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Mariana G Figueiro,1 Levent Sahin,1 Charles Roohan,1 Michael Kalsher,2 Barbara Plitnick,1 Mark S Rea11Lighting Research Center, Rensselaer Polytechnic Institute, Troy, NY 12180, USA; 2Department of Cognitive Sciences, Rensselaer Polytechnic Institute, Troy, NY 12180, USAIntroduction: Sleep inertia, broadly defined as decrements in performance and lowering of alertness following waking, lasts for durations ranging between 1 min and 3 hrs. This study investigated whether, compared to a dim light condition (the control), exposure to long-wavelength (red) light delivered to closed eyelids during sleep (red light mask) and to eyes open upon waking (red light goggles) reduced sleep inertia.Methods: Thirty participants (18 females, 12 males; mean age=30.4 years [SD 13.7]) completed this crossover, within-subjects, counterbalanced design study. Self-reported measures of sleepiness and objective measures of auditory performance and cortisol levels were collected on 3 Friday nights over the course of 3 consecutive weeks.Results: Performance improved significantly during the 30-min data collection period in all experimental conditions. Subjective sleepiness also decreased significantly with time awake in all experimental conditions. As hypothesized, performance of some tasks was significantly better in the red light mask condition than in the dim light condition. Performance scores in the red light goggles condition improved significantly after a few minutes of wearing the light goggles.Discussion: The results show that saturated red light delivered through closed eyelids at levels that do not suppress melatonin can be used to mitigate sleep inertia upon waking.Keywords: sleep inertia, cognitive performance, alertness, red light, closed eyelids

Mariana G Figueiro Lighting Research Center, Rensselaer Polytechnic Institute, Troy, NY, USAAbstract: Delayed sleep phase disorder (DSPD) is common among adolescents and further increases their susceptibility to chronic sleep restriction and associated detrimental outcomes, including increased risk of depression, drug and alcohol use, behavioral problems, and poor scholastic performance. DSPD is characterized by sleep onset that occurs significantly later than desired bedtimes and societal norms. Individuals with DSPD exhibit long sleep latencies when attempting to sleep at conventional bedtimes. Circadian sleep disorders such as DSPD can occur when there is misalignment between sleep timing and societal norms. This review discusses studies using light therapy to advance the timing of sleep in adolescents and college students, in particular on those suffering from DSPD. A discussion on how to increase effectiveness of light therapy in the field will also be provided.Keywords: circadian, melatonin, light, sleep, sleep phase disorder, adolescents

Mariana G Figueiro, Barbara Plitnick, Mark S Rea Lighting Research Center, Rensselaer Polytechnic Institute, Troy, NY, USA Abstract: Circadian rhythm disturbances parallel the increased prevalence of sleep disorders in older adults. Light therapies that specifically target regulation of the circadian system in principle could be used to treat sleep disorders in this population. Current recommendations for light treatment require the patients to sit in front of a bright light box for at least 1 hour daily, perhaps limiting their willingness to comply. Light applied through closed eyelids during sleep might not only be efficacious for changing circadian phase but also lead to better compliance because patients would receive light treatment while sleeping. Reported here are the results of two studies investigating the impact of a train of 480 nm (blue) light pulses presented to the retina through closed eyelids on melatonin suppression (laboratory study) and on delaying circadian phase (field study). Both studies employed a sleep mask that provided narrowband blue light pulses of 2-second duration every 30 seconds from arrays of light-emitting diodes. The results of the laboratory study demonstrated that the blue light pulses significantly suppressed melatonin by an amount similar to that previously shown in the same protocol at half the frequency (ie, one 2-second pulse every minute for 1 hour). The results of the field study demonstrated that blue light pulses given early in the sleep episode significantly delayed circadian phase in older adults; these results are the first to demonstrate the efficacy and practicality of light treatment by a sleep mask aimed at adjusting circadian phase in a home setting. Keywords: circadian phase, dim light melatonin onset, light through closed eyelids, blue light, sleep

Mariana G Figueiro,1 Barbara A Plitnick,1 Anna Lok,1 Geoffrey E Jones,1 Patricia Higgins,2,3 Thomas R Hornick,3,4 Mark S Rea1 1Lighting Research Center, Rensselaer Polytechnic Institute, Troy, NY, USA; 2School of Nursing, 3School of Medicine, Case Western Reserve University, 4Geriatric Research Education and Clinical Center, Louis Stokes Cleveland Veterans Affairs Medical Center, Cleveland, OH, USABackground: 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

Mariana G Figueiro, Andrew Bierman, Mark S ReaLighting Research Center, Rensselaer Polytechnic Institute, Troy, NY, USAAbstract: A model of circadian phototransduction was published in 2005 to predict the spectral sensitivity of the human circadian system to narrow-band and polychromatic light sources by combining responses to light from the spectral-opponent “blue” versus “yellow” cone bipolar pathway with direct responses to light by the intrinsically photosensitive retinal ganglion cells. In the model, depolarizing “blue” responses, but not hyperpolarizing “yellow” responses, from the “blue” versus “yellow” pathway are combined with the intrinsically photosensitive retinal ganglion cell responses. Intrinsically photosensitive retinal ganglion cell neurons are known to be much slower to respond to light than the cone pathway, so an implication of the model is that periodic flashes of “blue” light, but not “yellow” light, would be effective for stimulating the circadian system. A within-subjects study was designed to test the implications of the model regarding retinal exposures to brief flashes of light. The study was also aimed at broadening the foundation for clinical treatment of circadian sleep disorders by delivering flashing light through closed eyelids while people were asleep. In addition to a dark control night, the eyelids of 16 subjects were exposed to three light-stimulus conditions in the phase delay portion of the phase response curve while they were asleep: (1) 2-second flashes of 111 W/m2 of blue (λmax ≈ 480 nm) light once every minute for 1 hour, (2) 131 W/m2 of green (λmax ≈ 527 nm) light, continuously on for 1 hour, and (3) 2-second flashes of the same green light once every minute for 1 hour. Inferential statistics showed that the blue flash light-stimulus condition significantly delayed circadian phase and significantly suppressed nocturnal melatonin. The results of this study further our basic understanding of circadian phototransduction and broaden the technical foundations for delivering light through closed eyelids during sleep for treating circadian sleep disorders.Keywords: melatonin, dim light melatonin onset, eyelids, flashing blue light, circadian rhythms, sleep

Shift-working railway personnel are highly susceptible to fatigue stemming from poor-quality sleep that can impair the performance of safety-critical functions. Disruption of the 24-h light–dark pattern incident on the retinae appears to play a central role in exacerbating these problems, especially with respect to exposure to light at night and circadian rhythms disruption. This field study tested and demonstrated the effectiveness and acceptability of an intervention (red and blue light combined with white light) to (1) promote circadian alignment, (2) advance the timing of dim light melatonin onset, (3) improve objective and subjective sleep quality, and (4) reduce subjective sleepiness in an operational context compared to a baseline (conventional lighting) condition. The lighting intervention promoted significantly greater synchrony between day-shift participants' light–dark exposures and rest–activity patterns, but did not significantly advance the timing of dim light melatonin onset. The intervention was also associated with reduced objective sleep disturbances and improved (albeit less robustly) subjective sleep quality, but it only reduced participants' subjective sleepiness while they were off duty during the day shift. This research demonstrates that lighting interventions can promote circadian alignment and improve sleep quality among railway shift workers and may be translatable to other 24-h industries. [ABSTRACT FROM AUTHOR]

It is well established that light affects both visual and non-visual systems. Laboratory studies have shown that, depending upon the time of exposure, short-wavelength light of sufficient amount and exposure duration will either entrain or disrupt the synchrony between our biological clock and our local position on Earth. Laboratory studies have also shown that light across the entire visible spectrum can enhance alertness, both day and night. Scant attention has been given to testing the effects of light on building occupants' non-visual responses, and, consequently, lighting specifiers have been offered little guidance on the design and application of lighting for non-visual effects. The present study helps to fill that gap through field-testing of light exposures from a novel luminaire designed to promote entrainment and alertness throughout the day in actual office environments. The data support the inference that light exposures, when properly applied, can promote circadian entrainment and increase alertness. [ABSTRACT FROM AUTHOR]

Architectural lighting has traditionally addressed visual performance and horizontal illuminance on the work plane, later focussing on energy efficiency, while only recently paying particular regard to human health outcomes. The present study evaluated the effectiveness of several light-emitting diode lighting strategies for delivering circadian stimulus to occupants of a typical office space while minimizing energy use. The study employed photometric simulations in a typical open-office space, delivering a criterion circadian stimulus of 0.3 to calculation points modelled at the simulated occupants' eye level. Six luminaire types, two luminous intensity distributions, six spectral power distributions and two horizontal illuminances were evaluated, resulting in 144 unique lighting conditions. Additionally, the study calculated the discomfort glare for selected luminaires with the highest total lumen output, smallest aperture and direct-only luminous intensity distributions at the higher of the two horizontal illuminances (500 lx). The most impactful strategy involved supplementing common overhead lighting with a desktop luminaire delivering light directly to the simulated office occupants' eyes, which provided greater circadian stimulus and used less energy than overhead luminaires that were capable of delivering the criterion circadian stimulus of 0.3. [ABSTRACT FROM AUTHOR]

This study investigated how light exposure duration affects melatonin suppression, a well-established marker of circadian phase, and whether adolescents (13–18 years) are more sensitive to short-wavelength (blue) light than adults (32–51 years). Twenty-four participants (12 adolescents, 12 adults) were exposed to three lighting conditions during successive 4-h study nights that were separated by at least one week. In addition to a dim light (<5 lux) control, participants were exposed to two light spectra (warm (2700 K) and cool (5600 K)) delivering a circadian stimulus of 0.25 at eye level. Repeated measures analysis of variance revealed a significant main effect of exposure duration, indicating that a longer duration exposure suppressed melatonin to a greater degree. The analysis further revealed a significant main effect of spectrum and a significant interaction between spectrum and participant age. For the adolescents, but not the adults, melatonin suppression was significantly greater after exposure to the 5600 K intervention (43%) compared to the 2700 K intervention (29%), suggesting an increased sensitivity to short-wavelength radiation. These results will be used to extend the model of human circadian phototransduction to incorporate factors such as exposure duration and participant age to better predict effective circadian stimulus. [ABSTRACT FROM AUTHOR]

The increased use of self-luminous displays, especially in the evening prior to bedtime, has been associated with melatonin suppression, delayed sleep and sleep curtailment. The present study set out to investigate whether the Night Shift application provided by Apple Inc. for use on its portable electronic devices is effective for reducing acute melatonin suppression, a well-established marker of circadian phase. Participants experienced four experimental conditions: a dim light control, a high circadian stimulus true positive intervention and two Night Shift interventions delivering low and high correlated colour temperature light from the devices. Melatonin suppression did not significantly differ between the two Night Shift interventions, which indicates that changing the spectral composition of self-luminous displays without changing their brightness settings may be insufficient for preventing impacts on melatonin suppression. [ABSTRACT FROM AUTHOR]

Two studies were designed to investigate the carryover effects of daylight on performance and self-reports of sleepiness. The effects of daylight and of darkness were compared independent of the effects of simultaneous periodic, 1-hour exposures to narrow-band blue and red lights over the course of 26-hour sessions. Nighttime performance on a 54-minute tracking task, but not subjective sleepiness, was significantly better following exposure to daylight. There was no differential effect on performance or sleepiness from exposure to the blue or red lights. Eight of the participants returned to experience a completely dark session and a replication of the daylight and intermittent blue light session. Nighttime performance and subjective sleepiness were significantly worse for the dark session than for daylight-plus-blue sessions in both studies. [ABSTRACT FROM AUTHOR]

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. [ABSTRACT FROM AUTHOR]

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. [ABSTRACT FROM AUTHOR]

Light at night has been implicated in health risks associated with night-shift work and circadian system disruption, but there are scant data on actual nighttime light exposures experienced by night-shift workers and how they affect melatonin levels. This study analysed salivary melatonin assays and calibrated personal light-exposure measurements obtained from a larger 20-week field study to explore the association between nocturnal melatonin levels and light exposures in 27 nurses working night shifts at four hospitals in southern New York and southern Indiana. Complete time-matched melatonin and light exposure data were available for 20 of the participants, who underwent three 2-week data collection periods (6 weeks total), each separated by six weeks. A mixed-effects model was used to quantify the adjusted association between melatonin levels and light exposures. No significant association was found between the measures after adjusting for age and years working night shifts (β = −6.8, p = 0.26). This study demonstrates that healthcare environments can provide sufficient light for visual tasks without affecting melatonin levels, a marker of the circadian system. Our research also highlights the value of personal light-measurement devices when examining the relationship between exposure to light at night, melatonin levels and the potential for circadian disruption. [ABSTRACT FROM AUTHOR]

Objectives: To determine the feasibility of conducting a trial of a novel nighttime lighting system designed to support postural stability in assisted living (AL) residents, and to estimate intervention effectiveness by comparing the incidence of nighttime falls during the novel lighting condition to that in a control condition featuring a standard nightlight., Design: Randomized crossover trial. The intervention consisted of 3 custom-designed linear arrays of amber light-emitting diodes (LEDs) arranged in strips: 1 strip aligned horizontally across the top of the bathroom/entry doorframe containing 68 LEDs and 2 strips of 140 LEDs each aligned vertically down the sides of the doorframe. The control condition was 1 standard nightlight in the bedroom and 1 in the bathroom. Residents were randomized to treatment sequences, receiving each condition for 1-2 quarters., Setting and Participants: Five AL communities serving exclusively residents with dementia or having separate units for residents with dementia, with at least 30 beds and at least 5 residents in private rooms. Residents were eligible if they had dementia, were ambulatory, did not share a bedroom, were not on hospice or expected to die within the year, and were not expected to transfer to another setting within the year., Methods: Outcomes included recruitment, retention, incident falls, and satisfaction., Results: Thirty-eight residents of the 5 communities participated (56% recruitment rate), and 24 family members completed surveys about their satisfaction with the lighting system. Cameras captured falls data for 92% of 8591 resident nights. The incidence density for falls was 34% lower in the intervention condition than the control condition (incidence density ratio 0.66, 95% CI 0.35, 1.22), which did not reach statistical significance (P = .18)., Conclusions and Implications: This low-cost intervention was feasible with high satisfaction. Building on these results, the intervention is being evaluated in a larger clinical trial. A novel lighting system to reduce falls could ultimately benefit millions of older adults across all settings., Competing Interests: Disclosure The authors declare no conflicts of interest., (Copyright © 2024 Post-Acute and Long-Term Care Medical Association. Published by Elsevier Inc. All rights reserved.)

Proper alignment of activity-rest and light-dark patterns allows for healthy bodily functions to occur at optimal times of the day. Disruptions to this alignment may cause poor sleep as well as physical, mental, and cognitive problems. The purpose of this cross-sectional study was to determine if poorer circadian alignment was associated with decreased cognitive functioning among older (> 60 years) participants in the National Health and Nutrition Examination Survey. We utilized actigraphy-based rest-activity and dark-light measurements to calculate phasor magnitude (strength of circadian alignment coupling) and phasor angle (phase difference between activity-rest and light-dark cycles). Multiple linear regression models were used to determine associations of phasor magnitude and angle with performance in various cognitive tests, including Digit Symbol Substitution Test score (DSSS), CERAD Savings Percentage (CSP), and Animal Fluency Test (AFT) score. The results showed that a lower phasor magnitude (which indicates decreased strength of alignment coupling between rest-activity and dark-light cycles) was significantly associated with decreased DSSS (indicating slower processing speed and poorer working memory) when controlling for many important sociodemographic factors. However, this association became non-significant when accounting for sleep duration and total physical activity. Phasor angle did not have a significant association with any of the cognitive scores. Overall, we provided evidence indicating that circadian alignment may be a predictor of cognitive performance. Future studies should investigate whether improving circadian alignment may improve cognitive function and prevent cognitive decline., (© 2024. The Author(s).)

Postoperative cognitive dysfunction (POCD) is a neurological disorder characterized by the emergence of cognitive impairment after surgery. A growing body of literature suggests that the onset of POCD is closely tied to circadian rhythm disruption (CRD). Circadian rhythms are patterns of behavioral and physiological change that repeat themselves at approximately, but not exactly, every 24 h. They are entrained to the 24 h day by the daily light-dark cycle. Postoperative CRD affects cognitive function likely by disrupting sleep architecture, which in turn provokes a host of pathological processes including neuroinflammation, blood-brain barrier disturbances, and glymphatic pathway dysfunction. Therefore, to address the pathogenesis of POCD it is first necessary to correct the dysregulated circadian rhythms that often occur in surgical patients. This narrative review summarizes the evidence for CRD as a key contributor to POCD and concludes with a brief discussion of how circadian-effective hospital lighting can be employed to re-entrain stable and robust circadian rhythms in surgical patients., Competing Interests: The authors declare that the research was conducted in the absence of any commercial or financial relationships that could be construed as a potential conflict of interest., (Copyright © 2024 Campbell and Figueiro.)

Introduction: Sleep disturbance is a hallmark of Alzheimer's disease and related dementias, and caregiver stress caused by patients' nighttime wandering, injuries, and agitation are frequently at the root of decisions to move them to assisted living facilities, where typically dim institutional lighting can further exacerbate their sleep problems. This study explored the effects of a circadian-effective lighting intervention on actigraphic sleep measures and subjective assessments of sleep disturbance, depression, and sleep-disturbed behaviors. Methods: Fourteen older adult (≥60 years) participants (11 females, mean age = 84.1 [SD 8.9]), all diagnosed with moderate to severe dementia and sleep disturbance, were recruited from 3 assisted living and memory care facilities. Following a crossover, placebo-controlled design, 3 different lighting modes were used to deliver high levels of circadian stimulus to the participants' eyes for two 8-week intervention periods in a counter balanced order with a 4-week washout between the study's 2 conditions (dim light control vs. active intervention). Actigraphy and questionnaire data were collected over 7-day assessment periods that preceded (baseline weeks 1 and 9) and concluded (post-intervention week 9 and 22) the intervention periods. Actigraphic outcomes included sleep duration, sleep time, sleep efficiency, sleep start time, and sleep end time. Subjective assessments included the Cornell Scale for Depression in Dementia (CSDD), Pittsburgh Sleep Quality Index (PSQI), and Sleep Disorders Inventory (SDI) instruments. Results: Under the active condition, sleep duration significantly ( p = 0.018) increased and sleep start time significantly ( p = 0.012) advanced after the intervention compared to baseline. Also under the active condition, PSQI ( p = 0.012), CSDD ( p = 0.007), Sleep Disorders Inventory frequency ( p = 0.015), and SDI severity ( p = 0.015) scores were significantly lower after the intervention compared to baseline. Discussion: This study demonstrates that a circadian-effective lighting intervention delivering bright days and dark nights improves measures of sleep and mood in dementia patients living in controlled environments., Competing Interests: The authors declare that the research was conducted in the absence of any commercial or financial relationships that could be construed as a potential conflict of interest., (Copyright © 2023 Figueiro, Pedler, Plitnick, Zecena and Leahy.)

Objective: The alignment between environmental stimuli (e.g., dark, light) and behavior cycles (e.g., rest, activity) is an essential feature of the circadian timing system, a key contributor to metabolic health. However, no previous studies have investigated light-activity alignment in relation to glycemic control in human populations., Research Design and Methods: The analysis included ∼7,000 adults (aged 20-80 years) from the National Health and Nutrition Examination Survey (NHANES) (2011-2014) with actigraphy-measured, multiday, 24-h activity and light data. We used phasor analysis to derive phasor magnitude and phasor angle, which measures coupling strength and phase difference between the activity-rest and light-dark cycles, respectively. We used multinomial logistic regression and multiple linear regression to study phasor magnitude and phasor angle in relation to diabetes (primary outcome) and multiple secondary biomarkers of glycemic control., Results: Lower alignment strength (i.e., a shorter phasor magnitude) and more delayed activity relative to the light cycle (i.e., a larger phasor angle) were both associated with diabetes. Specifically, compared with individuals in the quintiles indicating the most proper alignment (Q5 for phasor magnitude and Q1 for phasor angle), those in the quintiles with the most impaired alignment had a >70% increase in the odds of diabetes for phasor magnitude (odds ratio 1.76 [95% CI 1.39, 2.24]) and for phasor angle (1.73 [1.34, 2.25]). Similar associations were observed for biomarkers for glucose metabolism. The results were generally consistent across diverse sociodemographic and obesity groups., Conclusions: The alignment pattern between 24-h activity-rest and light-dark cycles may be a critical factor in metabolic health., (© 2023 by the American Diabetes Association.)

Background: Older men with the worse alignment of activity and light may have lower levels of cognition and increased rates of cognitive decline., Methods: This cohort consisted of 1 036 older men (81.1 ± 4.6 years) from the MrOS Sleep Study (2009-2012). Light and activity levels were gathered by wrist actigraphy. Phasor analysis was used to quantify the alignment of light-dark and rest-activity patterns (magnitude) and their temporal relationship (angle). Global cognitive function (Modified Mini-Mental State examination [3MS]) and executive function (Trails B test) were measured, then repeated 4.2 ± 0.8 years later. Linear regression models examined the associations of phasor magnitude and angle with cognition and cognitive decline. Models were adjusted for age, clinic, race, education, and season., Results: Smaller phasor magnitude (worse aligned light and activity patterns) was associated with lower initial level and increased decline in executive function. Compared to those with higher phasor magnitude, those with lower magnitude took an average of 11.1 seconds longer to complete the Trails B test (quartile 1 vs quartile 4, p = .02). After follow-up, Trails B completion time increased an average of 5.5 seconds per standard deviation decrease in phasor magnitude (95% confidence interval [CI] 0.7-10.4, p = .03). There were no associations with phasor angle, and none with magnitude and global cognition (3MS)., Conclusion: Among older men, worse alignment of light and activity patterns was associated with worse initial performance and increased decline in executive function, but not related to global cognition. Interventions that improve the alignment of light and activity may slow cognitive decline in older adults., (© The Author(s) 2022. Published by Oxford University Press on behalf of The Gerontological Society of America. All rights reserved. For permissions, please e-mail: journals.permissions@oup.com.)

The advent of electric lighting in the built environment has radically transformed the human experience of light and darkness, which is often insufficient to stimulate and synchronize the circadian system to the day-night cycle. The lack of circadian system entrainment leads to poor sleep and could be an important biophysical mechanism underlying increased incidence of certain types of diseases, including cardiovascular (CV) disease (CVD). This contribution proposes to carve out a niche for including daily exposures to light and darkness among lifestyle factors for reducing the risk and progression of CVD. The fundamental workings of the human circadian system and its primary outputs are described. The discussion then progresses to light's effects on the circadian system and its outputs, and how threats to circadian health pose risks for CV health. The contribution concludes with simple recommendations for incorporating regular, robust daily exposures in lifestyle adjustments to combat CVD risks and progression., Competing Interests: Declaration of Competing Interest None., (Copyright © 2023 Elsevier Inc. All rights reserved.)