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Volume 2 Number 3
Copyright @1998 Rensselaer Polytechnic Institute

Light, Sight, and Photobiology

For more than 100 years, lighting research has pursued the obvious - that with light we can see, without it we cannot. This pursuit has produced a dramatic increase in the understanding of how lighting affects vision and led to the creation of a deluge of lighting equipment. Lighting designers and engineers assemble this equipment into the lighting installations that make modern life possible.

Circadian Lighting System installation at Northeast Utilities, Connecticut Yankee Atomic Power Company. The system comprises dimmable lighting fixtures, an electronic control unit, and software to regulate the intensity and timing of light,which shifts human biological rhythms. Courtesty of ShiftWork Systems, Inc., Cambridge, MA.

Lighting installations can be divided into two types - functional and inspirational. Primarily, functional installations enable people to see what they need to see, quickly, easily, and without discomfort. Examples of functional lighting installations are found in offices, factories, warehouses and motor vehicles.

Inspirational lighting installations ensure that people can see what designers want them to see; these installations attempt to create specific impressions and, therefore, generate specific feelings in the viewer. Examples of inspirational lighting installations are found in churches, restaurants, shops and hotels, and outdoors as floodlit buildings or urban enhancement projects.

We could argue that functional installations are engineered based primarily on science, while inspirational installations are based on art. Regardless of how the study of lighting is described, one thing is undeniable - it is based on the sense of vision.

More than vision?

Pursuing the effect of light on vision, have we - if you will excuse the pun - lost sight of other ways that light affects humans, namely its effects on biological functions other than vision?

That light can have such effects has been known for years. What has not been fully appreciated is how fundamental these effects are, how many aspects of everyday life are touched by them, and how much they can influence a person's quality of life.

Light and biological rhythms

Lighting designers will be soon faced with a basic question. "Is the fundamental objective of lighting to provide an appropriate level of visibility without discomfort, or should we consider the photobiological effects of lighting as equally, and sometimes more, important?"

To answer that question we need to consider the non-visual effects of light exposure on human physiology. The most fundamental and the best understood of these effects are associated with biological rhythms.

Biological rhythms are conventionally divided into ultradian, which have a period of much less than 24 hours; infradian, which last much longer than 24 hours; and circadian, which cycle about every 24 hours.

Tick, tock

Circadian rhythms are the biological rhythms that are most clearly affected by exposure to light. They can be studied using many physiological variables such as the daily patterns of core body temperature, level of melatonin, urine production, cortex activity, and alertness. In addition, they can be observed in the sleep/wake cycle. See Figure 1.

Even in subjects who were kept secluded in a room with no windows or clocks to provide information about the time or the length of day, circadian rhythms persist - but the length of the cycle expands to more than 24 hours. Exposure to light is the most important stimulus that synchronizes circadian rhythms to 24 hours.

Light influence

How is this synchronization by light achieved? For some creatures, such as chickens, the answer is exposure to light directly on the brain through the skull. In humans, synchronization is achieved by exposure to light through the eye.

Light reaching the retina of the eye is converted into electrical signals that are transmitted by the optic nerve. Most of these signals end up in the visual cortex of the brain and produce our sense of vision. However, some of the nerve fibers split off from the optic nerve soon after leaving the eye and send signals to the suprachiasmatic nucleus, which is the area of the brain where the main clock for the human body resides.

It is important to appreciate that this transfer of signals from the retina to the suprachiasmatic nucleus is not a "branch line" of the visual system. It is independent and much older in terms of development than the visual system.

The signals that reach the suprachiasmatic nucleus emanate equally from all parts of the retina without attempting to preserve their point of origin. In other words, the nerve fibers and signals do not attempt to preserve the image of the outside world reaching the retina. In contrast, most of the visual cortex is devoted to the central two degrees of the retina and the projections from the retina are organized to preserve the retinal image.

The visual system is an image-processing system. The circadian system is a retinal-illumination detector.

What lighting works?

Understanding that signals from the retina are important for synchronizing circadian rhythms, what characteristics of light are needed to achieve synchronization? The answer can be given in terms of "how much" and "when?"

One reason that the lighting community lost sight of the photobiological effects of light is that for many years scientists did not believe that such effects occurred in humans at the illuminances typically used indoors. Research-ers thought that illuminances of at least 2000 lx were necessary to have any photobiological effect. Recently it has been shown that this is not true, depending upon the time of day that the light is administered.

Using core body temperature as a marker and an illuminance of 10-15 lx as a steady-state condition, researchers at Harvard Medical School have shown that the phase shift in core body temperature following exposure to a light pulse at a given time is directly proportional to the cube root of the illuminance at the eye.

These results demonstrate that significant phase shifts in core body temperature can occur at illuminances as low as 180 lx and certainly at illuminances representative of interior lighting. And where core body temperature leads, other circadian rhythms follow.

As for "when," the effect of a light pulse producing a much higher retinal illumination in constant low-illuminance conditions is well established.

Phase response curve

Figure 2 shows the light phase response curve for core body temperature. This curve indicates that the effect of exposure to a light pulse depends heavily on the time of exposure. If the light pulse happens during the day, it causes very little phase shift. If the light pulse happens just before the lowest point in core body temperature, then the phase is maximally delayed. If the light pulse happens immediately after the lowest core body temperature, the phase is maximally advanced. Thus, the direction of phase shift depends upon timing. The magnitude of phase shift depends upon both the timing and the magnitude of the light pulse; at any given time, the brighter the pulse, the larger the phase shift.

Night lights

Do these results have any practical value beyond their interest to physiologists and lighting aficionados? The answer is yes - for some situations.

For example, one case where it is clearly of interest is shift work. We are becoming a 24-hour society. In the United States and Europe, approximately 20 percent of the workforce do shift work, usually some form of rotating-shift system.

Anyone who has worked a rotating-shift knows the difficulty of working on what is called the graveyard shift (typically, midnight to 8:00 a.m.). Basically, you are trying to work when your body is telling you to sleep, and trying to sleep when everyone else is awake.

Rapidly adjusting the sleep-wake cycle at the start and end of a series of night shifts, by using light to shift the phase of circadian rhythms, can alleviate this situation. Light exposure enables the worker's wake period to adjust quickly to the nighttime at the start of the series of night shifts and then rapidly changes the wake period back to daytime at the end of the series of night shifts.

Such phase shifts can make night-shift work more bearable for workers and reduce accidents as workers drive home in the morning. To make an economic case for the use of the photobiological effects of light on night-shift workers, however, it is necessary to demonstrate that being more awake at night affects task performance.

Over the last few years, a number of laboratory experiments, among them those reported by S. Campbell and D. Dawson in Psychology and Behavior, and J. French, et. al., in the Annual Review of Chromopharmacology, have demonstrated that exposure to bright light during the night does have such effects, not on how well we can see, but rather on how well we can think.

Following exposure to bright light, subjects who underwent cognitive performance tests showed improvements in complex cognitive tasks requiring logical reasoning and short- term memory.

Knowledge of these effects has led to the installation of lighting that provides high illuminances at times designed to shift circadian rhythms in a number of power station and chemical plant control rooms, where alert judgment can have major safety and economic impact.

The jet set

A common experience of unsynchronized circadian rhythms is called jet lag. People who have traveled across several time zones know the feeling of being out of step with the pattern of life at their destination. Timed light exposure is important for treating jet lag in business people, airline flight crews, and long distance tourists. The phase response curve can predict the best times for light and dark exposure in order to rapidly shift a person's circadian rhythm and, in particular, the sleep/wake cycle.

Special problems for the elderly

The timing and duration of sleep affects many more people than jet lag. The elderly frequently complain of early morning awakening and reduced daytime alertness.

Researchers believe that two factors contribute to these phenomena in the elderly: a decrease in the number of neurons in the suprachiasmatic nucleus and a reduction in exposure to light.

When suprachiasmatic neurons are destroyed in young animals, the amplitude and period of circadian rhythms diminish, much as they diminish with advancing age. The effect of decreased amplitude is that there is less difference between the states of sleep and wakefulness. The effects of a reduced circadian rhythm period is to desynchronize circadian rhythms.

As for everyday light exposure, field measurements reported by R.J. Cole, et. al., in the Journal of Biological Rhythms indicate that many older people do not stay outside for long periods and, because their homes are poorly lit, they have little exposure to high illuminances. This implies that many of the elderly would benefit from exposure to higher illuminances in the evening.

The phase response curve indicates that such exposure would delay the phase of the sleep/wake circadian rhythm so that people would be less likely to awaken in the early morning. Exposure to higher illuminances might also increase the amplitude of the sleep/wake circadian rhythm, thereby reducing the tendency to sleep during the day.

A good night's sleep

Another finding, described by A. Satlin, et. al., in the 1992 American Journal of Psychiatry, and by E.J.W. van Someren, et.al., in Sleep Research Abstracts, that is less clearly related to circadian physiology is that exposure to bright light during the day can make the rest-activity cycles of some people with Alzheimer's disease more regular. Typically, Alzheimer's patients have very irregular periods of rest and activity, but the people who care for them do not. Therefore, caregivers often have their sleep interrupted, leading to cumulative sleep deprivation and a feeling of being unable to handle difficult situations. Providing higher illuminances in places where Alzheimer's patients are cared for, hence making their rest-activity cycles more regular, might make ministering to these patients easier.

Waking up workers

The effects of light exposure on the synchronization of circadian rhythms are relatively well understood. We know much less about another effect of bright light exposure, the immediate effect. Immediate effects of light exposure are so called because they happen immediately after the bright light is initiated and disappear soon after it is extinguished. Among such effects are changes in EEG patterns and increases in core body temperature indicative of increased alertness (noted by P. Badia in Physiological Behav-ior), and in some circumstances, improvements in cognitive performance, as reported in Campbell and Dawson's report in Physiology and Behavior.

The immediate effects of light are independent of circadian rhythms. In principle, the immediate effects of light exposure could occur at any time, day or night, simply by introducing a bright light.

If energy could indeed be switched on and off by light, then lighting becomes a useful means of increasing alertness for boring or monotonous work. It could help people combat the times when attention tends to flag, such as reduced alertness in the early afternoon, in what is called the post-lunch dip.

For lighting practitioners, this is an exciting prospect with wide application. Unfortunately, several questions must be answered before the immediate effects of light can be applied with confidence.

Among these questions are: to what extent is the increase in concentration sustained over time if bright light exposure is used every day; what are the effects of an increase in attention on the performance of tasks of different levels of complexity; and is an increase in alertness, produced by exposure to bright light, followed by a period of enhanced relaxation when the bright light is extinguished.

Summer, Fall, Winter, Spring

In contrast to the immediate effects are the effects associated with the seasons. Probably the best established of these effects is seasonal affective disorder (SAD). This is a psychiatric condition in which people experience lethargy, depression, and food cravings during the winter, but not in the summer.

A study, published in the IESNA Lighting Handbook, 8th edition, 1993, of the prevalence of SAD suggests that about 10 percent of the population in New Hampshire shows some SAD symptoms compared with only 2 percent in Florida. From these figures, it is estimated that as many as 10 million Americans have SAD and some 25 million people are susceptible to it.

Fortunately, SAD can often be successfully treated with exposure to light. Light treatment uses two kinds of devices, the light box and the dawn simulator.

SAD remedies

The light box is a device that people diagnosed with SAD sit in front of for apredetermined period of time; the higher the illuminance at the eye, the shorter the time of exposure. Conditions for exposure to the eye are typically 2,500 lx at the eye for 2 to 4 hours or 10,000 lx for 30 minutes. The problem with using the light box is that it limits the patient's activities during the recommended exposure period, which is daily for several weeks.

The dawn simulator is, as its name implies, a device that slowly increases the amount of light in a bedroom while the person sleeps to improve their mood at awakening. According to a 1994 article by D.H. Avery, et.al., in Biological Psychiatry, the illuminance from the dawn simulator typically increases slowly over a period of 90 minutes, until it reaches a maximum value of about 250 lx on the pillow.

Both devices reduce seasonal depression for a number of the people who use them. However, the results of light exposure on seasonal depression provide no widely accepted explanation of the effect; light may be simply a very effective placebo. Although this is a possibility, scientists have demonstrated that both the light box and the dawn simulator alleviate depression more effectively than other relatively low-level lighting conditions designed to act as placebos.

The future of photobiology

The applications discussed earlier in this article are sufficient to indicate that lighting for photobiological purposes could have an impact on the lives of many people.

The photobiological effects of lighting should be considered equally as important as simply providing appropriate visibility without discomfort.

In some applications, visual conditions will dominate and photobiological considerations can be ignored. In others, such as night-shift work, photobiological considerations may be more important than visual ones. In yet others, such as increasing the amount of light in the homes of the elderly in the evening, benefits could include increased visual capabilities, improved sleep patterns and safety factors due to reduced home accidents.

More questions to answer

In the future, designing lighting solely for vision will be inadequate, but to meet this new challenge, lighting designers and physiologists will have to meet two additional needs.

The first is a clearer understanding of the mechanisms involved, particularly for the treatment of depression and for the immediate effects of light. Even for phase shifting of core body temperature, questions remain about how the pattern of light exposure occurring over the 24 hours is integrated.

Most of our understanding of the photobiological effects of light has been developed in the laboratory where the conditions of exposure are closely controlled. Will the effects that have been clearly demonstrated in the laboratory stand up to the complex conditions of everyday life?

On a narrower front, nowhere in this discussion has the effect of lamp spectrum been raised. This is because science has not yet proven which of the retinal photoreceptors are involved or even if the retina has an entirely different photoreceptor that is used for photobiological effects. It is only when we identify the spectral sensitivity of the active photoreceptors (and the requirements for vision) that we can derive optimum lamp spectrum.

The second need is a set of innovative methods to deliver the desired lighting conditions in a manner that is visually and financially attractive while being comfortable and easy to use.

These methods will depend on the circumstances. For treating seasonal depression, we require a method that allows people to go about their everyday business while being exposed to light. For night-shift work, we need lighting systems that provide the necessary light exposure without interfering with the visibility of VDT screens. For the elderly, an inexpensive method of providing bright light that blends with the rest of the furnishings in their homes is desirable.

What is ahead?

Clearly, incorporating the photobiological effects of light into lighting practice is not an established fact. Too many unanswered questions remain and more study needs to be conducted to determine if exposure to bright light carries the risk of any potential side effects.

However, the potential benefits for night shift workers, travelers, the elderly, patients with Alzheimer's disease, and the seasonally challenged are good reasons to start considering light that affects more than sight.
 

Meet the author

Peter Boyce is Group Leader of the Human Factors Program at the Lighting Research Center, where he conducts research on lighting for the elderly, security lighting, lighting evaluation, visibility of exit signs and the non-visual effects of light. Boyce is the author of Human Factors in Lighting, a fellow of the IESNA, and was awarded the CIBSE Silver Medal.




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