Chronobiol Int. 2017;34(3):303-317.

Acute effects of different light spectra on simulated night-shift work without circadian alignment

Markus Canazeia,b, Wilfried Pohla, Harald R. Bliemb, and Elisabeth M. Weissc

a Research Department, Bartenbach GmbH, Aldrans, Austria; b Department of Psychology, University of Innsbruck, Innsbruck, Austria; c Department of Psychology, University of Graz, Graz, Austria

 

ABSTRACT

Short-wavelength and short-wavelength-enhanced light have a strong impact on night-time working performance, subjective feelings of alertness and circadian physiology. In the present study, we investigated acute effects of white light sources with varied reduced portions of short wavelengths on cognitive and visual performance, mood and cardiac output. Thirty-one healthy subjects were investigated in a balanced cross-over design under three light spectra in a simulated night-shift paradigm without circadian adaptation. Exposure to the light spectrum with the largest attenuation of short wavelengths reduced heart rate and increased vagal cardiac parameters during the night compared to the other two light spectra without deleterious effects on sustained attention, working memory and subjective alertness. In addition, colour discrimination capability was significantly decreased under this light source. To our knowledge, the present study for the first time demonstrates that polychromatic white light with reduced short wavelengths, fulfilling current lighting standards for indoor illumination, may have a positive impact on cardiac physiology of night-shift workers without detrimental consequences for cognitive performance and alertness.

http://dx.doi.org/10.1080/07420528.2016.1222414

 

Supplement:

Authors: Lisa-Marie Neier a, Wilfried Pohl a, Markus Canazei a

a Research Department, Bartenbach GmbH, Aldrans, Austria

The circadian system controls the timing of a variety of human’s behaviour e.g. sleep, appetite or alertness. Anatomically it is projected in a complex network of central nervous system structures including, amongst others, the anterior hypothalamus and the pineal gland. It processes information of darkness and light collected by specific retinal ganglion cells, known as intrinsically photosensitive retinal ganglion cells (ipRGCs). The central pacemaker of the circadian system is the suprachiasmatic nucleus (SCN) of the hypothalamus, which integrates incoming photic information to respond with a broad range of physiological reactions, e.g. melatonin release (in darkness) or suppression (in light), changes in core body temperature or variations in heart rate. Light is one of the most important “zeitgebers” for our circadian system and alterations in environmental light conditions strongly affect our circadian rhythms. It was shown that under continuous light conditions circadian rhythms begin to shift and oscillation prolongs to more than our entrained 24 hours [1, 2]. Furthermore, it was shown that very low intensities (~80 lx) of nocturnal light suppresses melatonin production when applied with a colour temperature of 4000 Kelvin, respectively, representing light with increased short wavelengths [3]. This finding later finds expression in the spectral sensitivity curve of acute melatonin suppression in human beings, peaking close to 460 nm [4].

Melatonin is a hormone synthesized in the pineal gland that plays an important role in the regulation of sleep-wake cycle. Increasing melatonin levels in darkness and higher sleep pressure after increasing hours of wakefulness ease the transition from wakefulness to sleep. Beneath the melatonin suppressive effect of short wavelengths, it was shown that the circadian variation of heart rate and cardiac autonomic activity is also affected by increased light levels during the night [5]. Variations in heart beats are usually quantified as heart rate variability (HRV) and while resting in dark conditions at night, HRV is usually lower than during active periods during the day. In comparison to research on melatonin cycle disruptions, research on effects of night-time light exposure on the cardiovascular system is sparse.

Rotating shift work plays an important role in industry and health care facilities, but for a long time this kind of work was fulfilled without being aware of possible risks on human’s health. Our circadian rhythms naturally evolved over a very long time span and are still adapted to light-dark cycles provided by the presence and absence of sunlight. Research has shown recently, that higher intensities as well as increased short-wavelength light during nightshifts lead to chronic disruptions of the circadian system in the long run. Today furthermore, there is great evidence of increased risks to develop diabetes, obesity, cardiovascular diseases, sleep disorders, gastrointestinal disorders and some types of cancer in regular shift workers [6, 7, 8, 9].

To understand which effects light may provoke during the night, it is important to consider two different photometrical characteristics of light:

  • spectral distribution: describes the portion of different parts of the emitted light spectrum; for human beings, only a small part of the light spectrum is visible (380-780 nm) and even a much smaller part is highly effective in suppressing melatonin (short wavelengths: 460-480 nm); a proxy measure of the spectral distribution of light is given by its colour temperature [unit: Kelvin].
  • light intensity: describes the perceived brightness of light [unit: lux]

Modifications in these photometrical characteristics are necessary to avoid harm and furthermore to provide positive effects on human’s health.

The night-shift study conducted at the research department of Bartenbach GmbH demonstrated that changes in the colour temperature of light, as a measure of proportion of shorter or longer wavelengths in its spectral distribution, greatly affects the physiological response of humans to light at night. Three colour temperatures varying from 2166 Kelvin (very low amount of short wavelengths) over 3366 Kelvin (moderate amount of short wavelengths) to 4667 Kelvin (high amount of short wavelengths) were tested while horizontal (501 lux; at desk level) and vertical (149 lux; at eye level) illuminance levels remained the same. All investigated lighting conditions were in line with current indoor lighting standards. Our findings showed that alerting effects can also be provoked in the absence of short wavelengths in light sources and that physiological parameters, i.e. heart rate and heart rate variability, are sensitive markers of light exposure with increased short wavelengths. Additionally, our results showed that colour discrimination performance seems to be decreased under light sources with reduced short wavelengths. Practically this means, that in workplaces, where high colour discrimination capabilities are needed, full spectrum light sources should be recommended. In contrast, in night shift workplaces with normal to reduced colour perception demands, light sources with reduced short wavelengths would provide advantages on employees’ health during night shifts.

Findings from this study have already found their implementation. Bartenbach successfully planned and installed lighting systems in three hospitals, that change colour temperatures and portions of short wavelengths: the psychiatric hospital in Hall, Austria, the psychiatric hospital in Slagelse, Denmark (Figure 1) and the Helmut-G.-Walther-Klinikum in Lichtenfels, Germany. Furthermore, in 2015 this lighting concept was implemented in the research & development office of Bartenbach itself (Figures 2). To provide a more natural lighting surrounding without disrupting the circadian system, it contains light with lower colour temperature (2200 Kelvin) and reduced portion of short wavelengths in the evening and during the night. In the early morning colour temperature and portion of short wavelengths unrecognizable change to higher levels (4000 Kelvin), remaining the same during the whole day. In the evening a reduction in colour temperature and portion of short wavelengths occurs again, closing the cycle. The provided rhythmicity, imitating natural lighting conditions, enables the entrainment of the circadian system. It is expected, that in the long term this lighting concept provokes positive effects on patients’ and staff’s health as well as on office workers. Analysis of current research projects as well as future projects are necessary to evaluate the expected health effects of this lighting design.

 

 

Figure 1. Short-wavelength reduced lighting (2200 Kelvin) at night in the psychiatric hospital in Slagelse, Denmark [Lead Consultant and Architect: Karlsson Arkitekter / VLA, Photographer: Jens Lindhe]

 

 

Figure 2. Short-wavelength reduced lighting (2200 Kelvin) during the evening and at night (left), full spectrum lighting containing short wavelengths (4000 Kelvin) during the day (right) in the R&D office, Bartenbach GmbH, Austria

 

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