Non-Visual Lighting Effects and Their Impact on Health and Well-Being
Biological, non-visual effects of lighting constitute a new field within lighting research and education. In addition to visual effects, light is known to affect other biological rhythms, most notably the circadian system, which generates and regulates a number of rhythms that run with a period close to 24 h. Light/dark patterns incident on the retina are the major synchronizer of circadian rhythms to the 24-h solar day. Lighting characteristics (quantity, spectrum, timing, duration, and distribution) affecting the visual and circadian systems differ. Symptoms of circadian sleep disorders, such as seasonal affective disorder, jet lag, and delayed sleep phase disorder, can be mitigated by timed light exposure.
The neurophysiology and neuroanatomy of the human visual system is largely understood: vision is the result of complex interactions between light sources, objects and surfaces, the eye, and the brain. Lighting engineers and practitioners have developed technologies, standards, measurement devices, and applications based on this knowledge, and one of the primary goals of lighting design is to increase performance and productivity by improving vision and perception. Recent scientific discoveries, however, have added another dimension to lighting: namely, that light is not just for vision anymore. Specifically, scientists have discovered that daily light/dark patterns reaching the retina (back of the eye) play an important role in human health and well-being because they regulate our bodies’ circadian rhythms. Circadian rhythms are every rhythm in our body that repeat approximately every 24 h. The circadian system is profoundly important for many human behaviors, as well as human well-being.
This entry provides basic knowledge about the non-visual effects of light entering the eye, focusing exclusively on the effects of light on human circadian and diurnal rhythms. Also discussed here are the lighting characteristics that affect these rhythms, as well as applications in which light/dark cycles have been experimentally shown to affect them.
Overview of the Circadian System
The Earth rotates around its axis, and, as a result, all creatures exposed to daylight on Earth experience a 24-h cycle of light and dark. Living organisms have adapted to this daily rotation of the Earth by developing biological rhythms that repeat at approximately 24-h intervals . These rhythms are called circadian rhythms, from the Latin circa (about) and dies (day).
Circadian rhythms are generated internally by the body, yet they are constantly aligned with the environment by factors that are external to the body, mainly light/dark cycles. In mammals, circadian rhythms are regulated by an internal biological clock located in the suprachiasmatic nuclei (SCN) of the hypothalamus of the brain. The biological clock in humans has a natural period that is slightly greater than 24 h, and environmental cues, such as light/dark cycles, social activities, and meal times, can reset and synchronize the clock daily, ensuring that our behavioral and physiological rhythms are synchronized with the external environment.
Output rhythms are behavioral and physiological rhythms regulated by the biological clock, such as the sleep/wake cycle, alertness, core body temperature, locomotor activity, feeding and drinking behavior, and hormone production. Of special interest here are the sleep/wake cycle, alertness, core body temperature, and hormone production, especially melatonin production.
Sleep/Wake Cycle and Alertness
The sleep/wake cycle is one of the most prominent circadian rhythms. We are under the influence of two opposing systems: the sleep drive (homeostatic) and the alerting force (circadian). The sleep drive and the alerting force are distinct forces, independent from each other, though complementary, ensuring that we are asleep at night and awake during the day. The sleep drive, for example, is low when we get up, increasing steadily throughout the waking day, before diminishing rapidly within the first hours of sleep. The alerting force is regulated by the biological clock and follows a circadian rhythm, reaching a peak during the early evening and a trough during the second half of the night. The interaction between the sleep drive and the alerting force determines when we fall asleep and how well we sleep at night. Alertness is strongly correlated with the sleep/wake cycle.
Core Body Temperature
Core body temperature also follows a circadian pattern. It is high during the day, reaching a peak in the early evening, and low at night, reaching its low point about 2 h before one naturally wakes up. Core body temperature is typically in phase with alertness and has a negative correlation with the hormone melatonin.
Although there is evidence that the biological clock influences a variety of hormones, this discussion will be limited to melatonin production by the pineal gland. The pineal gland is located near the center of the brain, is about the size of a pea, and has a shape resembling that of a pinecone. It is believed that the primary function of the pineal is to convey light/dark information to the body via secretions of the hormone melatonin . Melatonin is easily absorbed into the bloodstream, which makes it an ideal chemical messenger of time of day information for the entire body. Melatonin also participates in the transmission of information concerning day length or photoperiod for the organization of seasonal responses in animals (e.g., breeding).
Melatonin is produced during the subjective night (i.e., inactive period in diurnal species and active period in nocturnal species) and under conditions of darkness. As a result, melatonin levels are low during the day and high at night (in the dark). Although not yet well established, many researchers believe that melatonin can induce sleep in diurnal mammals, including humans, by acting on the SCN and reducing the alerting force, or the “wake-promoting” signal sent by the biological clock during the daytime. In addition, melatonin has an inverse relationship with core body temperature, which is closely linked to waking and sleep times. Peak melatonin levels typically occur slightly before core body temperature bottoms out. When external cues are absent, melatonin starts being endogenously produced by the body at around 9–10 p.m. (or about 2 h prior to sleeping) and stops being produced at around 7–9 a.m. Maximum melatonin levels occur between 2 and 4 a.m. .
Regulation of Circadian Rhythms by Light
The 24-h light/dark cycle is the biological clock’s main synchronizer to the solar day. Light can phase advance or phase delay human circadian rhythms, depending upon when it is applied . For example, light that is applied before the minimum core body temperature, which is reached approximately 1.5–2.5 h before we naturally awaken, will delay the clock (e.g., one will wake up later the following day), and light applied after minimum core body temperature is reached will advance the clock (e.g., one will wake up earlier the following day). Although light is the main synchronizer of the biological clock to the solar day, it is not the only one. Exercise, social activities, timing of the sleep/wake cycle, and scheduled meals have also been shown to shift and synchronize the clock, although their impact on circadian rhythms seems to be weaker than the impact of light/dark cycles.
Lighting Characteristics Affecting Circadian Rhythms
Measuring Light for the Circadian System
In the following section, practical situations in which the effects of light on the circadian system could be beneficial to human health and well-being will be discussed. It is important to note that light is not currently known to cure any diseases. Further, individual differences need to be considered when designing lighting for the circadian system, so the lighting recommendations discussed below should be used as a framework only, and adjustments must be considered. It is also important to emphasize that few of the studies using light as a non-pharmacological treatment to various diseases and disorders reported the spectral power distribution of the light source, which makes it difficult to quantify the impact of light on the retina for circadian effectiveness. Consequently, generalizations from these studies are often more qualitative (e.g., bright vs. dim) than quantitative.
Tentative application tips will be made and the quantity, spectrum, duration, and timing of the light presented at the eye will be discussed. In order to simplify and standardize the tips offered here, bright white light will be defined as 1-h exposure of at least 600 lx measured at the eye level from a 6,500 K light source, unless otherwise stated in the text. This light level is about six times the amount of light one typically finds in an office environment without a window. “Blue” light is defined here as 1-h exposure to 40 lx at the cornea from a narrowband light source peaking at 470 nm. If duration of exposure is increased, light levels may be decreased, but it is recommended that it is never below 300 lx at the cornea for the white light and 20 lx at the cornea for the blue light. The lighting recommendations offered here are based on current knowledge from research findings, but results of future research will refine these values.
Seasonal Affective Disorder
Seasonal affective disorder (SAD) is a subtype of depression, with episodes occurring during winter months and remitting during summer months. Symptoms of SAD include depression, hypersomnia, and weight gain due to increased carbohydrate cravings, social withdrawal, and even suicidal thoughts. It is believed that because daylight availability decreases in the winter at high latitudes, the number of people experiencing SAD increases as the latitude increases. “The winter blues” is an even more common subtype of SAD. The mechanisms of SAD are still unknown, and there are several competing hypotheses as to what causes SAD and how light can be used as a treatment. One of these is that late daybreak during winter months delays the circadian rhythms of those more susceptible to SAD; in this case, morning light is believed to be effective in treating symptoms of SAD. Another hypothesis is that the overall melatonin production of those suffering from SAD is greater during winter months than during summer months, which extends the amount of time during the 24-h day that their bodies think it is nighttime. In this case, light in the early morning or evening is recommended.
Encourage SAD patients to go for a half-hour walk outdoors in the morning (right after daybreak).
Expose individuals to bright white light (or “blue” light) in the morning or evening.
Eastward travel: Upon arrival at your destination, avoid bright white light (including daylight) during the morning, until about noon local time, and seek exposure to bright white (or “blue”) light in the afternoon. Upon arrival, wear glasses that filter out radiation below 520 nm (“orange” glasses; Fig. 4) to avoid circadian effective light because the circadian system is maximally sensitive to short-wavelength (“blue”) light.
- Westward travel: Upon arrival at your destination, seek exposure to bright white light (or “blue” light) during the daylight hours and avoid bright white light in the evening.
Delayed Sleep Phase Disorder
Expose individuals to bright white (or “blue”) light in the morning, after the minimum core body temperature. Note that minimum core body temperature of people with DSPD may occur as late as 9 a.m.; therefore, an understanding of the characteristics of the biological clock of those using the light treatment is needed before recommending the timing of the light exposure. Just as a tip, minimum core body temperature generally occurs 1.5–2.5 h prior to waking without an alarm clock.
Wear glasses that filter out radiation below 520 nm (“orange” goggles; Fig. 4) in the evening and in the morning, before minimum core body temperature is reached. Because the circadian system is maximally sensitive to short-wavelength light, elimination of optical radiation below 520 nm at these specific times will reduce the chances of phase shifting the biological clock to a later time, aggravating symptoms of DSPS even further.
- Avoid the use of very bright self-luminous electronic devices (e.g., computer screens, tablets, and cell phones) during the evening hours. At a minimum, dim them down or filter their screens with “orange” filters.
Sleep in Older Adults
For older adults who want to fall asleep later in the evening, expose them to bright white (or “blue”) light in the late afternoon/early evening (before 7 p.m.), before minimum core body temperature. It is recommended that the duration of the exposure be increased to 2 h or the quantity of light be increased by a factor of two because of the optical changes to the aging eye.
Wear glasses that filter out radiation below 520 nm (“orange” goggles) in the very early morning hours (before 7–8 a.m.).
For AD patients who do not have a consistent sleep/wake schedule, expose them to bright white (or “blue”) light at any time during the day and reduce evening light exposures. Be consistent with the timing of the light exposure, however. Because the circadian phase of AD patients is not known and because they tend to remain in continuous dim light all day, it is expected that a robust light/dark pattern will positively impact their sleep.
Install safe nightlighting systems that provide low-level illumination and perceptual (horizontal/vertical) cues to aid in controlling postural control and stability (Fig. 5).
Entrainment for the General Population
As shown in the examples above, lighting can help reduce or mitigate symptoms associated with various circadian sleep disorders. Lighting can also aid in maintaining entrainment in the general, healthy population. Because humans have a biological clock that runs with a period slightly greater than 24 h, we need daily morning light exposure to maintain entrainment to the solar day. In winter months, some of us can go to and come back from work or school in darkness; therefore lighting in our work environments should provide enough circadian stimulation during at least the morning hours. Although the link between circadian entrainment and productivity is yet to be established in larger, clinical studies, one study  to date showed that compared with a 4,000 K light source, blue-enriched white light (17,000 K) improved subjective measures of alertness, positive mood, performance, evening fatigue, irritability, concentration, and eye discomfort. Daytime sleepiness was reduced, and the quality of subjective nocturnal sleep was improved under blue-enriched white light. Although future studies should confirm and extend these results, it is suggested that a dynamic lighting system that delivers bright white light (or “blue” light) to office workers during the morning hours can help maintain entrainment and possibly improve well-being, alertness, and performance, especially in winter months, when duration of daylight availability is short .
This overview of the impact of light on the circadian system and its effects on our health and well-being underscores the importance of developing a new framework for lighting practices that includes not only those lighting characteristics that affect the visual system but also those that affect the circadian system. Because there are great differences between the two systems’ responses to the quantity of light, its spectral composition, spatial distribution, timing, and duration, generalizations about “quality lighting” will have to be assessed by two very different sets of criteria in the future. Although the information and recommendations presented here will certainly be refined as more research is undertaken, little progress will be made in delivering “healthy lighting” to society until researchers and practitioners begin to consider, measure, calculate, and control the fundamental characteristics of light for the circadian system. Hopefully, this entry will serve as an important step toward that goal.
Finally, it is important to remember that we have no practical way to intuitively know when and what type of light we need for circadian entrainment. Like other medical monitors where we have no conscious access to what we need for good health (e.g., glucose monitors for diabetes), we need technologies to measure and track the state of our circadian system. A wireless technology connecting the Daysimeter to a smartphone is currently under development. A smartphone (or similar technology) will be able to interpret the light/dark patterns to which a person is exposed and then be able to recommend, based on models of human circadian entrainment , both current and future light exposure patterns for the user to maintain circadian entrainment. One day, the smartphone might be able to interface with the building lighting system itself so that children in a school, nurses in a hospital, or people at home would be exposed to the proper lighting conditions for maintaining well-being and for increasing productivity.
The author would like to acknowledge the projects’ sponsors (National Institute on Aging, National Institute of Nursing Research, National Institute on Drug Abuse, National Cancer Institute, and Office of Naval Research). Mark Rea, PhD, of the Lighting Research Center is acknowledged for his technical assistance, and Nicholas Hanford of Rensselaer Polytechnic Institute is acknowledged for his editorial assistance.
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