Biology

Evening use of light-emitting eReaders negatively affects sleep, circadian timing, and next-morning alertness Anne-Marie Changa,b,1,2, Daniel Aeschbacha,b,c, Jeanne F. Duffya,b, and Charles A. Czeislera,b

aDivision of Sleep and Circadian Disorders, Departments of Medicine and Neurology, Brigham and Women’s Hospital, Boston, MA 02115; bDivision of Sleep Medicine, Harvard Medical School, Boston, MA 02115; and cInstitute of Aerospace Medicine, German Aerospace Center, 51147 Cologne, Germany

Edited by Joseph S. Takahashi, Howard Hughes Medical Institute, University of Texas Southwestern Medical Center, Dallas, TX, and approved November 26, 2014 (received for review September 24, 2014)

In the past 50 y, there has been a decline in average sleep duration and quality, with adverse consequences on general health. A representative survey of 1,508 American adults recently revealed that 90% of Americans used some type of electronics at least a few nights per week within 1 h before bedtime. Mounting evidence from countries around the world shows the negative impact of such technology use on sleep. This negative impact on sleep may be due to the short-wavelength–enriched light emitted by these electronic devices, given that artificial-light exposure has been shown experimentally to produce alerting effects, suppress melatonin, and phase-shift the biological clock. A few reports have shown that these devices suppress melatonin levels, but little is known about the effects on circadian phase or the following sleep episode, exposing a substantial gap in our knowledge of how this increasingly popular technology affects sleep. Here we compare the biological effects of reading an electronic book on a light-emit- ting device (LE-eBook) with reading a printed book in the hours before bedtime. Participants reading an LE-eBook took longer to fall asleep and had reduced evening sleepiness, reduced melatonin secretion, later timing of their circadian clock, and reduced next- morning alertness than when reading a printed book. These results demonstrate that evening exposure to an LE-eBook phase-delays the circadian clock, acutely suppresses melatonin, and has important implications for understanding the impact of such technologies on sleep, performance, health, and safety.

sleep | chronobiology | phase-shifting | digital media | electronics

The use of electronic devices for reading, communication, andentertainment has greatly increased in recent years. Greater portability, convenience, and ease of access to reading materials in electronic form add to the popularity of these devices. The use of light-emitting devices immediately before bedtime is a con- cern because light is the most potent environmental signal that impacts the human circadian clock and may therefore play a role in perpetuating sleep deficiency (1). The circadian-timing system synchronizes numerous internal physiological and biochemical processes, including the daily rhythm of sleep propensity (2), to external environmental time cues. For optimal sleep duration and quality, the timing of the sleep episode must be appropri- ately aligned with the timing of the circadian clock. In humans, exposure to light in the evening and early part of the night, even at low intensity, suppresses the release of the sleep-facilitating hormone melatonin (3–5) and shifts the circadian clock to a later time (3, 6), both of which make it more difficult to fall asleep at night. Light exposure in the biological evening/night also acutely increases alertness (7, 8), but not much is known about its impact on alertness the following day. Here we present results from a randomized study comparing the effects of reading before bed- time using a light-emitting eReader (LE-eBook) with reading a printed book by reflected light. We examined circadian timing and suppression of melatonin, polysomnographic (PSG) recordings of

sleep, and subjective and objective measures of sleepiness both in the evening while reading and the following morning.

Results Twelve healthy young adults (mean ± SD: 24.92 ± 2.87 y; six women) completed a 14-d inpatient protocol. The randomized, crossover design (shown in Fig. 1) consisted of two conditions: (i) reading an LE-eBook in otherwise very dim room light for ∼4 h before bedtime for five consecutive evenings, and (ii) reading a printed book in the same very dim room light for ∼4 h before bedtime for five consecutive evenings. All participants completed both reading conditions but were randomized to the order. Hourly blood samples were collected during portions of

Significance

The use of light-emitting electronic devices for reading, com- munication, and entertainment has greatly increased recently. We found that the use of these devices before bedtime pro- longs the time it takes to fall asleep, delays the circadian clock, suppresses levels of the sleep-promoting hormone melatonin, reduces the amount and delays the timing of REM sleep, and reduces alertness the following morning. Use of light-emitting devices immediately before bedtime also increases alertness at that time, whichmay lead users to delay bedtime at home. Overall, we found that the use of portable light-emitting devices immedi- ately before bedtime has biological effects that may perpetuate sleep deficiency and disrupt circadian rhythms, both of which can have adverse impacts on performance, health, and safety.

Author contributions: A.-M.C., J.F.D., and C.A.C. designed research; A.-M.C. performed research; A.-M.C. and D.A. analyzed data; and A.-M.C. and C.A.C. wrote the paper.

Conflict of interest statement: Dr. Czeisler has received consulting fees from or served as a paid member of scientific advisory boards for: Boston Celtics; Boston Red Sox; Citgo Inc.; Cleveland Browns; Merck; Novartis; Purdue Pharma LP; Quest Diagnostics, Inc.; Teva Pharmaceuticals Industries Ltd.; Valero Inc.; Vanda Pharmaceuticals, Inc. Dr. Czeisler currently owns an equity interest in Lifetrac, Inc.; Somnus Therapeutics, Inc.; Vanda Phar- maceuticals, Inc., and between October 2012 and October 2013, Apple, Inc. and Microsoft, Inc. Dr. Czeisler received royalties from McGraw Hill, Penguin Press/Houghton Mifflin Harcourt, and Philips Respironics, Inc. and has received grants and research support from Cephalon Inc., National Football League Charities, Philips Respironics, ResMed Founda- tion, San Francisco Bar Pilots and Sysco. Dr. Czeisler is the incumbent of an endowed professorship provided to Harvard University by Cephalon, Inc. and holds a number of process patents in the field of sleep/circadian rhythms (e.g., photic resetting of the human circadian pacemaker). Since 1985, Dr. Czeisler has also served as an expert witness on various legal cases related to sleep and/or circadian rhythms, including matters involving Bombardier, Inc.; Delta Airlines; FedEx; Greyhound; Michael Jackson’s mother and chil- dren; Purdue Pharma, L.P.; United Parcel Service and the United States of America.

This article is a PNAS Direct Submission.

Freely available online through the PNAS open access option.

See Commentary on page 946. 1Present address: Department of Biobehavioral Health, Pennsylvania State University, University Park, PA 16802.

2To whom correspondence should be addressed. Email: amchang@psu.edu.

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