Circadian Light

Lighting Research Center, Rensselaer Polytechnic Institute, 21 Union Street, Troy, NY 12180, USA. .
Journal of Circadian Rhythms 02/2010; 8(1):2. DOI: 10.1186/1740-3391-8-2
Source: PubMed


The present paper reflects a work in progress toward a definition of circadian light, one that should be informed by the thoughtful, century-old evolution of our present definition of light as a stimulus for the human visual system. This work in progress is based upon the functional relationship between optical radiation and its effects on nocturnal melatonin suppression, in large part because the basic data are available in the literature. Discussed here are the fundamental differences between responses by the visual and circadian systems to optical radiation. Brief reviews of photometry, colorimetry, and brightness perception are presented as a foundation for the discussion of circadian light. Finally, circadian light (CLA) and circadian stimulus (CS) calculation procedures based on a published mathematical model of human circadian phototransduction are presented with an example.

Download full-text


Available from: Mariana Figueiro, Apr 07, 2015
  • Source
    • "To specify light in the circadian system, it would be attractive to develop a one-dimensional unit to account for multiple photoreceptor inputs. Attempts have been made to model the human circadian spectral sensitivity function based on nocturnal photoreceptor transduction (Rea et al., 2005, 2010) and a unit called ''CS'' (for circadian stimulus), which is proposed to quantify light information for the circadian system. However, this model has assumed an S-ON input, which was not consistent with an S-OFF input to ipRGCs based on in vitro recordings in the primate retina (Dacey et al., 2005). "
    [Show abstract] [Hide abstract]
    ABSTRACT: The intrinsic circadian clock requires photoentrainment to synchronize the 24-hour solar day. Therefore, light stimulation is an important component of chronobiological research. Currently, the chronobiological research field overwhelmingly uses photopic illuminance that is based on the luminous efficiency function, V(λ), to quantify light levels. However, recent discovery of intrinsically photosensitive retinal ganglion cells (ipRGCs), which are activated by self-containing melanopsin photopigment and also by inputs from rods and cones, makes light specification using a one-dimensional unit inadequate. Since the current understanding of how different photoreceptor inputs contribute to the circadian system through ipRGCs is limited, it is recommended to specify light in terms of the excitations of five photoreceptors (S-, M-, L-cones, rods and ipRGCs)(Lucas, Peirson et al., 2014). In the current study, we assessed whether the spectral outputs from a commercially available spectral watch (i.e., Actiwatch Spectrum) could be used to estimate photoreceptor excitations. Based on the color sensor spectral sensitivity functions from a previously published work, as well as from our measurements, we computed spectral outputs in the long-wavelength range (R), middle-wavelength range (G), short-wavelength range (B) and broadband range (W) under 52 CIE illuminants (25 daylight illuminants, 27 fluorescent lights). We also computed the photoreceptor excitations for each illuminant using human photoreceptor spectral sensitivity functions. Linear regression analyses indicated that the Actiwatch spectral outputs could predict photoreceptor excitations reliably, under the assumption of linear responses of the Actiwatch color sensors. In addition, R, G, B outputs could classify illuminant types (fluorescent vs. daylight illuminants) satisfactorily. However, the assessment of actual Actiwatch recording under several testing light sources showed that the spectral outputs were subject to great nonlinearity, leading to less accurate estimation of photoreceptor excitations. Based on our analyses, we recommend that each spectral watch should be calibrated to measure spectral sensitivity functions and linearization characteristics for each sensor to have an accurate estimation of photoreceptor excitations. The method we provided to estimate photoreceptor excitations from the outputs of spectral watches could be used for chronobiological studies that can tolerate an error in the range of 0.2-0.5 log units. Our method can be easily expanded to incorporate linearization functions to have more accurate estimations.
    Full-text · Article · Oct 2014 · Chronobiology International
  • Source
    • "A great deal of research and development has been undertaken to accurately characterize ''circadian light'' for humans and for nocturnal rodents. We are also now able to measure light-dependent circadian disruption in humans living their normal lives and are able to quantify light-dependent circadian disruption in different species (Figueiro & Rea, 2010; Figueiro et al., 2012, 2013a; Radetsky et al., 2013; Rea et al., 2014). Through the Daysimeter technology and supporting science, we are now able to relate ecological measurements of human light-dependent circadian disruption to controlled laboratory studies of the impact of light-dependent circadian disruption in animal models (Radetsky et al., 2013). "
    [Show abstract] [Hide abstract]
    ABSTRACT: Although circadian disruption is an accepted term, little has been done to develop methods to quantify the degree of disruption or entrainment individual organisms actually exhibit in the field. A variety of behavioral, physiological and hormonal responses vary in amplitude over a 24-h period and the degree to which these circadian rhythms are synchronized to the daily light–dark cycle can be quantified with a technique known as phasor analysis. Several studies have been carried out using phasor analysis in an attempt to measure circadian disruption exhibited by animals and by humans. To perform these studies, species-specific light measurement and light delivery technologies had to be developed based upon a fundamental understanding of circadian phototransduction mechanisms in the different species. When both nocturnal rodents and diurnal humans, experienced different species-specific light–dark shift schedules, they showed, based upon phasor analysis of the light–dark and activity–rest patterns, similar levels of light-dependent circadian disruption. Indeed, both rodents and humans show monotonically increasing and quantitatively similar levels of light-dependent circadian disruption with increasing shift-nights per week. Thus, phasor analysis provides a method for quantifying circadian disruption in the field and in the laboratory as well as a bridge between ecological measurements of circadian entrainment in humans and parametric studies of circadian disruption in animal models, including nocturnal rodents.
    Full-text · Article · Sep 2014 · Chronobiology International
  • Source
    • "The Daysimeter is a small device that continuously records light (using red, green, and blue [RGB] sensors) and activity levels.32 Upon downloading, the RGB values are converted into illuminance (lux), circadian light (CLA), and CS levels.33 Briefly, illuminance is irradiance weighted by the photopic luminous efficiency function, V(λ), an orthodox measure of the spectral sensitivity of the human fovea, peaking at 555 nm. "
    [Show abstract] [Hide abstract]
    ABSTRACT: 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.
    Full-text · Article · Sep 2014 · Clinical Interventions in Aging
Show more