A review of the studies on nonvisual lighting effects in the field of physiological anthropology

Review on age-related differences in non-visual effects of light: melatonin suppression, circadian phase shift and pupillary light reflex in children to older adults

Physiological effects of light exposure in humans are diverse. Among them, the circadian rhythm phase shift effect in order to maintain a 24-h cycle of the biological clock is referred to as non-visual effects of light collectively with melatonin suppression and pupillary light reflex. The non-visual effects of light may differ depending on age, and clarifying age-related differences in the non-visual effects of light is important for providing appropriate light environments for people of different ages. Therefore, in various research fields, including physiological anthropology, many studies on the effects of age on non-visual functions have been carried out in older people, children and adolescents by comparing the effects with young adults. However, whether the non-visual effects of light vary depending on age and, if so, what factors contribute to the differences have remained unclear. In this review, results of past and recent studies on age-related differences in the non-visual effects of light are presented and discussed in order to provide clues for answering the question of whether non-visual effects of light actually vary depending on age. Some studies, especially studies focusing on older people, have shown age-related differences in non-visual functions including differences in melatonin suppression, circadian phase shift and pupillary light reflex, while other studies have shown no differences. Studies showing age-related differences in the non-visual effects of light have suspected senile constriction and crystalline lens opacity as factors contributing to the differences, while studies showing no age-related differences have suspected the presence of a compensatory mechanism. Some studies in children and adolescents have shown that children's non-visual functions may be highly sensitive to light, but the studies comparing with other age groups seem to have been limited. In order to study age-related differences in non-visual effects in detail, comparative studies should be conducted using subjects having a wide range of ages and with as much control as possible for intensity, wavelength component, duration, circadian timing, illumination method of light exposure, and other factors (mydriasis or non-mydriasis, cataracts or not in the older adults, etc.).

Blue Light-Ocular and Systemic Damaging Effects: A Narrative Review

Light is a fundamental aspect of our lives, being involved in the regulation of numerous processes in our body. While blue light has always existed in nature, with the ever-growing number of electronic devices that make use of short wavelength (blue) light, the human retina has seen increased exposure to it. Because it is at the high-energy end of the visible spectrum, many authors have investigated the theoretical harmful effects that it poses to the human retina and, more recently, the human body, given the discovery and characterization of the intrinsically photosensitive retinal ganglion cells. Many approaches have been explored, with the focus shifting throughout the years from examining classic ophthalmological parameters, such as visual acuity, and contrast sensitivity to more complex ones seen on electrophysiological assays and optical coherence tomographies. The current study aims to gather the most recent relevant data, reveal encountered pitfalls, and suggest future directions for studies regarding local and/or systemic effects of blue light retinal exposures.

Intake of l-serine before bedtime prevents the delay of the circadian phase in real life

Background

It has been shown in laboratory experiments using human subjects that ingestion of the non-essential amino acid l-serine before bedtime enhances the advance of circadian phase induced by light exposure the next morning. In the present study, we tested the effect of ingestion of l-serine before bedtime on circadian phase in real life and whether its effect depends on the initial circadian phase.

Methods

The subjects were 33 healthy male and female university students and they were divided into an l-serine group (n = 16) and a placebo group (n = 17). This study was conducted in a double-blind manner in autumn and winter. After a baseline period for 1 week, the subjects took 3.0 g of l-serine or a placebo 30 min before bedtime for 2 weeks. Saliva was collected twice a week at home every hour under a dim light condition from 20:00 to 1 h after habitual bedtime. Dim light melatonin onset (DLMO) was used as an index of phase of the circadian rhythm.

Results

DLMO after intervention was significantly delayed compared to the baseline DLMO in the placebo group (p = 0.02) but not in the l-serine group. There was a significant difference in the amount of changes in DLMO between the two groups (p = 0.04). There were no significant changes in sleeping habits after intervention in the two groups. There were significant positive correlations between advance of DLMO and DLMO before intervention in the l-serine group (r = 0.53, p < 0.05) and the placebo group (r = 0.69, p < 0.01). There was no significant difference in the slopes of regression lines between the two groups (p = 0.71), but the intercept in the l-serine group was significantly higher than that in the placebo group (p < 0.01). The levels of light exposure were not significantly different between the two groups.

Conclusions

Our findings suggest that intake of l-serine before bedtime for multiple days might attenuate the circadian phase delay in the real world and that this effect does not depend on the initial circadian phase.

Trial registration

This study is registered with University Hospital Medical Information Network in Japan (UMIN000024435. Registered on October 17, 2016).

Ambient-task combined lighting to regulate autonomic and psychomotor arousal levels without compromising subjective comfort to lighting

Background

Although evidence of both beneficial and adverse biological effects of lighting has accumulated, biologically favorable lighting often does not match subjectively comfortable lighting. By controlling the correlated color temperature (CCT) of ambient lights, we investigated the feasibility of combined lighting that meets both biological requirements and subjective comfort.

Methods

Two types of combined lightings were compared; one consisted of a high-CCT (12000 K) light-emitting diode (LED) panel as the ambient light and a low-CCT (5000 K) LED stand light as the task light (high-low combined lighting), and the other consisted of a low-CCT (4500 K) LED panel as the ambient light and the same low-CCT (5000 K) stand light as the task light (low-low combined lighting) as control. Ten healthy subjects (5 young and 5 elderly) were exposed to the two types of lighting on separate days. Autonomic function by heart rate variability, psychomotor performances, and subjective comfort were compared.

Results

Both at sitting rest and during psychomotor workload, heart rate was higher and the parasympathetic index of heart rate variability was lower under the high-low combined lighting than the low-low combined lighting in both young and elderly subject groups. Increased psychomotor alertness in the elderly and improved sustainability of concentration work performance in both age groups were also observed under the high-low combined lighting. However, no significant difference was observed in the visual-analog-scale assessment of subjective comfort between the two types of lightings.

Conclusions

High-CCT ambient lighting, even when used in combination with low-CCT task lighting, could increase autonomic and psychomotor arousal levels without compromising subjective comfort. This finding suggests the feasibility of independent control of ambient and task lighting as a way to achieve both biological function regulation and subjective comfort.

Is the use of high correlated color temperature light at night related to delay of sleep timing in university students? A cross-country study in Japan and China

Background

Blue-enriched white light at night has the potential to delay the circadian rhythm in daily life. This study was conducted to determine whether the use of high correlated color temperature (CCT) light at home at night is associated with delay of sleep timing in university students.

Methods

The survey was conducted in 2014–2015 in 447 university students in Japan and 327 students in China. Habitual sleep timing and type of CCT light at home were investigated by using a self-administered questionnaire. The Japanese students were significantly later than the Chinese students in bedtime, wake time, and midpoint of sleep. They were asked whether the lighting in the room where they spend most of their time at night was closer to warm color (low CCT) or daylight color (high CCT). The amount of light exposure level during daily life was measured for at least 1 week by the use of a light sensor in 60 students in each country.

Results

The percentages of participants who used high CCT lighting at night were 61.6% for Japanese students and 80.8% for Chinese students. Bedtime and sleep onset time on school days and free days were significantly later in the high CCT group than in the low CCT group in Japan. The midpoint of sleep in the high CCT group was significantly later than that in the low CCT group on free days but not on school days. On the other hand, none of the sleep measurements on school days and free days were significantly different between the high CCT and low CCT groups in China. Illuminance level of light exposure during the night was significantly higher in Japanese than in Chinese, but that in the morning was significantly higher in China than in Japan.

Conclusions

The use of high CCT light at night is associated with delay of sleep timing in Japanese university students but not in Chinese university students. The effects of light at night on sleep timing and circadian rhythm may be complicated by other lifestyle factors depending on the country.

Supplementary Information

The online version contains supplementary material available at 10.1186/s40101-021-00257-x.

Crystalline lens transmittance spectra and pupil sizes as factors affecting light-induced melatonin suppression in children and adults

PURPOSE

To investigate the contributions of ocular crystalline lens transmittance spectra and pupil size on age-related differences in the magnitude of light-induced melatonin suppression at night. The first aim was to demonstrate that spectral lens transmittance in children can be measured in vivo with a Purkinje image-based system. The second aim was to test the hypothesis that the magnitude of melatonin suppression in children is enhanced by larger pupils and higher lens transmittance of short wavelengths.

METHODS

Fourteen healthy children and 14 healthy adults participated in this study. The experiment was conducted for two nights in our laboratory. On the first night, the participants spent time under dim light conditions (<10 lux) until one hour after their habitual bedtime (BT+1.0). On the second night, the participants spent time under dim light conditions until 30 min before their habitual bedtime (BT-0.5). They were then exposed to LED light for 90 min up to BT+1.0. Individual pupil sizes were measured between BT and BT+1.0 for both conditions. Lens transmittance spectra were measured in vivo using the Purkinje image-based system during the daytime. Non-visual photoreception was calculated from lens transmittance and pupil size. This was taken as an index of the influence of age-related ocular changes on the non-visual photopigment melanopsin.

RESULTS

Measured lens transmittance in children was found to be higher than for adults, especially in the short wavelength region (p < 0.001). Pupil size in children was significantly larger than that of adults under both dim (p = 0.003) and light (p < 0.001) conditions. Children's non-visual photoreception was 1.48 times greater than that of adults, which was very similar to the finding that melatonin suppression was 1.52 times greater in children (n = 9) than adults (n = 9).

CONCLUSIONS

Our Purkinje image-based system can measure children's lens transmittance spectra in vivo. Lens transmittance and pupil size may contribute to differences in melatonin suppression between primary school children and middle-aged adults.

Photobiomodulation of the Visual System and Human Health

Humans express an expansive and detailed response to wavelength differences within the electromagnetic (EM) spectrum. This is most clearly manifest, and most studied, with respect to a relatively small range of electromagnetic radiation that includes the visible wavelengths with abutting ultraviolet and infrared, and mostly with respect to the visual system. Many aspects of our biology, however, respond to wavelength differences over a wide range of the EM spectrum. Further, humans are now exposed to a variety of modern lighting situations that has, effectively, increased our exposure to wavelengths that were once likely minimal (e.g., “blue” light from devices at night). This paper reviews some of those biological effects with a focus on visual function and to a lesser extent, other body systems.

A Purkinje image-based system for an assessment of the density and transmittance spectra of the human crystalline lens in vivo

A method for rapid and objective assessment of ocular lens density and transmittance is needed for research and clinical practice. The aim of this study was to determine whether the Purkinje image-based technique can be used for objective and accurate quantification of spectral density and transmittance of ocular media (the mainly crystalline lens) in visible light. Twenty-six individuals (10 young, 9 middle-aged and 7 older individuals) participated in this study. Spectral lens density was evaluated by detecting the intensity of the IVth Purkinje image for different wavelengths. Subsequently, optical density index (ODI), the area under the curve in the lens density spectrum, was calculated and ODIs were compared with clinical lens opacification scales assessed subjectively using a slit lamp. Spectral lens transmittance was estimated from the lens density spectrum. Lens densities were higher in the short wavelength region of the visible spectrum across all age groups. ODI was highly correlated with the clinical opacification scale, while lens transmittance decreased with aging. Our results showed that spectral transmittance of the human crystalline lens can be easily estimated from optical density spectra evaluated objectively and rapidly using the Purkinje image-based technique. Our results provide clinicians and scientists with an accurate, rapid and objective technique for quantification of lens transmittance.

Difference in autonomic nervous effect of blue light depending on the angle of incidence on the eye

Objective

Blue light has been attributed to the adverse biological effects caused by the use of smartphones and tablet devices at night. However, it is not realistic to immediately avoid nighttime exposure to blue light in the lifestyle of modern society, so other effective methods should be investigated. Earlier studies reported that inferior retinal light exposure causes greater melatonin suppression than superior retinal exposure. We examined whether the autonomic responses to blue light depends on the angle of incidence to the eye.

Results

In eight healthy subjects, blue light from organic electroluminescent lighting device (15.4 lx at subjects’ eye) was exposed from 6 angles (0º, 30º, 45º, 135º, 150º, and 180º) for 5 min each with a 10-min interval of darkness. After adjusting the order effect of angles, however, no significant difference in heart rate or autonomic indices of heart rate variability with the angle of incidence was detected in this study.

Salivary melatonin suppression under 100-Hz flickering blue light and non-flickering blue light conditions

Bright light at night has been known to suppress melatonin secretion. Photoreceptors, known as intrinsically photosensitive retinal ganglion cells (ipRGCs), project dark/bright information into the superchiasmatic nucleus, which regulates the circadian system. Electroretinograms of ipRGCs show fluctuation that is synchronized with light ON-OFF stimulation. This finding suggests that the flickering condition of light may have an impact on our circadian system. In this study, we evaluate light-induced melatonin suppression under flickering and non-flickering light conditions. Fifteen male subjects between the ages of 20 and 23 years (mean ± SD, 21.9 ± 1.9) were exposed to three light conditions (dim, 100-Hz flickering and non-flickering light) from 1:00 a.m. to 2:30 a.m. Saliva samples were taken just before 1:00 and at 1:15, 1:30, 2:00, and 2:30 a.m. Repeated-measure t-test with Bonferroni correction showed a significant decrease in melatonin levels under both 100-Hz and non-flickering light conditions compared to dim light conditions after 2:00 a.m. Moreover, at 2:30 a.m., the rate of change in melatonin level under 100 Hz of flickering light was significantly lower than that under non-flickering light. Our present findings suggest that 100-Hz flickering light may suppress melatonin secretion more than non-flickering light.

Non-visual effects of diurnal exposure to an artificial skylight, including nocturnal melatonin suppression

BACKGROUND

Recently, more consideration is being given to the beneficial effects of lighting on the maintenance and promotion of the health and well-being of office occupants in built environments. A new lighting technology using Rayleigh scattering has made it possible to simulate a blue sky. However, to date, no studies have examined the possible beneficial effects of such artificial skylights. The aims of this study were to examine the non-visual effects of artificial skylights and conventional fluorescent lights in a simulated office environment and to clarify the feature effects of the artificial skylights.

METHODS

Participants were 10 healthy male adults. Non-visual effects were evaluated based on brain arousal levels (α-wave ratio and contingent negative variation [CNV]), autonomic nervous activity (heart rate variability [HRV]), work performance, and subjective responses during daytime exposure to either an artificial skylight or fluorescent lights, as well as nocturnal melatonin secretion.

RESULTS

Subjective evaluations of both room lighting-related "natural" and "attractive" items and the "connected to nature" item were significantly higher with the skylight than with the fluorescent lights. Cortical arousal levels obtained from the early component of the CNV amplitude were significantly lower with the skylight than with the fluorescent lights, whereas α-wave ratio and work performance were similar between the two light sources. The HRV evaluation showed that sympathetic nerve tone was lower and parasympathetic nerve tone was higher, both significantly, for the skylight than for the fluorescent lights during daytime. Nocturnal melatonin secretion was significantly greater before and during light exposure at night under the daytime skylight than under the fluorescent lights.

CONCLUSIONS

Our results suggest that artificial skylights have some advantages over conventional fluorescent lights in maintaining ordinary work performance during daytime with less psychological and physiological stress. The findings also suggest that the artificial skylights would enable built environments to maintain long-term comfort and productivity.