Differences in twenty-four-hour profiles of blue-light exposure between day and night shifts in female medical staff

Highlights

• Twenty-four-hour blue light exposure profiles during periods with either day or night are described.

• A new approach to select time-windows of the day for the description of light exposure is presented.

• Night work was associated with light exposure in all time-windows except for evening between 7 and 9 p.m.

• Duration of blue-light darkness per 24 hours differs by almost four hours between day- and night-shift periods.

• Shift work and also individual factors are associated with light exposure levels.

Abstract

Light is the strongest zeitgeber currently known for the synchronization of the human circadian timing system. Especially shift workers are exposed to altered daily light profiles. Our objective is the characterization of differences in blue-light exposures between day and night shift taking into consideration modifying factors such as chronotype. We describe 24-hour blue-light profiles as measured with ambient light data loggers (LightWatcher) during up to three consecutive days with either day or night shifts in 100 female hospital staff including 511 observations. Linear mixed models were applied to analyze light profiles and to select time-windows for the analysis of associations between shift work, individual factors, and log mean light exposures as well as the duration of darkness per day. Blue-light profiles reflected different daily activities and were mainly influenced by work time. Except for evening (7–9 p.m.), all time windows showed large differences in blue-light exposures between day and night shifts. Night work reduced the duration of darkness per day by almost 4 h ($\widehat{\beta }$ = −3:48 hh:mm, 95% CI (−4:27; −3.09)). Late chronotypes had higher light exposures in the morning and evening compared to women with intermediate chronotype (e.g. morning $\widehat{\beta }$ = 0.50 log(mW/m²/nm), 95% CI (0.08; 0.93)). Women with children had slightly higher light exposures in the afternoon than women without children ($\widehat{\beta }$ = 0.48, 95% CI (−0.10; 1,06)). Time windows for the description of light should be chosen carefully with regard to timing of shifts. Our results are helpful for future studies to capture relevant light exposure differences and potential collinearities with individual factors. Improvement of well-being of shift workers with altered light profiles may therefore require consideration of both – light at the workplace and outside working hours.

Introduction

The individual light environment in the 24/7 society differs markedly from several hundred years as well as from some decades ago with changes in the temporal pattern, intensity, and light spectrum. It now often involves minimum time spent outside under sunlight, illuminated nights and light emitted from electronic screens (Smolensky et al., 2015). Light is the strongest zeitgeber for the synchronization of human circadian timing system. Humans' endogenous circadian rhythms are synchronized by light/dark cycles via the suprachiasmatic nucleus (SCN) in the hypothalamus which has been identified as the master circadian pacemaker in the brain (Buijs et al., 1998; Saper, 2013). Intrinsically photosensitive retinal ganglion cells (ipRGCs) mediate light signals via melanopsin to the suprachiasmatic nucleus (SCN) to align the body's endogenous rhythms with external environment (Berson et al., 2002; Hattar et al., 2002). ipRGCs are most sensitive to the blue portion of the visible spectrum (~475–505 nm), which in natural settings is specifically high in morning sunlight (Lucas et al., 2014).

Changes in daily patterns of light spectrum, duration, timing, and intensity due to e.g. work in rotational shift systems, imply perturbations of the circadian systems and have been suggested to negatively affect human health. Numerous studies investigated potential associations between shift work that may involve circadian disruption and chronic diseases such as diabetes, cardiovascular diseases, gastrointestinal problems, neuropsychological issues, and cancer (Behrens et al., 2017; Cordina-Duverger et al., 2018; Harvey et al., 2017; Kecklund and Axelsson, 2016; Vetter et al., 2018; Vyas et al., 2012; Yuan et al., 2018). However, the International Agency for Research on Cancer classified long-term shift work that involves circadian disruption as probably carcinogenic (group 2A) (Straif et al., 2007). According to the light-at-night hypothesis (Fritschi et al., 2011; Stevens, 1987), light at night changes daily rhythms of neurohormone melatonin which may subsequently change reproductive hormone levels such as oestrogens. Experimental studies have shown that light at night, even at low intensity suppresses the production of melatonin (Brainard et al., 2001; Chang et al., 2015; Rüger et al., 2013). However, studies have yet to establish the specific characteristics of light such as duration, brightness, timing, and spectrum that alter melatonin levels (Price, 2014; Rahman et al., 2018). Hébert and colleagues demonstrated that not only light at night but also the prior light history may be relevant for the suppression of melatonin (Hébert et al., 2002). Besides the effect of light on melatonin, changes in light can also affect many other marker signals such as alertness or cortisol (Cajochen, 2007; James et al., 2004).

Although it is intuitive that a sufficient amount of natural sunlight is required per day, it remains unclear what exactly constitutes a healthy 24-hour light profile. Open questions include the right amount of daily light dose as well as the right amount of darkness. Light should be rich with regard to the spectrum that results in biological responses, but adequate timing of specific light exposures is important. Healthy lighting characteristics in settings like night shift work have yet to be fully explored. Therefore, the relevant time windows per day for the description of light exposures and how these differ when women are working during the day or at night must be explored.

A panel of experts recently concluded that the impact on public health from either direct or indirect electric lighting practices is of upmost concern (Lunn et al., 2017). Studies that described light exposures of shift workers at workplace and in domestic settings primarily measured light as illuminance in lux (Daugaard et al., 2017; Dumont et al., 2012; Grundy et al., 2011; Papantoniou et al., 2014). In the majority of studies, the rationale for the choice of time windows and exposure metrics was rather arbitrary, such as average light exposures over a period of four to 8 h including the hours around midnight.

This motivated us to describe individual light exposures during consecutive days, including light specifically in the blue range among female staff of Bergmannsheil University Hospital in Bochum, Germany. We identify time-windows for the comparison of light exposures with regard to work schedule and investigate individual factors that contribute to differing lighting habits.