Carlat Psychiatry Report
