Screens versus paper books before bed

What we know

Many of us look at a phone or tablet right before bed. The research team behind this study cites a US survey in which 90% of 1,508 adults used some kind of electronic device within an hour of bedtime at least a few nights a week.

A team from a Harvard-affiliated hospital (Chang and colleagues, 2015) tested what this habit does to sleep under carefully controlled conditions. Twelve adults stayed in a hospital research unit. For several hours before bed, they spent one stretch of days reading on a light-emitting e-reader (an iPad) and another stretch reading a printed book. The order was randomized, and each person was compared with themselves (a randomized crossover trial).

Compared with the printed book, reading on the glowing screen led to:

• taking about 10 minutes longer to fall asleep
• feeling less sleepy in the evening
• lower levels of melatonin, the hormone that helps bring on sleep
• a later body clock
• lower alertness the next morning

Earlier lab studies had already shown that bluish light from screens signals “it’s still daytime” to the body. This study showed that the same thing happens under conditions close to ordinary bedtime reading.

How to read it

This is an experiment designed to test cause and effect. Because the same people tried both conditions in random order, we can say that reading on the screen delayed their sleep.

There are limits, though.

• Only 12 people took part. The direction of the result is clear, but the exact size of the effect is uncertain.
• It took place in a research unit. Light and daily routines were controlled, which is not how life at home works.
• It compared reading. Social media or videos may add other effects, such as excitement from the content, on top of the light. This experiment did not test that.

In Japan, sleep data from about 80,000 workers studied by the University of Tsukuba showed that people who took longer to fall asleep tended to have lower work productivity. That is a correlational finding, but it suggests that small nightly delays can add up.

One more note: a well-known study claimed that just having your smartphone nearby makes you think less clearly. A preregistered replication did not reproduce it. What UPSHIFT can say is narrower: glowing screens before bed can delay sleep.

A small step for today

• Log “put my phone down an hour before bed”. Leaving it outside the bedroom makes this easier to keep up.
• Decide what you will do instead. A printed book, some stretching, or writing tomorrow’s plan: pick one thing without a screen.
• If you must use a screen, dim it. The experiment pointed to light as the problem. It did not test dimming, but dimming does reduce the light you take in.

Track it for about a week and see whether falling asleep or waking up feels different, in your own data. Your own log will not prove anything scientifically, but it will show you what fits your evenings.

Sources

• [B4] Chang et al. (2015) Evening use of light-emitting eReaders negatively affects sleep, circadian timing, and next-morning alertness. PNAS.
• [B3] Seol, Iwagami & Yanagisawa (2025) Association of Sleep Patterns Assessed by a Smartphone Application with Work Productivity Loss Among Japanese Employees. npj Digital Medicine.
• [B6] Ruiz Pardo & Minda (2022) Reexamining the “brain drain” effect: A replication of Ward et al. (2017). Acta Psychologica.