A systematic review of the amount and timing of light in association with objective and subjective sleep outcomes in community-dwelling adults

Abstract

Light is considered the dominant environmental cue, or zeitgeber, influencing the sleep-wake cycle. Despite recognizing the importance of light for our well-being, less is known about the specific conditions under which light is optimally associated with better sleep. Therefore, a systematic review was conducted to examine the association between the amount and timing of light exposure in relation to sleep outcomes in healthy, community-dwelling adults. A systematic search was conducted of four databases from database inception to June 2016. In total, 45 studies met the review eligibility criteria with generally high study quality excepting for the specification of eligibility criteria and the justification of sample size. The majority of studies involved experimental manipulation of light (n = 32) vs observational designs (n = 13). Broad trends emerged suggesting that (1) bright light (>1000 lux) has positive implications for objectively assessed sleep outcomes compared to dim (<100 lux) and moderate light (100-1000 lux) and (2) bright light (>1000 lux) has positive implications for subjectively assessed sleep outcomes compared to moderate light (100-1000 lux). Effects due to the amount of light are moderated by the timing of light exposure such that, for objectively assessed sleep outcomes, brighter morning and evening light exposure are consistent with a shift in the timing of the sleep period to earlier and later in the day, respectively. For subjectively assessed sleep outcomes, brighter light delivered in the morning was associated with self-reported sleep improvements and brighter evening light exposure was associated with worse self-reported sleep.

Introduction

Light is considered the dominant environmental cue, or zeitgeber, influencing the sleep-wake cycle.¹ Light input enters the retina; passes through the retinohypothalamic tract; and entrains the central circadian pacemaker, the suprachiasmatic nucleus, to a daily cycle.² Under natural conditions, the suprachiasmatic nucleus is reset daily by exposure to light. However, with advances in technology, exposure to light has become desynchronized from the natural light-dark cycle. As research on jet lag and the effects of shift work illustrates, this desynchronization can have a detrimental impact on sleep and overall well-being.3, 4, 5 Despite recognizing the importance of light for our well-being, less is known about the specific conditions under which light is optimally associated with healthy sleep. Although the specific components of light in association with sleep have been examined in reviews focused on light interventions for specific disorders,6, 7 no systematic review to date has examined the role of light in relation to the sleep of healthy individuals.⁸ As such, the objective of this review is to increase understanding of the association between light exposure and sleep outcomes in healthy community-dwelling adults.

As light is a multifaceted construct, multiple characteristics of light are important for the entrainment of the sleep-wake cycle. For example, facets of light exposure such as amount, timing, duration, rate of change, prior history, and spectrum have the potential to influence sleep outcomes.9, 19 Specifically, light exposure of insufficient amount or duration can result in unstable circadian rhythms that are desynchronized from the light-dark cycle.10, 11 Light exposure in the beginning of the night can induce a delay in the timing of the sleep period, whereas light exposure at the end of the night can induce a shift in the opposite direction, resulting in earlier timing of the sleep period.12, 13 The nature of the appearance of light (eg, more gradual vs sudden onset) can differentially affect sleep outcomes, and the impact of light can depend on prior light exposure history.¹⁴ Lastly, the spectral composition of light has important implications for sleep, with light enriched in blue wavelengths being more impactful than wavelengths outside of this range.15, 16

The current review will focus on 2 specific components of light exposure: amount and timing. These 2 components of light exposure are selected for specific focus given that they are salient to individuals and, consequently, are amenable to modification within a home environment. As such, any conclusions drawn from the review of these 2 characteristics of light have the potential to be tested and applied within the daily lives of community-dwelling adults. Conversely, the average individual is less likely to be aware of other components of light exposure such as the rate of change or spectrum of light. Furthermore, the amount and timing of light have a longer history of being examined or manipulated in studies of light exposure and sleep compared to the other characteristics of light which have been investigated only more recently. Unfortunately, a comprehensive review of the literature examining multiple characteristics of light in association with sleep is prohibited by the lack of studies including multiple characteristics of light.

The amount of light exposure, or illuminance level, is defined as the amount of light measured in a specified area. Light can be quantified in “lux” units (equivalent to a 0.0929 foot-candle), with lux levels approximately corresponding to the illumination of objects and surfaces by the following indoor and outdoor light sources: >100,000 lux = sunlight, ~1000 lux = overcast day, ~500 lux = indoor office, ~300 lux = living area within a home, and ~10 lux = twilight.17, 18 The amount of light required to entrain the sleep-wake cycle depends on a complex combination of the above-mentioned characteristics of light. To organize the literature and enable comparison across multiple levels of light, for the purpose of this review, the amount of light will be compared across 3 broad categories: dim (<100 lux), moderate (100- 1000 lux), and bright light (>1000 lux). These categories are selected based on their face validity such that natural outdoor daylight is visually distinguishable from indoor lighting to the naked eye.²⁰ Consequently, any conclusions drawn from the review will have ecological validity within natural environments with individuals able to discern whether they are being exposed to dim, moderate, or bright light (eg, 1000 lux is considered a reliable level for differentiating between artificial and natural light).²⁰ As a result, any summary conclusions based on the review could have real-world applicability for individuals interested in how the amount of light affects their sleep. Furthermore, these categories are congruent with many clinical recommendations for light exposure (eg, limit exposure to even dim light at night).²¹ As such, the use of these categories can facilitate communication between providers and clients/patients Lastly, this categorization schema will be adopted to facilitate abstraction across studies, as the vast majority of the reviewed articles used some combination of these categories (31 of 45 studies).

Timing of light exposure is categorized as broadly occurring during the morning, afternoon, evening, or across the full 24-hour day of the solar day. The time of day that individuals are exposed to light is an important factor to examine in association with sleep outcomes, as the proximity of light exposure to the sleep period has the potential to shift the timing of sleep period. Light presented early in the night is associated with shifts in timing to a later hour (delay), and light presented in the late night or early morning is associated with shifts in timing to an earlier hour (advance).²² Furthermore, light exposure during the day also shows potential to shift sleep timing.²²

In sum, given the theoretical and empirical research linking light exposure to sleep outcomes, there is a need to systematically summarize the existing body of literature in this area. Furthermore, there is a need to collate literature on this topic within healthy, community-dwelling adults. Therefore, this systematic review has the following aims:

• To describe the association between the amount of light exposure and objective and subjective sleep outcomes in healthy, community-dwelling adults. Specifically, to assess whether dim (<100 lux), moderate (100-1000 lux), or bright (>1000 lux) light is differentially associated with objective and subjective sleep outcomes in experimental and observational designs.

• To describe the association between the timing of light exposure and objective and subjective sleep outcomes in healthy, community-dwelling adults. Specifically, to assess whether the time of day of light exposure (eg, morning, evening, or night) is differentially associated with sleep outcomes within experimental and observational studies.