Mariana G. Figueiro

Measuring and using light in the melanopsin age

Light is a potent stimulus for regulating circadian, hormonal, and behavioral systems. In addition, light therapy is effective for certain affective disorders, sleep problems, and circadian rhythm disruption. These biological and behavioral effects of light are influenced by a distinct photoreceptor in the eye, melanopsin-containing intrinsically photosensitive retinal ganglion cells (ipRGCs), in addition to conventional rods and cones. We summarize the neurophysiology of this newly described sensory pathway and consider implications for the measurement, production, and application of light. A new light-measurement strategy taking account of the complex photoreceptive inputs to these non-visual responses is proposed for use by researchers, and simple suggestions for artificial/architectural lighting are provided for regulatory authorities, lighting manufacturers, designers, and engineers.

Circadian photobiology: an emerging framework for lighting practice and research

A century of research and practice have optimized the use of electric lighting in buildings to support human vision. However, recent lines of research show that light is also important to human circadian regulation, as reflected in such diverse phenomena as depression, sleep quality, alertness, and, perhaps, even health. Although light is essential to both vision and circadian regulation, research shows that the biophysical processes that govern circadian regulation are very different from those that govern vision. This growing body of research will probably influence the architectural lighting community and manufacturers to reoptimize the use of electric lighting in buildings to support both human vision and circadian functions. The present paper is concerned with establishing a framework for lighting practice and applied research that will assist lighting practitioners and manufacturers in interpreting this emerging research.

Modelling the spectral sensitivity of the human circadian system

It is now well established that the spectral, spatial, temporal and absolute sensitivities of the human circadian system are very different from those of the human visual system. Although qualitative comparisons between the human circadian and visual systems can be made, there still remains some uncertainty in quantitatively predicting exactly how the circadian system will respond to different light exposures reaching the retina. This paper discusses attempts to model the spectral sensitivity of the circadian system. Each of the models discussed here varies in terms of its complexity and its consideration of retinal neuroanatomy and neurophysiology. Future testing to validate or improve any of these computational models will require a targeted hypothesis, as well as a suitably high level of experimental control before one model can be rejected in favour of another. Until specific hypotheses are formulated and tested, it would be premature to recommend international acceptance of any model or system of circadian photometry.

Preliminary evidence that both blue and red light can induce alertness at night

BackgroundA variety of studies have demonstrated that retinal light exposure can increase alertness at night. It is now well accepted that the circadian system is maximally sensitive to short-wavelength (blue) light and is quite insensitive to long-wavelength (red) light. Retinal exposures to blue light at night have been recently shown to impact alertness, implicating participation by the circadian system. The present experiment was conducted to look at the impact of both blue and red light at two different levels on nocturnal alertness. Visually effective but moderate levels of red light are ineffective for stimulating the circadian system. If it were shown that a moderate level of red light impacts alertness, it would have had to occur via a pathway other than through the circadian system.MethodsFourteen subjects participated in a within-subject two-night study, where each participant was exposed to four experimental lighting conditions. Each night each subject was presented a high (40 lx at the cornea) and a low (10 lx at the cornea) diffuse light exposure condition of the same spectrum (blue, λmax = 470 nm, or red, λmax = 630 nm). The presentation order of the light levels was counterbalanced across sessions for a given subject; light spectra were counterbalanced across subjects within sessions. Prior to each lighting condition, subjects remained in the dark (< 1 lx at the cornea) for 60 minutes. Electroencephalogram (EEG) measurements, electrocardiogram (ECG), psychomotor vigilance tests (PVT), self-reports of sleepiness, and saliva samples for melatonin assays were collected at the end of each dark and light periods.ResultsExposures to red and to blue light resulted in increased beta and reduced alpha power relative to preceding dark conditions. Exposures to high, but not low, levels of red and of blue light significantly increased heart rate relative to the dark condition. Performance and sleepiness ratings were not strongly affected by the lighting conditions. Only the higher level of blue light resulted in a reduction in melatonin levels relative to the other lighting conditions.ConclusionThese results support previous findings that alertness may be mediated by the circadian system, but it does not seem to be the only light-sensitive pathway that can affect alertness at night.

Preliminary evidence for spectral opponency in the suppression of melatonin by light in humans.

Human adult males were exposed to light from blue light emitting diodes (18 lux; 29 μW/cm2) and from clear mercury vapor lamps (450 lux; 170 μW/cm2) during night-time experimental sessions. Both conditions suppressed nocturnal melatonin concentrations in blood plasma with the blue light more effective than mercury at melatonin suppression. No additive model incorporating opsin photopigments either alone or in combination could explain the results, but a model incorporating an opponent mechanism was consistent with the present data as well as data from previously published studies.

Comparisons of three practical field devices used to measure personal light exposures and activity levels

This paper documents the spectral and spatial performance characteristics of two new versions of the Daysimeter, devices developed and calibrated by the Lighting Research Center to measure and record personal circadian light exposure and activity levels, and compares them to those of the Actiwatch Spectrum. Photometric errors from the Daysimeters and the Actiwatch Spectrum were also determined for various types of light sources. The Daysimeters had better photometric performance than the Actiwatch Spectrum. To assess differences associated with measuring light and activity levels at different locations on the body, older adults wore four Daysimeters and an Actiwatch Spectrum for seven consecutive days. Wearing the Daysimeter or Actiwatch Spectrum on the wrist compromises accurate light measurements relative to locating a calibrated photosensor at the plane of the cornea.

Retinal mechanisms determine the subadditive response to polychromatic light by the human circadian system

Light is the major synchronizer of circadian rhythms to the 24-h solar day. The intrinsically photosensitive retinal ganglion cells (ipRGCs) play a central role in circadian regulation but cones also provide, albeit indirectly, input to these cells. In humans, spectrally opponent blue versus yellow (b-y) bipolar cells lying distal to the ganglion cell layer were hypothesized to provide direct input to the ipRGCs and therefore, the circadian system should exhibit subadditivity to some types of polychromatic light. Ten subjects participated in a within-subjects 3-night protocol. Three experimental conditions were employed that provided the same total irradiance at both eyes: (1) one unit of blue light (lambda(max)=450 nm, 0.077 W/m(2)) to the left eye plus one unit of green light (lambda(max)=525 nm, 0.211 W/m(2)) to the right eye, (2) one unit of blue light to the right eye plus one unit of green light to the left eye, and (3) 1/2 unit of blue light plus 1/2 unit of green light to both eyes. The first two conditions did not differ significantly in melatonin suppression while the third condition had significantly less melatonin suppression than conditions 1 and 2. Furthermore, the magnitudes of suppression were well predicted by a previously published model of circadian phototransduction incorporating spectral opponency. As was previously demonstrated, these results show that the human circadian system exhibits a subadditive response to certain polychromatic light spectra. This study demonstrates for the first time that subadditivity is due to spectrally opponent (color) retinal neurons.

The Effects of Red and Blue Lights on Circadian Variations in Cortisol, Alpha Amylase, and Melatonin

The primary purpose of the present study was to expand our understanding of the impact of light exposures on the endocrine and autonomic systems as measured by acute cortisol, alpha amylase, and melatonin responses. We utilized exposures from narrowband long-wavelength (red) and from narrow-band short-wavelength (blue) lights to more precisely understand the role of the suprachiasmatic nuclei (SCN) in these responses. In a within-subjects experimental design, twelve subjects periodically received one-hour corneal exposures of 40 lux from the blue or from the red lights while continuously awake for 27 hours. Results showed-that, as expected, only the blue light reduced nocturnal melatonin. In contrast, both blue and red lights affected cortisol levels and, although less clear, alpha amylase levels as well. The present data bring into question whether the nonvisual pathway mediating nocturnal melatonin suppression is the same as that mediating other responses to light exhibited by the endocrine and the autonomic nervous systems.

Phototransduction for human melatonin suppression

Human adult males were exposed to combinations of two illuminances and two broadband spectral power distributions over the course of four night‐time sessions. Results showed that melatonin suppression is dominated by short visible wavelengths (420–520 nm), consistent with recently published studies. Although the authors of these recent studies suggest that a novel opsin underlies melatonin suppression, the present paper offers a more conservative interpretation of the data based on what is known about existing photoreceptors and associated neuroanatomy and neurophysiology.

Tailored lighting intervention improves measures of sleep, depression, and agitation in persons with Alzheimer’s disease and related dementia living in long-term care facilities

Background Light therapy has shown great promise as a nonpharmacological method to improve symptoms associated with Alzheimer’s disease and related dementias (ADRD), with preliminary studies demonstrating that appropriately timed light exposure can improve nighttime sleep efficiency, reduce nocturnal wandering, and alleviate evening agitation. Since the human circadian system is maximally sensitive to short-wavelength (blue) light, lower, more targeted lighting interventions for therapeutic purposes, can be used. Methods The present study investigated the effectiveness of a tailored lighting intervention for individuals with ADRD living in nursing homes. Low-level “bluish-white” lighting designed to deliver high circadian stimulation during the daytime was installed in 14 nursing home resident rooms for a period of 4 weeks. Light–dark and rest–activity patterns were collected using a Daysimeter. Sleep time and sleep efficiency measures were obtained using the rest–activity data. Measures of sleep quality, depression, and agitation were collected using standardized questionnaires, at baseline, at the end of the 4-week lighting intervention, and 4 weeks after the lighting intervention was removed. Results The lighting intervention significantly (P<0.05) decreased global sleep scores from the Pittsburgh Sleep Quality Index, and increased total sleep time and sleep efficiency. The lighting intervention also increased phasor magnitude, a measure of the 24-hour resonance between light–dark and rest–activity patterns, suggesting an increase in circadian entrainment. The lighting intervention significantly (P<0.05) reduced depression scores from the Cornell Scale for Depression in Dementia and agitation scores from the Cohen–Mansfield Agitation Inventory. Conclusion A lighting intervention, tailored to increase daytime circadian stimulation, can be used to increase sleep quality and improve behavior in patients with ADRD. The present field study, while promising for application, should be replicated using a larger sample size and perhaps using longer treatment duration.