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Daylighting Resources - Health


Increased use of daylight can reduce eyestrain which may in turn lead to improved productivity, as long as the daylight is controlled by simple devices such as window blinds, light shelves or reveals.

The symptoms and causes of eyestrain occur as:

  • irritation of the eyes, evident as inflammation of the eyes and lids
  • breakdown of vision, evident as blurring or double vision
  • referred effects such as headaches, indigestion, etc.

Eyestrain has not been shown to cause long-term ill health, but the symptoms can be disabling while they last, and certainly reduce worker productivity. Eyestrain can be brought about by poor lighting, the inherent features of the task and its surroundings, inadequacies in the individual's visual system, or any combination of these factors.

Lighting conditions which have been shown to lead to eyestrain include:

Daylight and eyestrain

Daylighting usually provides more light on the task than electric lighting would, and is certainly free of flicker. Daylight also tends to provide more diffuse task lighting than would be provided by electric light alone; this reduces excessive contrasts that can be caused by shadows on the task area.

However, windows with inadequate shading devices can lead to excessive contrast between window and the interior walls surrounding the window. Architectural features such as window reveals (Hopkinson, 1971) and light shelves, and devices such as window blinds can be used to reduce excessive contrast.

Heerwagen et al. (1992) found that office workers used a variety of tactics to relieve visual discomfort including walking around, going to get a drink, complaining to colleagues, and introducing ad-hoc modifications to the environment. Some of these activities are likely to subtract from productivity.

Other visually-induced conditions: migraine and photoepilepsy

Daylight is a mixed blessing for sufferers of these conditions; the absence of flicker in daylight makes it unlikely to trigger symptoms. However, the high brightnesses produced by daylight tend to exacerbate the disturbing effects of any large-area, regular, high-contrast patterns. Providing occupants with control over their window blinds should be sufficient to alleviate symptoms.

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Visual Performance

There is no unique physical characteristic of daylight that separates it from all other light sources, although it does generally outperform electric lighting. The copious quantities of light provided by daylight, its diffuse character and good spectral content can lead to high visual performance, but glare, shadows and reflections in glossy surfaces must be controlled.

Reading text

An office desk situated near a window typically receives five times as much light from daylight as it would from electric lighting alone. However, this increased amount of light does not significantly improve human visual performance, since the performance of the human visual system follows a ‘plateau and escarpment’ curve that plateaus at much lower light levels (Rea and Ouellette, 1988). High levels of light only allow improved visual performance for reading tasks that have low levels of contrast or contain small details close to the threshold of detectability (Boyce et al., 2003).

Discriminating color

Daylight allows better color discrimination than most electric lamps. The human eye’s ability to discriminate colors is dependent on the spectral content of light, and daylight contains light of almost all visible wavelengths, whereas most electric lamps emit most of their light over a limited wavelength range.

Nevertheless, the human visual system is able to discriminate most colors adequately under electric lighting, and in most situations daylight provides little additional benefit (Santamaria and Bennett, 1981, Smith and Rea, 1978). Daylight only provides a benefit over electric lighting for tasks that require fine color discrimination (Nickerson, 1948; ASTM, 1996a and b, Williams, 1966), especially when contrast levels are low (Eklund, 1999).


In potentially dangerous industrial environments it is prudent to ensure that shadows do not make essential details invisible. Shadowing can be reduced by providing light from several different directions, either from several windows or skylights, or from several electric lights with wide cut-off angles.

Reflections in glossy surfaces

These ‘veiling’ reflections can reduce the legibility of text, images or features on glossy surfaces, and cause visual discomfort as the reader or worker strains to resolve details. The slight vertical polarization of daylight can reduce veiling reflections on glossy horizontal surfaces such as magazines, thereby increasing task contrast, which leads to higher visual performance (Rea, 1986). The vertical polarization of daylight does not reduce reflections on vertical surfaces such as computer screens, although modern computer screens have bright screen images and low surfaces reflectances that make the image visible despite veiling reflections (Howlett, 2003).

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The Daylight Dividends program has sponsored the LRC’s development of a unique device for measuring the daylighting effect on the human circadian system. The device called a “daysimeter” measures photopic and light affecting the circadian system illuminance at the eye. It also measures and records head movements. It records the information within the device for later downloading onto a computer. Click here for further information.

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Alertness and Sleep

Bright light during the day and darkness at night act to synchronize the circadian system, which is the body’s internal time clock. The circadian system regulates alertness, concentration, digestion and sleep, and so is essential to all human activity. Daylight in the workplace may be the best practical way to provide bright light exposure and maximally synchronize the circadian system.

Problems with sleep are common among the working population. These problems include insufficient sleep duration, difficulty maintaining sleep, and sleep phase disorders including difficulty initiating sleep and early morning awakening. Exposure to bright light at an appropriate time may alleviate sleep problems.

Consequences of sleep problems

It is estimated that the cost of insomnia to the US economy exceeds $30 billion (1994 USD) (Chilcott and Shapiro, 1996). Insomnia is correlated with high degrees of work stress and job dissatisfaction (Rosmond, Lapidus and Bjorntorp, 1998), and lower job performance (Kupperman, Lubeck and Mazonson, 1995). Poor sleep has been linked to reduced concentration, listlessness, and difficulties in making decisions (Linton and Bryngelsson, 2000)

Poor sleep has direct and measurable consequences; 5 percent of chronic insomniacs report having an automobile accident due to chronic sleepiness—more than twice the rate of those with no insomnia (Roth and Ancoli-Israel, 1999). Poor sleepers miss five more days of work per year than those with no sleep problems (Schweiter et al., 1992).

Prevalence of sleep problems

Sleep problems are defined in a variety of ways, and so many measures of prevalence exist. Linton and Bryngelsson (2000) report that 35% of Swedish adults claim to have had difficulty sleeping within the past 3 months, and that 8% met the criteria for clinical insomnia. Liu et al. (2000) report that 29% of Japanese adults sleep less than six hours per night on average, and that 15% suffer from excessive daytime sleepiness. 4.3% of the US adult population use sleep enhancing medication (Mellinger et al., 1995).

Daylight as a treatment for chronic sleep debt

Exposure to bright relight has been shown to be an effective treatment for sleep disorders. Czeisler et al. (1988) have demonstrated that exposure to 10,000 lux (less than half as much light as is typically available outdoors) at appropriate times is effective in resynchronizing sleep phase. Studies of elderly patients with various sleep disorders have shown that exposure to bright light in the evening produces longer and better quality sleep (Campbell et al., 1993; Campbell and Dawson, 1991; and Lack and Schumacher, 1993.

Figueiro et al. (2002) found that workers in windowless offices spend a small but statistically significant greater amount of time talking to others, either directly or by telephone; and a small but statistically significant lesser amount of time working on their computer, relative to workers in windowed offices. One explanation for this is that workers in windowless offices do not receive sufficient daylight to entrain their circadian system, and therefore seek additional daylight and social interactions.

Heschong et al. (2002b) conducted an epidemiological study that showed that children in well-daylit classrooms scored better on standardized tests for reading and mathematics. Daylight explained only 0.3% of the variation in test scores, which indicates that daylight has only a small effect. However, other factors believed to be educationally important also showed only small effects. These included participation in a gifted and talented program (0.3%), the number of absences (0.1%), and the size of the school (0.1%). Whether this means that all or none of these factors are really important is an interesting question. If daylight is an important factor, this may be due to early-morning daylight entraining the circadian systems of students with delayed sleep phase.

How much light is required?

A wide range of illuminances, from 2,500 lux to 10,000 lux, a range of times, from 15 minutes to 4 hours, and a wide range of spectra, from fluorescent lamps to sunlight, have been shown to be effective in the treatment of sleep disorders (Terman et al. , 1995). What is clear is that daylight through windows, assuming it is available at the required time, is a convenient means to provide the high light levels apparently required to sustain the operation of the circadian system. The spectral requirements of the circadian system are also more closely matched by daylight than by most artificial light sources (Rea et al., 2002).

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Seasonal Affective Disorder

Seasonal affective disorder (SAD) is a seasonal form of depression. Exposure to bright light has been shown to alleviate this condition. Daylight is a convenient means to provide such exposure.

Prevalence of SAD

SAD is experienced by about 5% of the population, and about 10% to 20% have sub-syndromal symptoms, the percentages tending to increase with an increase in latitude (Kasper et al., 1989; Wehr and Rosenthal,, 1989). SAD is more common in females than males (Rosen et al., 1990). Its prevalence increases with age until about the sixth decade, after which it declines dramatically.

Symptoms of SAD

SAD can be recognized by the increase in feelings of depression and a reduced interest in all or most activities, typical of depression, together with such atypical symptoms as increased sleep, increased irritability, and increased appetite with carbohydrate cravings and consequent weight gain. These symptoms disappear in summer. Some patients suffer from a rare variant, summer SAD, in which the symptoms occur during the summer (Wehr et al., 1991).

Treatment of SAD

The cause of SAD remains unknown. Explanations based on disturbances to the circadian system, melatonin concentration, and regulation of the hormone serotonin have been proposed but none have been proven. While the cause of SAD is unclear, what is clear is that exposure to bright light is often an effective treatment (Rosenthal et al., 1985; Terman et al., 1989; Tam et al., 1995), although whether this is because of some physiological effect or a placebo effect is open to question (Eastman, 1990; Eastman et al., 1992).

“Bright light” is often provided by a light box containing fluorescent tubes, placed close to the eyes during working hours. Exposure durations range from 2 hours to 30 minutes depending on the intensity of light. The timing of the exposure to "bright light" is relatively unimportant (Wirz-Justice et al., 1993). Working close to a window provides at least as much light as is provided by a light box, so daylight is also an effective treatment for SAD.

Side effects of treatment

As with most medical treatments, there are side effects of prolonged exposure to the high intensity of a light box. Typically they are mild disturbances of vision and headaches that subside over time. These symptoms are not usually experienced when daylight is used as the source of light.

Care should be taken with patients who have a tendency towards mania, and whose skin is photosensitive or who already have retinal damage or who have a medical condition that makes retinal damage likely (Levitt et al., 1993; Gallin et al., 1995; Kogan and Guilford, 1998). General guidance on the use of light in the treatment of SAD is available from a number of sources (Saeed and Bruce, 1998; Lam, 1998; Lam and Levitt, 1999).

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Tissue Damage

Daylight contains ultraviolet (UV) radiation. Exposure to low levels of UV has several health benefits, whereas exposure to high levels of UV causes damage to the tissues of the eyes and skin.

Daylight levels in buildings are typically only 1%-5% of those found outdoors, and , window glass screens out most UV radiation. Therefore, UV levels inside buildings are safe, although people who are sensitive to ultraviolet radiation can still experience adverse effects. UV sensitivity can be caused by lupus erythematosus, and exposure to certain pharmaceuticals and chemical agents.

Amount of UV admitted into buildings

Very little UV radiation penetrates into a typical building. Windows usually constitute only 20%-50% of the wall area of a building; consequently, interior daylight levels are usually only 0.5%-5% of outdoor levels.

Furthermore, window glass allows almost no UV radiation through into the building interior (though of course window glass is mostly transparent to visible frequencies). UV-B protection is often referred as a ‘sun protection factor’ (SPF), as displayed on bottles of sunscreen; in these terms, 6mm clear float glass reduces provides protection equivalent to a factor 30 sunscreen (National Institute of Water & Atmospheric Research Ltd, 2001). Tinted windows and those with additional coatings provide increased protection.

Consequently, UV radiation in buildings is typically reduced to between 0.01% and 0.2% of levels found outside, equivalent to wearing a sunscreen with an SPF of between 500 and 10,000.

Acute effects

There are several acute forms of tissue damage known to be caused by short-term exposure to daylight, though none of these conditions is likely to be caused by the comparatively low levels of daylight inside buildings. These include:

  • Erythema (sunburn): Caused mainly by exposure to UV-B (mid-range ultraviolet radiation) radiation. Erythema can occur inside buildings as a result of exposure to direct sunlight through windows which have a high transmission of UV-B.
  • Photokeratitis: A temporary irritation of the eye caused by prolonged exposure to high levels of UV radiation (Pitts and Tredici, 1971). Such high levels of UV are generally found at high altitude, or when UV is reflected by snow or water, and are not generally experienced inside buildings.
  • Photoretinitis and chorio-retinal injury: Injuries to the retina caused (respectively) by visible light and infra-red radiation. Both can be caused by staring directly at the sun for a sustained period, although the eye’s normal aversive reaction, i.e. blinking or looking away, is sufficient to protect against these injuries.

Chronic effects

There are several forms of tissue damage known to be caused by long-term exposure to daylight. These include:

  • Skin ageing: Prolonged exposure to UV radiation is well established as a cause of skin ageing. The most striking feature of severely photoaged skin is the presence of massive quantities of thickened, degraded elastic fibers which degenerate into amorphous masses. The result is a thicker skin resembling a crust. Photoageing is most commonly seen on the parts of the body that are not usually protected by clothing. UV radiation is the dominant cause of skin photoageing (Cesarini, 1998). It is unclear whether the low levels of UV found inside buildings contribute to photoageing.
  • Skin cancer: There is a strong correlation between exposure of the skin to the sun during childhood and the development of skin cancer in later life (Moan and Dahlback , 1993). Exposure to sunlight during adulthood is less strongly correlated to the incidence of cancer.

The potential for sunlight in buildings to contribute to the formation of cancers is limited by three factors. First, the sun is at its strongest while overhead, but sunlight typically enters buildings through side windows and is therefore weaker. Second, building occupants typically use blinds to shield themselves from direct sunlight, since direct sunlight causes glare and thermal discomfort. Finally, UV radiation (both UV-B and UV-A) is significantly reduced by window glass.

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Other Health Effects

Exposure to the low levels of ultraviolet radiation (UV) found in buildings has a variety of important health benefits, although it may contribute to ageing of the skin and retina. Windows and the view out may help to alleviate workplace stress. The high levels of light provided by daylight may trigger symptoms in sufferers of autism.

Vitamin D production

Exposure to UV radiation is important for the production of vitamin D in the skin. Vitamin D deficiency causes bone softening diseases such as rickets in children and osteomalacia in adults, but it also has an important function in regulating cell growth and differentiation (Holick, 2002). Vitamin D deficiency has been associated with an increased risk of dying from colon, breast, prostate and ovarian cancer, as well as developing diabetes and multiple sclerosis (Garland et al., 1989; (Malabanan et al., 1998; Ahonen et al., 2000; Hypponen et al., 2001).

Current lifestyles, which involve long periods indoors and the wearing of sunscreens when outdoors have increased the risk of vitamin D deficiency (Tangpricha et al., 2002). People who live in areas where sunlight is limited for several months, or those who have limited exposure and very dark skin, must depend on dietary sources and vitamin supplements to meet their vitamin D requirement. The low levels of UV transmitted through window glass may help to maintain vitamin D levels.

Healing of psoriasis and eczema

Low levels of UV radiation, such as those transmitted through window glass, are effective in reducing symptoms of several common skin conditions, such as psoriasis and eczema (Parrish et al., 1985).


Natural scenes viewed through windows have been suggested as a means of relieving stress (Kaplan, 1992; Ulrich, 1993). If this is so, then given the increased pace of change in much modern business and the consequent increase in stress having some windows may be a useful countermeasure. (Sutherland and Cooper, 2002).

Prolonged experience of stress can have undesirable health effects such as headaches, stomach ulcers, and high blood pressure. Stress can also cause changes in behavior that affect productivity such as absenteeism, high turnover of employees, and poor job performance (Cooper and Payne, 1988).


Autistic people have a chronically high level of psychological arousal. The repetitive behavior they typically exhibit is believed to be a means of dealing with this problem. The high light levels often provided by daylight are inherently arousing, and so may trigger symptoms. Further, the variation in lighting of an interior that is associated with daylighting, especially when there are scattered clouds in the sky, can be regarded as a form of environmental stimulation.

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