Screen time and your brain

Attention and working memory

The clearest finding is also the strangest: the phone doesn't have to be on to cost you. In 2017, researchers at the University of Texas ran two experiments with almost 800 participants. People did the same tasks with their phone on the desk, in a pocket or bag, or in another room. Those with the phone on the desk did worst on working memory and fluid intelligence, and those with it in another room did best, even though nobody used the phone and most said they hadn't thought about it. The authors called it "brain drain": some part of your attention is spent on not checking the phone.¹

Then there is how long we stay on anything. Gloria Mark at the University of California, Irvine, has timed people's attention on screens since 2004 using logging software, not self-report. In 2004 the average stretch on one screen was two and a half minutes. By 2012 it was 75 seconds. In her most recent measurements it was 47 seconds, and half of all stretches were under 40.¹³ Each switch has a cost: it takes time to reload where you were, and the residue of the last task stays with you.

Heavy media multitasking, meaning using several streams at once, is consistently associated with worse performance on tests of sustained attention and working memory. The direction of cause is still argued over, but a 2018 review in PNAS concluded the association is reliable across studies.¹⁶

Sleep

This is where the evidence is strongest and least disputed. A 2016 meta-analysis in JAMA Pediatrics pooled 20 studies covering 125,198 children and teenagers. Using a device at bedtime roughly doubled the odds of getting too little sleep (odds ratio 2.17), raised the odds of poor sleep quality by about half, and more than doubled the odds of daytime sleepiness. Simply having the device in the bedroom, unused, still raised all three.²

Two things are happening. The light matters: a tightly controlled study at Harvard had people read either a printed book or a light-emitting tablet before bed for five nights each. On tablet nights it took longer to fall asleep, melatonin was suppressed, the body clock drifted later, and people were less alert the next morning.¹⁴ The content matters more. A feed has no end, and every swipe might be the one worth staying up for.

Mood and wellbeing

Large surveys find a dose-response pattern. Across 40,337 American children and teenagers, wellbeing was similar for up to an hour a day of screens and then dropped with each additional hour: less curiosity, less self-control, more distractibility, more difficulty making friends. High users were twice as likely as low users to have been diagnosed with anxiety or depression. The association was larger in teenagers than in younger children.⁶ Surveys can't prove cause, so the interesting studies are the ones that change behaviour and watch what happens.

Two randomised trials did exactly that. At the University of Pennsylvania, 143 students were assigned to either use social media as usual or limit it to 10 minutes per platform per day for three weeks. The limited group reported significantly less loneliness and depression.⁴ In Germany, 642 adults were split into four groups; the group that cut social media by 30 minutes a day for two weeks showed fewer depressive symptoms and more life satisfaction, with the effects still present six months later. A group that added 30 minutes of exercise instead did about as well, and the group that did both did best.³

Phones also change the quality of time we spend with people. In a field experiment in a restaurant, groups asked to keep their phones on the table enjoyed the meal measurably less than groups asked to put them away, and reported more distraction and boredom.²¹

Children's developing brains

This is the area with the most alarming headlines and the least settled science, so it's worth being precise. A 2019 study in JAMA Pediatrics scanned 47 healthy three- to five-year-olds with diffusion tensor imaging. Children with more screen use, measured against the American Academy of Pediatrics' guidance, had lower measures of white-matter integrity in tracts that support language and emerging literacy, and lower scores on language tests.⁵ It's a small, cross-sectional study: it shows an association at one moment, not that screens caused the difference.

The larger picture comes from the Adolescent Brain Cognitive Development study, which is following more than 11,000 American children. Early analyses found that screen media activity is associated with differences in cortical structure, and those patterns partly overlap with the ones associated with lower academic performance. Again, the direction of cause is not yet known.²²

The honest summary: children who spend a lot of time on screens differ, on average, in brain structure and in language and attention measures. Whether screens cause this, or whether the same households and temperaments lead to both, is still being worked out. Sleep displacement, which is well established, is likely part of the mechanism.

When use becomes a problem

"Problematic smartphone use" is the research term for use that feels out of control and gets in the way of life. A 2019 meta-analysis of 41 studies covering more than 40,000 children and young people estimated that roughly one in four met a threshold for it, and that problematic use was associated with higher odds of depression, anxiety, stress and poor sleep.⁷ It isn't a formal diagnosis, and the thresholds vary between questionnaires, but the pattern is consistent: the harm tracks loss of control more than raw hours.

A 2019 review in World Psychiatry put the mechanism plainly. The internet is unusually good at capturing attention because it offers a constant stream of small, unpredictable rewards, and it lets us outsource memory to search, which changes how we store what we learn.¹⁵

What actually helps

Most of the tools people reach for are untested. A few have been tested and work.

• Take the colour away. In a randomised trial of 161 university students, phones switched to greyscale for a week cut daily screen time by about 38 minutes, mostly from social media and browsing. Video was unaffected.⁸ Offly's grey screen is this, per app or after a set hour.
• Add a pause with an exit. A six-week field study of the app one sec, with 280 users, found a short breathing pause before a chosen app opened led people to abandon 36% of opens and to attempt opening those apps 37% less often.⁹ The pause has to be escapable. Hard blocks get uninstalled.
• Put a number on it. Both the Pennsylvania and German trials worked with simple daily limits around 30 minutes.^{4, 3}
• Move the phone, not just the app. The brain-drain study suggests the cheapest fix for focused work is distance: another room beats a pocket.¹
• Protect the last hour. Given the sleep evidence, a phone out of the bedroom, or a home screen that goes quiet at a set hour, is the highest-value change most people can make.^{2, 14}

Sources

1. Ward, A. F., Duke, K., Gneezy, A., & Bos, M. W. (2017). Brain drain: the mere presence of one's own smartphone reduces available cognitive capacity. Journal of the Association for Consumer Research, 2(2), 140–154. journals.uchicago.edu
2. Carter, B., et al. (2016). Association between portable screen-based media device access or use and sleep outcomes: a systematic review and meta-analysis. JAMA Pediatrics, 170(12), 1202–1208. jamanetwork.com
3. Brailovskaia, J., et al. (2022). Experimental longitudinal evidence for causal role of social media use and physical activity in COVID-19 burden and mental health. Journal of Public Health. ncbi.nlm.nih.gov
4. Hunt, M. G., Marx, R., Lipson, C., & Young, J. (2018). No more FOMO: limiting social media decreases loneliness and depression. Journal of Social and Clinical Psychology, 37(10), 751–768. guilfordjournals.com
5. Hutton, J. S., et al. (2020). Associations between screen-based media use and brain white matter integrity in preschool-aged children. JAMA Pediatrics, 174(1). jamanetwork.com
6. Twenge, J. M., & Campbell, W. K. (2018). Associations between screen time and lower psychological well-being among children and adolescents. Preventive Medicine Reports, 12, 271–283. sciencedirect.com
7. Sohn, S. Y., et al. (2019). Prevalence of problematic smartphone usage and associated mental health outcomes amongst children and young people. BMC Psychiatry, 19, 356. springer.com
8. Holte, A. J., & Ferraro, F. R. (2020). True colors: grayscale setting reduces screen time in college students. The Social Science Journal. tandfonline.com
9. Grüning, D. J., Riedel, F., & Lorenz-Spreen, P. (2023). Directing smartphone use through the self-nudge app one sec. PNAS, 120(8). pnas.org
10. Mark, G. (2023). Attention Span. Hanover Square Press. Interview summary at apa.org
11. Chang, A.-M., Aeschbach, D., Duffy, J. F., & Czeisler, C. A. (2015). Evening use of light-emitting eReaders negatively affects sleep, circadian timing, and next-morning alertness. PNAS, 112(4), 1232–1237. pnas.org
12. Firth, J., et al. (2019). The "online brain": how the Internet may be changing our cognition. World Psychiatry, 18(2), 119–129. wiley.com
13. Uncapher, M. R., & Wagner, A. D. (2018). Minds and brains of media multitaskers: current findings and future directions. PNAS, 115(40), 9889–9896. pnas.org
14. Dwyer, R. J., Kushlev, K., & Dunn, E. W. (2018). Smartphone use undermines enjoyment of face-to-face social interactions. Journal of Experimental Social Psychology, 78, 233–239.
15. Small, G. W., et al. (2020). Brain health consequences of digital technology use. Dialogues in Clinical Neuroscience, 22(2), 179–187. tandfonline.com

This article is for general information and is not medical advice.