Dose\u2013response effects of light therapy on sleepiness and circadian phase shift in shift workers: a meta-analysis and moderator analysis

Calvin Lam,Min-Huey Chung

doi:10.1038/s41598-021-89321-1

Abstract

Light therapy has been considered to be effective in mitigating sleepiness and regulating circadian phase shift in shift workers. However, the effective treatment dose of light therapy remains undetermined. We performed a meta-analysis of randomized experimental studies to determine the effect of light therapy doses on sleepiness and circadian phase shift in shift workers. An article search was performed in 10 electronic databases from inception to June 2020. Two raters independently screened and extracted data and reached consensus. Twenty-one eligible studies were included. Analyses were performed using random-effects models. Light therapy exerted significantly small to medium effects on sleepiness and large treatment effects on circadian phase shift. Moderator analyses performed with subgroup and metaregression analyses revealed that medium-intensity light therapy for a shorter duration more effectively reduced sleepiness at night, whereas higher-intensity light therapy more effectively induced phase shifting, but the required treatment duration remained inconclusive. This study provides evidence regarding the effect of light therapy in reducing sleepiness and shifting circadian phase in shift workers. Exposure to medium-intensity light for a short duration at night reduced sleepiness, whereas exposure to high-intensity light improved sleep by shifting their circadian phase.

Highlights

Light therapy has been considered to be effective in mitigating sleepiness and regulating circadian phase shift in shift workers
These studies recorded the outcome by using the following subjective scales: the Karolinska Sleepiness Scale (KSS) with 9-point scales from “very alert” to “very sleepy, fighting sleep, an effort to stay awake;” the Stanford Sleepiness Scale (SSS) with 7-point scales from “feeling active and vital, alert, wide awake” to “lost struggle to remain awake;” and a visual analog scale (VAS) using 10-cm or 100-cm scales with adjectives from “sleepy” to “alert”
Thirteen studies recorded the outcome of circadian phase shift, which was measured by determining melatonin secretion by using a radioimmunoassay or enzyme-linked immunosorbent assay or by examining core temperature

Summary

Introduction

Light therapy has been considered to be effective in mitigating sleepiness and regulating circadian phase shift in shift workers. We performed a meta-analysis of randomized experimental studies to determine the effect of light therapy doses on sleepiness and circadian phase shift in shift workers. Moderator analyses performed with subgroup and metaregression analyses revealed that medium-intensity light therapy for a shorter duration more effectively reduced sleepiness at night, whereas higher-intensity light therapy more effectively induced phase shifting, but the required treatment duration remained inconclusive. This study provides evidence regarding the effect of light therapy in reducing sleepiness and shifting circadian phase in shift workers. Light therapy is a nonpharmacological treatment that has been considered effective in mitigating the sleepiness of shift workers. Shift workers may exhibit improved alertness at work and improved sleep after work and on days off. Light therapy may adjust the timing of melatonin secretion to coincide with the sleep phase

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Discussion

Conclusion