Use Within Hour of Bedtime Has Greatest Impact, UCSF Researchers Find
Smartphone use directly correlates with sleep, with greater use demonstrating a significant association with shorter sleep duration and worse sleep efficiency, according to researchers at UC San Francisco.
Their study appears online Nov. 9, 2016, in PLOS ONE.
“Our study found that, not surprisingly, people spend a lot of time interacting with their phones,” said senior author Gregory Marcus, MD, MAS, a UCSF Health cardiologist and director of clinical research in the UCSF Division of Cardiology. “This was the first study to examine such use in a broad population, directly measuring screen time rather than relying on self-reported use. And, those with more screen time use had poorer sleep.”
About 64 percent of American adults owned a smartphone in 2015, and an estimated 68 percent of owners store their phone on a bedside table while sleeping. Screen time differs across age and race but is similar across socioeconomic groups.
Previous studies have associated exposure to smartphone screens, particularly around bedtime, with poor sleep, but their frequency of use has not been objectively measured and compared to demographics, health information and sleep quality.
Poor sleep – either quantity or quality – has been shown as a risk factor for obesity, diabetes, cardiovascular disease, depression and overall mortality. Light in the blue spectrum, such as that produced by a smartphone, can suppress production of melatonin, leading to decreased drowsiness, difficulty initiating sleep and non-restorative sleep, the authors said.
App Recorded Screen Time
In their PLOS ONE study, Marcus and his colleagues used a volunteer sample of 653 people (average age 42) from the more than 23,000 individuals enrolled worldwide in the UCSF Health eHeart Study as of Sept. 1, 2015. The ongoing Health eHeart Study, which currently includes nearly 80,000 consented participants toward a goal of 1 million, harnesses the power of online and mobile technology to gather cardiovascular data through devices such as smartphone apps, ECG smartphone cases and portable blood pressure cuffs.
The participants downloaded an app developed by Ginger.io for Android-based smartphones. The app recorded screen time continuously as the number of minutes in each hour the screen was on, as long as the phone was not in “airplane” mode and the app not turned off. Resulting data then was transmitted daily over the Internet to the study database.
Smartphone screen time was measured between Sept. 1, 2014, and Sept. 30, 2015. The total and average screen time was computed over selected 30-day windows for each participant. Participants provided baseline information on demographics, alcohol and tobacco use, mood, physical activity, and cardiovascular diagnoses.
In selected 30-day windows, total screen time exposure averaged 38.4 hours at an average 3.7 minutes per hour. After taking other factors into account, those who were younger and self reported as Black, Hispanic or “Other” race/ethnicity had longer average screen time.
As Smartphone Use Increased, Sleep Duration Decreased
Utilizing the Pittsburg Sleep Quality Index (PSQI) to assess sleep duration and quality, as smartphone use increased – particularly around bedtime – sleep duration and quality decreased, said Marcus, endowed professor of atrial fibrillation research in the UCSF School of Medicine. PSQI is a self-reporting 19-item questionnaire that generates scores in subjective sleep quality, sleep latency, sleep duration, habitual sleep efficiency, sleep disturbances, use of sleeping medication and daytime dysfunction.
“Increased screen time in the hour of and after bedtime, but not the hour before, was associated with greater sleep onset latency,” said lead author Matthew Christensen, UCSF research fellow. “This agrees with the notion that screen use just before attempting to fall asleep may be particularly problematic.”
Based on the International Physical Activity Questionnaire (IPAQ), a self-reporting survey that measures health-enhancing physical activity in adults, smartphone screen time was not associated with any change in physical activity level or body mass index (BMI). This could be attributable to the multi-functionality of smartphones or their use during physical activities, such as in running apps.
The researchers note these results may not be representative of the general population, and that study participants were predominantly female (75 percent) and Caucasian (75 percent). The authors said iPhones also were not used, multiple people could have used a single smartphone, and the screen could have been on while a participant was unaware.
Other UCSF contributors to the PLOS ONE study were Laura Bettencourt, Health eHeart Study analyst; Kaylin Nguyen, cardiology medical student; Jeffrey Olgin, MD, professor of medicine and chief of cardiology; and Mark Pletcher, MD, MPH, professor of epidemiology and biostatistics. Leanne Kaye, PhD, MPH, and Sai Moturu, PhD, MS, of Ginger.io Inc. also contributed.
Research was supported by the Office of Behavioral Social Sciences, National Institute of Biomedical Imaging and Bioengineering, National Institute of Neurological Disorders and Stroke, National Heart Lung and Blood Institute, and National Institute on Alcohol Abuse and Alcoholism of the National Institutes of Health under award 1U2CEB021881. Christensen is supported by the Sarnoff Cardiovascular Research Foundation, and Nguyen is supported by the National Institute on Minority Health and Health Disparities under award R25MD006832.
Ginger.io Inc. provided support in the form of salaries for Kaye and Moturu, but it and the other funders did not have any additional role in the study design, data collection and analysis, decision to publish, or preparation of the manuscript.
UC San Francisco (UCSF) is a leading university dedicated to promoting health worldwide through advanced biomedical research, graduate-level education in the life sciences and health professions, and excellence in patient care. It includes top-ranked graduate schools of dentistry, medicine, nursing and pharmacy; a graduate division with nationally renowned programs in basic, biomedical, translational and population sciences; and a preeminent biomedical research enterprise. It also includes UCSF Health, which comprises three top-ranked hospitals, UCSF Medical Center and UCSF Benioff Children’s Hospitals in San Francisco and Oakland, and other partner and affiliated hospitals and healthcare providers throughout the Bay Area.