Biological Effects of Light: Can Self-Luminous Displays Play a Role?
Light is the major synchronizer of circadian rhythms with local time on earth, and can also promote alertness in humans. Self-luminous displays built in a variety of form factors can be used to optimize circadian synchronization and alertness.
by Mariana G. Figueiro
Humans have a biological clock located in the brain’s hypothalamus that generates and regulates circadian rhythms, which repeat in a cycle approximately every 24 hours. These rhythms include processes such as sleeping and waking, body temperature regulation, hormone production, and alertness. Light is the main input for synchronizing the biological clock to the solar day. If we are not exposed to a sufficient amount of light in the appropriate spectrum, for a sufficient amount of time, and at the right time, our biological clock becomes desynchronized with the solar day and we may experience disturbances in physiological functions, neurobehavioral performance, and sleep.1,2
The circadian and homeostatic systems influence the sleep–wake cycle. Sleep homeostasis (i.e., regulation of the need for sleep) increases with time awake, contributing to strong pressure to sleep at night. The circadian system sends an alerting signal to the body during the day, counteracting the increase of sleep pressure with time spent awake, and sends a sleeping signal during the night, promoting a consolidated night of sleep. A person is more likely to experience a good night of sleep when the circadian and homeostatic systems are aligned. Misalignment between these two systems can also lead to health problems such as metabolic and cardiovascular diseases, depression, and cancer.
Another well-known circadian rhythm is the cyclical production of melatonin, a hormone that is produced by the pineal gland at night and under conditions of darkness. For diurnal species, such as humans, melatonin signals that it is time to sleep. The timing of the onset of melatonin secretion in the evening, referred to as dim light melatonin onset (DLMO), occurs approximately two hours prior to natural bedtimes and is used as a marker of the biological clock. Evening exposure to sufficient amounts of light will delay DLMO, thereby delaying sleep times.
Lighting Characteristics Affecting the Circadian System. The characteristics of light affecting the circadian system, as measured by acute melatonin suppression and phase shifting of DLMO, are different from those affecting the performance of visual tasks like reading a book. Unlike the visual system, which consistently responds to light at any time of day or night, the circadian system’s response varies according to the timing and duration of light exposure, one’s personal history of such exposure, the light’s spectral properties, and the amount or level of light received. After bedtime, for example, a warm-color nightlight delivering low levels of light will permit safe navigation without the need to increase room and hallway lighting, but will not suppress melatonin production. On the other hand, because humans are especially sensitive to short-wavelength (e.g., 460 nm) “blue” light,3–5 exposure to high levels of such light in the middle of the night will stimulate the circadian system at a time when it should be providing cues for melatonin production and the maintenance of proper metabolic functioning.
Light’s effects on the circadian system vary over the course of the 24-hour day. Morning light from any source habitually received after the trough of core body temperature, which tends to occur in the second half of thenight, will advance the timing of the sleep cycle to come. Light received in the evening (e.g., from self-luminous displays) prior to the trough of core body temperature will delay the onset of sleep.6
Our research has also shown that it is important to accurately measure light exposure over the 24-hour day, as opposed to taking just a “snapshot” measurement of light exposure at a specific place and time.7,8 The circadian system appears to keep track of light exposure, and knowing an individual’s history of light exposure over the past 24 hours can help determine the best light prescription for the next 24 hours.9 For this reason, a light treatment or intervention designed to promote earlier bedtimes should not be limited to providing exposure to blue light in the morning, but should instead control the total circadian light exposure during all waking hours.
Developing Circadian Light Metrics. Architectural lighting has traditionally been designed and specified primarily to meet the needs of the visual system, but it has become apparent that lighting design for the circadian system requires different metrics. To that end, the Lighting Research Center at Rensselaer Polytechnic Institute (LRC) has developed the circadian light (CLA) and circadian stimulus (CS) metrics for characterizing both the spectral and absolute sensitivities of the human circadian system.10,11 These metrics are based upon fundamental knowledge of retinal physiology as well as the measured operating characteristics of circadian phototransduction (i.e., the conversion of light into electrical signals received by the biological clock), from response threshold to saturation. The CLA metric represents irradiance weighted by the spectral sensitivity of every retinal phototransduction mechanism that stimulates the biological clock, as measured by nocturnal melatonin suppression. The CS metric is a transformation of CLA into relative units from 0.1 (the threshold for circadian system activation) to 0.7 (response saturation), and is directly proportional to nocturnal melatonin suppression after a one-hour exposure (10 to 70 percent). The units and quantities for these metrics have also been published.12
The LRC has released an open-access CS calculatora to help lighting professionals increase the potential for circadian-effective light exposure in designs for architectural spaces. Developed by LRC researchers, the calculator is designed to facilitate the calculation of CLA and CS for several example light source spectra as well as for user-supplied light-source spectra. To obtain the CLA and CS values for a given source, the user simply selects the supplied source and its spectral power distribution (i.e., the radiant power emitted by a light source as a function of its spectral power distribution), or enters their own unique source data, and then enters the illuminance value (in lux) measured at the eye.
Self-Luminous Displays and the Circadian System
The use of self-luminous displays in the evening and nighttime hours may deliver sufficient light to the eye to suppress melatonin and delay sleep times. In fact, many recent reports suggest that the use of self-luminous displays before bed curtails sleep duration. We have completed three laboratory studies and one field study investigating the significant impact of various self-luminous displays on melatonin suppression.
Potential Detrimental Effects of Light from Self-Luminous Displays. The three lab studies followed a broadly similar protocol involving three or four experimental conditions while viewing cathode ray tube (CRT) computer monitors,13 tablet computers,14 and LED-backlit LCD flat-panel TVs15 over periods ranging from 90 minutes (TVs) to two hours (monitors and tablets) on each night of the study (separated by at least one week). Salivary melatonin samples were taken from participants at the beginning, midpoint, and conclusion of each experimental session. In the CRT monitor and tablet studies, the participants viewed the devices: (1) alone without an intervention, (2) while wearing orange-tinted glasses that filtered out optical radiation approximately <525 nm, and (3) while wearing goggles equipped with blue LEDs that directed 40 lux of 470-nm or 475-nm light at the participants’ eyes. For the TV experiment, participants wore orange-tinted glasses on one night and viewed the same TVs set at progressively increasing correlated color temperatures (CCTs) (2,700 K, 6,500 K, and 12,000 K) on the remaining three successive nights of the study.
As our researchers expected for the CRT monitor and tablet experiments, exposure to the blue-light goggles significantly suppressed melatonin. Also as expected, viewing both devices without the orange-tinted glasses also suppressed melatonin. A one-hour viewing of the CRT monitor suppressed melatonin by a median value of 11 percent. Consistent with the CS model’s predictions, suppression levels after a one-hour exposure to the tablets-only condition were not statistically different from zero. This difference reached significance after two hours, however, with an average melatonin suppression of 22 percent. The TV experiment showed that sitting six or nine feet from the device resulted in no significant suppression of melatonin after a 90-minute exposure, irrespective of the device’s CCT setting, compared to the orange-tinted glasses control.
Using a protocol adapted from the lab studies, we collaborated with a high school student researcher to collect field data on the effects of self-luminous displays on melatonin suppression in high school students (aged 15–17 years) during the evening.16 On two separate nights, the participants viewed their personal devices with and without orange-tinted glasses while also wearing a Daysimeter,17 a device that measures personal light exposures and rest–activity levels, from the time they woke that day until the end of that night’s data collection period. They also collected saliva samples at hourly intervals during the three-hour data collection period. On the first night, the participants wore the orange-tinted glasses for the entire three-hour data collection period. On the second study night, they wore the orange-tinted glasses only during the first hour, and then viewed their personal devices without the orange-tinted glasses during the remaining two hours of the data collection period.
Compared to when participants wore the orange-tinted glasses, viewing their devices without the glasses resulted in mean melatonin suppression of 23 percent after one hour and 38 percent after a two-hour exposure. The Daysimeters, however, indicated that the participants received extremely low circadian stimulus (CS = 0.01, equivalent to a 1 percent predicted melatonin suppression) during the experiment. By comparison, mean melatonin suppression among the college students (mean ± SD age = 28 ± 9.9 years) who wore Daysimeters during the CRT monitor lab study was 16 percent after a one-hour exposure that exposed them to a mean CS of 0.19, which is a log unit greater than that recorded for the adolescents. These data suggest that adolescents are much more sensitive to acute melatonin suppression from light in the evening than college students.
Potential Beneficial Effects of Light from Self-Luminous Displays. Light from self-luminous displays, if provided at sufficient levels and delivered at the right time, can benefit outcomes of sleep and mood. Alzheimer’s disease patients, for example, suffer from conditions such as disrupted sleep, depression, and agitated behavior. Some of these problems are associated with age-related changes to the eye that permit less light (especially short-wavelength light) to reach the retinae, thereby reducing input to the biological clock. Alzheimer’s disease patients can also experience neuronal degeneration that reduces the biological clock’s sensitivity to light, and the situation is worsened by the dim, constantly energized lighted environment typical of nursing homes and assisted-living facilities.
Our research has demonstrated that self-luminous tables can be used to improve sleep, behavior, and mood in Alzheimer’s disease patients.18 While our previous research had shown that a robust 24-hour pattern of light and dark improves sleep, while also reducing depression and agitation in this population,19,20 a major challenge remained in delivering light to patients’ eyes. Given that it is common practice in these facilities to gather residents in a common area during the day, frequently in groups around tables, we hypothesized that a practical way to deliver the light would be to install LED lighting in those tables (Fig. 1). In a pilot study, we worked with Sharp Corp. to build tables incorporating 70-in. LED edge-lit LCD TVs that delivered a large amount of CS by providing 2,000 lx of 25,000 K light at the eye.
Fig. 1: A light table used to deliver circadian-effective light to Alzheimer’s disease patients in a nursing home is shown with the author.
In the facility where we conducted this research, residents typically had their meals and remained seated at the light tables for the entire day in a room that provided no direct access to daylight. The tables were programmed to operate from 7:00 am to 6:00 pm. Baseline data were collected during the first study week, and the residents experienced the daily light exposure for four weeks. Post-intervention data were collected at the beginning of the fourth week. The study’s outcome measures included objective (actigraphy) and subjective sleep (Pittsburgh Sleep Quality Index [PSQI]),21 depression (Cornell Scale for Depression in Dementia [CSDD]),22 and agitation (Cohen-Mansfield Agitation Index [CMAI])23 scores. Results from the six residents participating in the experiment showed a significant improvement in sleep quality and a reduction in depression and agitation scores. Data collection using additional experimental participants is under way.
Can Displays Play a Role in Light and Health?
Lighting systems using color-tunable LEDs are now widely available on the market, and are typically installed in ceilings to deliver direct, direct-indirect, or indirect light into the space. As light needs to reach the back of the eye to support the circadian system, however, and ceiling lights are not always ideal for accomplishing that, we have found that self-luminous displays can more practically and effectively deliver circadian light to users’ eyes. Effective form factors include light tables (see Fig. 1) and vertical displays attached to walls or office cubicle partitions (Fig. 2), both of which could also be used to deliver information, or personal light therapy goggles (Fig. 3). The light table used in the Alzheimer’s disease study, for example, could also function as a touch screen that displays games or entertainment to attract attention and ensure that light is reaching the back of the recipients’ eyes. We are presently developing personal light sensors that can provide a
prescription for when to deliver, and when not to deliver, circadian-effective light to individual users. These sensors can then communicate wirelessly with the self-luminous displays to ensure that the appropriate light is being delivered at the right time.
Fig. 2: These renderings show how self-luminous displays might be used to deliver circadian effective (blue) or circadian ineffective (red) light in light oases, depending on time of day.
Fig. 3: Personal light-therapy goggles delivering red light can be used to promote alertness in the afternoon and evening without disrupting the circadian phase. Blue-light goggles may also be used early in the day to promote circadian entrainment.
Potential Impact of Research
Light–dark patterns received at the back of the eye are the major synchronizers of the biological clock to the local time on Earth. Self-luminous displays can be designed and used to practically and effectively deliver light to promote circadian entrainment or deliver an alerting stimulus without affecting circadian phase. Taking into account that the timing of exposure also needs to be considered, new sensors that are now being developed can determine the appropriate time for delivery of the prescribed light. Manufacturers are invited to use the open-access CS calculator to identify the optimum spectrum and light level needed to successfully deliver the desired circadian-effective light. These are exciting times, and self-luminous displays can play an important role in delivering the right light at the right time to promote health and well-being among all age groups.
What Can Developers and Display Engineers Do Now?
A crucial next step in advancing display technologies for circadian health is for developers and engineers to rigorously quantify the effects of all new applications and devices. Several applications for promoting circadian entrainment or reducing circadian disruption among users of self-luminous displays have come to market over the past several years, for example, but to date their effectiveness remains unproven at best.
The LRC recently investigated one of these applications, Apple Inc.’s Night Shift, which offers users display adjustment options ranging from a “less warm” (high CCT, 5997 K as measured via spectrometer) setting to a “more warm” (low CCT, 2837 K as measured via spectrometer) setting, the latter being designed to minimize stimulation of the circadian system. Night Shift also offers a time setting, which activates the low-CCT display mode at a user-defined interval before bedtime in accordance with proven sleep hygiene principles. Our study involving 12 young adult participants who viewed iPads between 10:30 pm and 1:00 am, however, found no significant difference between the two modes in terms of acute melatonin suppression when the device was set to full brightness.24 We concluded that regardless of Night Shift setting, selecting low light levels, limiting device use to one-hour sessions, and avoiding displays at least two hours before bedtime would be more effective for reducing nighttime CS exposures.
Use of the CLA and CS metrics could help to avoid these pitfalls and provide prospective users with products that are proven to deliver what is promised, and far more importantly, what is urgently needed in our around-the-clock lighted environment. Unfortunately, at least for anyone who might be interested in an easy solution, beyond these metrics and sound scientific practice there exists no tried and true formula for success other than continued research and development. For only then will we see devices that strike an optimal balance between CS exposure and factors that are very important for user satisfaction and device appeal, such as color rendering. By quantifying CS and using the data to design innovative products that can do things like track exposure and deliver personalized, 24-hour lighting prescriptions, we will then be able to provide users with devices that are great to use while also being better for them.
The author would like to acknowledge Sharp Laboratories of America, which funded the self-luminous displays studies, and the National Institute on Aging (R01AG034157), which funded the Alzheimer’s disease studies. The author would also like to thank David Pedler for his technical and editorial assistance.
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