J Sleep Med.  2021 Apr;18(1):29-36. 10.13078/jsm.210003.

Effect of Light Emitted by Smartphones at Bedtime on Circadian Rhythm and Sleep: Is It Different between Day Worker and Shift Worker?

• Kim D
• Jo H
• Choi SJ
• Joo EY

• KMID: 2515507
• DOI: http://doi.org/10.13078/jsm.210003

Abstract

Objectives
Light at night (LAN) can suppress melatonin secretion and thus disturb normal sleep. The aim of this study was to investigate how the illumination of a smartphone at bedtime affects the circadian rhythm and sleep in patients with insomnia.
Methods
We recruited two middle-aged patients (one day worker and one shift worker) with insomnia. They used a smartphone more than 12 hours a day, particularly at bedtime. This was a crossover design study, and each patient spent a night at the light control unit twice at a one-week interval, with or without smartphone use. Patients were instructed to look at a smartphone (5–10 lux) under 150 lux of ceiling illumination from 18:00 until lights-off. During the night, without a smartphone, they read a book or newspaper. Saliva was collected every 30 minutes and analyzed for melatonin. Sleep was monitored by polysomnography.
Results
The day worker showed a delayed dim light melatonin onset time (DLMO) (21:30 vs. 22:00) and a 38.7% decrease in melatonin levels with smartphone use. For the shift worker, both melatonin and cortisol showed abnormal patterns, and thus DLMO was not determined in either condition. In the day worker, shorter rapid eye movement (REM) latency and increased REM were observed with smartphone use.
Conclusions
This study demonstrates that the use of smartphones at bedtime acutely suppresses melatonin secretion and delays the sleep-wake cycle. However, the effect of LAN on melatonin secretion was not apparent in the shift worker with already misaligned circadian rhythm.