Harvard researchers have determined that reading before bedtime from a light-emitting e-reader -- but not from a book -- delays sleep onset, reduces deep sleep, and causes morning drowsiness.
In the study, published in the Proceedings of the National Academy of Sciences, 12 men and women with an average age of 25 years spent 14 days as inpatients in a sleep laboratory. Each participant spent five days reading from an e-reader before sleep, and another five days reading from a printed book in dim light. Each reading session was four hours long. Melatonin levels were measured from hourly blood samples, and various methods were used to measure sleep onset, depth of sleep, and evening and morning alertness and drowsiness.
The results were unequivocal: The use of e-readers suppressed melatonin by 55% and delayed the onset of its release by 1.5 hours, delayed sleep onset by 10 minutes, reduced the duration of REM sleep by 8%, and significantly increased self-reported sleepiness the next morning.
The authors note that these results do not apply to the original Kindle e-reader or other e-ink-based reflective technology readers -- just to those that emit light from backlighting LEDs. The figure shows the measured power-spectrum of the light emitted from the e-readers used in the study, with the reflected light from a book shown for comparison.
The researchers reinforced a point we have discussed before: All these negative effects from light-emitting e-readers will be magnified in teenagers, who naturally tend to get sleepy later, yet must rise early for school.
The researchers summarize the import of the study this way:
If you were waiting for a smoking gun before conceding that LED-backlight devices have implications for health and well-being, this study provides it.
— Keith Dawson , Editor-in-Chief, All LED Lighting