Restscreens and sleepAdults

Reading on a Backlit Screen Before Bed Delays Your Body Clock

A controlled study found that reading on a light-emitting e-reader before bed suppressed melatonin, delayed the circadian clock, and reduced next-morning alertness compared with reading a printed book.

Study title: Reading on a Backlit Screen Before Bed Delays Your Body Clock — Chang, A.-M., Aeschbach, D., Duffy, J. F., & Czeisler, C. A., Proceedings of the National Academy of Sciences, 2015

Study type: Experimental study
Population: 12 healthy young adults, USA
Published: 2015
Evidence confidence: Moderate confidence

The short version

Reading on a backlit device before bed suppressed melatonin and delayed the body clock compared with reading a printed book, in a controlled lab study.

Why it matters

It isn't just what's on the screen that matters for sleep, but the light itself — enough evening light exposure from self-luminous devices can measurably push the body's internal clock later and blunt the sleep-promoting rise in melatonin.

The question

Does reading on a light-emitting e-reader before bed, compared with reading a printed book, affect melatonin levels, circadian timing, sleep, and next-morning alertness?

How it was studied

In a within-subjects laboratory study, participants spent five consecutive evenings reading either on a light-emitting e-reader or a printed book for four hours before bed, under tightly controlled lighting conditions, with the two conditions completed in counterbalanced order weeks apart. Researchers measured melatonin levels, sleep timing and architecture, circadian phase, and next-morning alertness and cognition.

What they found

Compared with reading a printed book, reading on the e-reader suppressed evening melatonin secretion, delayed the timing of the circadian clock, took longer to fall asleep, reduced REM sleep, and left participants feeling less alert and more sleepy the next morning, despite having the same time in bed.

Limitations

The sample was small (12 participants) and limited to healthy young adults in a highly controlled lab setting reading for a longer duration (four hours) than most people spend on devices before bed, so the size of the effect may not directly translate to typical everyday phone or tablet use.

What it may mean

In real life

In real life that might mean the last hour before bed spent on a bright phone or tablet is doing more than just delaying the moment you close your eyes — it may be nudging your whole internal clock later, making the next morning harder regardless of how many hours you actually slept.

Try this

Try switching to a printed book, an e-ink reader without backlighting, or a dimmed device with night-mode filtering for the last hour before bed, and notice whether falling asleep feels easier.

Evidence confidence

Moderate confidence

This is a rigorous, tightly controlled experimental study, but with a small sample and an evening screen exposure longer and brighter than most typical device use, so effect sizes for everyday habits may differ.

Source

Chang, A.-M., Aeschbach, D., Duffy, J. F., & Czeisler, C. A. (2015). Reading on a Backlit Screen Before Bed Delays Your Body Clock. Proceedings of the National Academy of Sciences.

Peer-reviewed · Free full text available · Last reviewed 2026-08-14

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