Reading on a tablet before bed pushed people's body clocks back by over 90 minutes
A tightly controlled lab study compared four hours of evening reading on an iPad with the same reading from a printed book. The screen changed melatonin, sleep timing and next-morning alertness.
At a glance
- Reading on an iPad for about four hours before bed cut evening melatonin by about 55% compared with a printed book.
- The body clock's evening melatonin signal started more than 1.5 hours later after five nights of screen reading.
- People took about 10 minutes longer to fall asleep, had slightly less REM sleep and felt sleepier the next morning.
- Study type
- Randomised crossover trial, 14 days living in a sleep lab
- Who took part
- 12 healthy adults, average age 25, six women
- Published
- Proceedings of the National Academy of Sciences (PNAS), 2015
- Strength of evidence
- Early evidence A well-controlled experiment, but only 12 young people in lab conditions. It shows what can happen, not how big the effect is in everyday life.
What the researchers did
Twelve healthy young adults lived in a sleep laboratory for two weeks, so researchers controlled their light, meals and bedtimes.
For five evenings in a row, each person read for about four hours before bed on an iPad set to full brightness. For another five evenings they read printed books under very dim light. The order was randomised, so everyone did both.
The researchers measured melatonin in blood samples, recorded sleep with brain-wave monitoring, and asked about sleepiness at night and the next morning.
What they found
After reading on the iPad, evening melatonin was about 55% lower than after reading the printed book. Melatonin is the hormone that tells the body it's night.
The timing of the body clock moved too. The evening rise in melatonin came more than an hour and a half later after the screen evenings.
People took about 26 minutes to fall asleep after the iPad, against about 16 minutes after the book. They also had around 12 minutes less REM sleep, felt less sleepy in the evening, and felt sleepier the next morning.
The caveats
- Twelve people is a small group, and all were in their twenties. Eyes let through less blue light as they age, so the effect may be different in older adults. This study can't tell us.
- Four hours at full brightness is more than most people read. Shorter or dimmer screen use may have a smaller effect.
- The tablet was compared with a printed book under very dim light, not with a normally lit room. At home, room lighting also plays a part.
- Devices now have night modes that warm the screen colour. These weren't tested here, and at least one later study found night mode on its own made little difference to sleep.
What this could mean for you
If you read in bed, a printed book or an e-ink reader without a bright backlight is the lower-light choice.
If you do use a phone or tablet late, turning the brightness down and holding it further away reduces how much light reaches your eyes.
Ten minutes longer to fall asleep is modest, but a body clock that drifts later night after night can make early alarms harder. Bright light in the morning helps pull it back.
The study
Chang AM, Aeschbach D, Duffy JF, Czeisler CA. Evening use of light-emitting eReaders negatively affects sleep, circadian timing, and next-morning alertness. Proceedings of the National Academy of Sciences (PNAS), 2015; 112(4):1232-1237.
Read the paper (DOI 10.1073/pnas.1418490112) · PubMed
Declared interests: One author declared consulting fees, equity holdings and research funding from a range of companies, including Apple and Microsoft shares held in 2012 to 2013. The full statement is in the paper.