Science: Deep-Dive

Introduction to a Deeper Dive on Science: “The Impact of Lighting on Human Circadian Rhythms" – A White Paper

The light that we are exposed to can have significant impacts on our circadian rhythms; these impacts can, in turn, significantly affect our health. Generally, these correlations are true for humans, but can be even more acute in older adults with dementia, as these conditions themselves can be serious disruptors to their circadian cycles.

Over the past two decades, researchers have made significant advances in the understanding of our circadian systems, how these systems are impacted by the lighting conditions we are exposed to, and how we measure these conditions and control these impacts. Most importantly, Blue Iris Labs is focused on how lighting can be used to support our circadian systems and improve health outcomes for healthy populations, as well as those with specific cognitive diseases.

This White Paper, “The Impact of Lighting on Human Circadian Rhythms”, provides an overview of these issues with a special emphasis placed on the measurement and control of lighting to support the health and well-being of those with Alzheimer’s disease and related dementia (ADRD).

Older Adults and Alzheimer’s Disease

Older Americans (65+) numbered 44.7 million in 2013, representing 14.1% of the U.S. population,1 and by 2030 this older population is estimated to grow to about 72 million. Furthermore, this population is living longer; people reaching age 65 have an average life expectancy of an additional 19.3 years.1 Of the 65+ population, 3.4% lived in nursing homes in 2013. If current trends continue, by 2030 this number will rise to about 10%.

Dementia is a progressive, degenerative disease of the brain. There is no known cure, and there are very few effective treatments. Alzheimer’s disease (AD), the most common form of dementia, is the sixth-leading cause of death in the U.S. and the fifth-leading cause of death for those over the age of 65.2 It is projected that 13.8 million Americans will have AD or a related dementia disorder (ADRD) by 2050.2 More than 70% of people with this disease live at home, and family members and friends provide almost 75% percent of the required care.2 As the disease progresses, families are often forced to move loved ones from home to assisted living facilities. Often, the precipitating factor is disturbed sleep-wake (circadian) cycles, where the person with AD/ADRD is awake at night, causing stress and fatigue to caregivers.3

Light therapy has shown great promise as a nonpharmacological treatment in helping to regulate sleep and in improving cognition in older adults with AD/ADRD. Studies have demonstrated that daytime light exposure can consolidate and increase nighttime sleep efficiency, while increasing daytime wakefulness and reducing evening agitation.4, 5

Download a Presentation on “Lighting Intervention for Alzheimer’s Patients”

(Click here) if you want to download Mariana G. Figueiro’s PhD presentation to the AAIC on
“Lighting Intervention for Alzheimer’s Patients”.

Sleep, Circadian Rhythms and Light

The sleep/wake pattern is directly driven by the timing signals generated by the suprachiasmatic nuclei (SCN), which is known to be compromised by aging and AD. Studies have shown a reduced circadian rhythm amplitude after the age of 50.6, 7 It is hypothesized that some of the neural processes involved in entrainment might be dysfunctional or less effective as we age.8 Disturbances in circadian rhythms leading to poor sleep in older adults can be the result of dysfunctional circadian pathways or a pathway that cannot process light information with as much fidelity. Moreover, older adults not only have reduced optical transmission at short wavelengths, which is maximally effective for the circadian system, they also lead a more sedentary indoor lifestyle, with less access to bright light during the day. In fact, research demonstrates that middle-aged adults receive approximately 58 minutes of bright light per day9 while older adults in assisted-living facilities receive bright light for only 35 minutes per day.10 Finally, changes in the amplitude and timing of melatonin and core body temperature rhythms may occur in older adults. Lower amplitude of melatonin rhythms may be associated with reduced sleep efficiency and deterioration of internal circadian rhythms, such as hormone production, alertness, and performance.11, 12, 13

In order to maintain synchronization in the face of these physiological changes, it is necessary both to increase the strength of the light stimulus and to design an intervention that is maximally effective for entraining the circadian systems of those with AD/ADRD. A 24-hour light-dark pattern incident on the retina is the most efficacious stimulus for entraining the circadian system in humans.14 In fact, a carefully orchestrated light-dark pattern has been shown in several controlled studies of older populations, with and without ADRD, to be a powerful nonpharmacological tool to improve sleep efficiency and consolidation.15-17

More recently, work from our collaborators at the Lighting Research Center at Rensselaer Polytechnic Institute (LRC) showed increased sleep efficiency and reduced depression and agitation behavior after only 4 weeks of a tailored lighting intervention (TLI).18 However, current approaches to light therapy for reducing sleep disturbances, depression and agitation in older adults do not consider the complete 24-hour light-dark pattern they experience, nor do they integrate light treatment into a practical delivery system, thus compromising their therapeutic value.19 More importantly, the light dose is never measured because, except for the calibrated personal light exposure devices developed by our collaborators, there is no other personal or ambient light measuring device that allows users or researchers to measure the dose actually experienced by patients.

Our work under our NIH grant aims to address need by building upon a patented lighting control system we have developed20 so that it measures the ambient circadian light and provides feedback to the user and to the lighting in the space to assure the correct dose is being delivered at all times.

Lighting Characteristics Affecting the Circadian System

Light is formally defined as optical radiation reaching the human retina that provides visual sensation. All current light sources and light meters are calibrated based on the characteristics of the light affecting our visual system. There are five important characteristics of light for both the human visual and circadian systems: quantity (or level), spectrum, timing, duration, and distribution. The ideal characteristics for vision are, however, quite different from those that are most effective for stimulating the circadian system. In brief, the quantity of polychromatic white light necessary to activate the circadian system is significantly greater than the amount that activates the visual system (measured through melatonin suppression or phase shift). The spectral sensitivity of the circadian system peaks at short wavelengths,21, 22 while the visual system is most sensitive to the middle wavelength portion of the visible spectrum. Operation of the visual system does not depend significantly on the timing of light exposure; it responds well to a light stimulus at any time of the day or night. However, depending on the timing of light exposure, light can phase advance or phase delay the biological clock.23 In addition, while the visual system responds to a light stimulus very quickly (less than one second), the duration of light exposure needed to affect the circadian system can take much longer. For example, to achieve measurable melatonin suppression from exposure to a moderate amount of light in young adults, the required duration of light exposure is at least 5 to 10 minutes.24, 25 For the visual system, spatial light distribution is critical (e.g., when reading black text on white paper), while the circadian system does not respond to spatial patterns.26, 27 It is also important to note that the short-term history of light exposure affects the circadian system’s sensitivity to light. For example, the higher the exposure to light during the day (e.g., 4 hours per day for one week to outdoor light), the lower the sensitivity of the circadian system to light at night, as measured by nocturnal melatonin suppression.28

Measuring and Characterizing Circadian Light and Circadian Stimulus

In 2005, Rea et al. proposed a mathematical model of human circadian phototransduction based on the neuroanatomy and neurophysiology of the human retina and using published action spectrum data for acute melatonin suppression.29 30 Using this model, the spectral irradiance at the cornea is first converted into circadian light (CLA), which is comparable to photopic illuminance but weighted by the spectral sensitivity of the human circadian system as measured by acute melatonin suppression after a 1-hour exposure. Then the circadian stimulus (CS) is determined, which reflects the effectiveness of the spectrally weighted irradiance at the cornea from threshold (CS = 0.1) to saturation (CS = 0.7). While CS was developed using data from studies that measured acute melatonin suppression, Zeitzer et al.31 showed that acute melatonin suppression and phase shifting of melatonin rhythms followed similar threshold and saturation response characteristics. The model has been successfully used to predict the effectiveness of various light sources for activating the circadian system in laboratory32-37 and field studies.38-42


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