By Judith Heerwagen | Photography by Gerry Johansson

A considerable body of research shows that people prefer daylit spaces to those lacking natural light. Why should this be? If there is sufficient light to see, why would people prefer one source to another? To answer this question, we need to understand the evolved relationship between humans and natural light.

Imagine you are on a camping trip that lasts a lifetime. You and your small band of hunter-gatherers wake up with the first light of day. You huddle around the fading embers of the camp fire, eat leftovers from the night before and discuss the day’s foraging activities. By the time the sun is fully up, you head into the bush using the bright light of day to identify edible fruits and berries and to track animals. By noon, the sun is hot and you seek the shade of a tree canopy for refuge and rest. As you snack on nuts and berries, conversation is drawn to the horizon where large storm clouds have begun to gather. You’re a long way from camp and are concerned about getting back. Dark clouds gather, cutting out the sun overhead but providing dramatic shafts of light in the distance. It rains hard, but briefly, as you huddle under a rock outcropping for protection. As you head back, the clouds begin to break and a rainbow lights up the sky signalling the end of the storm. You get back to camp just in time for dinner. You discuss the day’s events around the campfire as the sun begins to set, lighting the sky orange and pink. Dusk brings with it a greyness that hides the details of the landscape, making it more difficult to discern what is happening beyond the campsite. Soon it is dark and everyone gathers around the fire for warmth, light and companionship before going to sleep under the soft light of the moon.


Prior to the advent of buildings, humans lived immersed in nature. Daily activities were aided or constrained by the presence or absence of daylight and by qualities of light that signalled time and weather. Our physiological systems – especially our sleep-wake cycles – were in synch with the diurnal rhythms of daylight, as were our emotional responses to light and darkness. The strong, consistent preference for daylight in our built-up environments today suggests that evolutionary pressures are likely to be influencing our responses. Although all our sensory systems acting together were important to survival, the visual system is our primary mode of gathering information. Thus, light must have played a powerful role in information processing and survival. In ancestral habitats, light was likely to have had several key functions that are relevant to the design and operation of built-up environments. These include:

• Indicator of time. Natural light changes significantly over the course of the day, providing a signal of time that has been crucial to survival throughout human history. Being in a safe place when the sun was setting was not a trivial matter for our ancestors, and it is still important to human well-being.

• Indicator of weather. Light also changes with weather, from the dark, ominous colour of storms to rainbows and beams of light as clouds break up and recede. Attending to the variability in light and its relationship to changes in weather would have been highly adaptive (Orians and Heerwagen, 1992).

• Signal of prospect and refuge. The sense of prospect is signalled by distant brightness and refuge is signalled by shadow (Appleton, 1975, Hildebrand, 1999). Brightness in the distance aids assessment and planning because it allows for information to be perceived in sufficient time for action to be taken. High prospect environments include open views to the horizon and a luminous sky (‘big sky’). A sense of refuge is provided by shadows from tree canopies, cliff overhangs, or other natural forms. Mottram (2002) suggests that allowing the eyes to rest on infinity (which the horizon represents visually) may be beneficial, even if the view is perceptually manipulated through visual images rather than actual distant views. Thus, our natural attraction to the horizon could be satisfied in many ways through the manipulation of light and imagery applied to vertical surfaces.

• Signal of safety, warmth, and comfort. Although we usually think of the sun as the primary source of light in the natural environment, fire also served as a source of light and comfort, both physical and psychological. Anthropologist and physician Melvin Konner (1982) suggests that the campfire served important cognitive and social functions in developing human societies. The campfire extended the day, allowing people to focus their attention not only on the daily grind of finding food and avoiding predators but also on thinking about the future, planning ahead and cementing social relationships through story-telling and sharing the day’s experiences.

• Peripheral processing aid. Light also provides information about what is happening beyond the immediate space one occupies. It illuminates the surrounding environment that impinges continually on our peripheral processing system. The importance of peripheral light is evident from the discomfort many people feel when they
are in a lighted space with low lighting at the edges, leading to a perception of gloom. Lighting researchers suggest that negative responses to gloom may be associated with its natural function as an early warning signal that visual conditions are deteriorating (Shepherd et al, 1989).

• Synchronisation of bio- and social rhythms. As a diurnal species, light plays a critical role in our sleep-wake cycles and also synchronises social activities. Although we can now alter our activity cycle with the use of electric light, research evidence nonetheless shows that night work is still difficult and often results in drowsiness, difficulty sleeping, mood disturbances and increased cognitive difficulties at work (Golden et al, 2005). Some night work facilities are using bright interior light to shift biological rhythms and increase alertness. There is also evidence that people who experience seasonally-related depression prefer to be in brightly lit spaces (Heerwagen, 1990).

To summarise, light provides information for orientation, safety and surveillance, interpretation of social signals, identification of resources and awareness of hazards. Whether it is the changing colour of light associated with sunset or storms, the movement of fire or lightning, the brightness in the distance that aids planning and movement, or the sparkle of light off water – all these aspects of light have played a role in helping our ancestors make decisions about where to go, how to move through the environment, what to eat, and how to avoid dangers.

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Given Homo sapiens immersion in a naturally-lit environment during our evolutionary history, it is not surprising that building occupants enjoy the very features that characterise daylight in natural landscapes.

Research on office buildings shows a high preference for daylit spaces and for specific features of daylight. A study of seven office buildings in the Pacific Northwest (Heerwagen et al, 1992) shows that more than 83% of the occupants said they “very much” liked daylight and sunlight in their workspace, and valued the seasonal changes in daylight. Interestingly, daylight design generally aims to eliminate direct beam sunlight from entering work areas due to glare and heat gain.

When the data were looked at with respect to the occupant’s location, 100% of those in corner offices said the amount of daylight was “just right,” as did more than 90% of those along the window wall in spaces other than the corner offices. Even those located in more interior positions were satisfied with the daylight, as long as they could look into a daylit space.


We know that people like to be in daylit spaces and that they like indoor sunlight. However, when occupants in the above study were asked about light for work purposes, only 20% said daylight was sufficient for work. The vast majority said they used electric ceiling light “usually” or “always” to supplement daylight.

Even those who rated daylight as “just right” also used electric lights regularly. Although the reasons for this situation are not clear, anecdotal evidence suggests that occupants also supplement daylight with task lamps. It is possible that electric light, whether ceiling or task, reduces lighting contrasts on work surfaces that make some visual work difficult. A post-occupancy evaluation of the first LEED Platinum building in the US, the Philip Merrill Environmental Center, shows very high satisfaction with daylight, despite concerns with visual discomfort (Heerwagen and Zagreus, 2005). This suggests that people may value the psychological benefits of daylight even when daylight creates difficulties for work due to glare and uneven light distribution. Certainly, the kinds of visual tasks we perform in today’s work environments are very different from our ancestors’ daily tasks. Cooking, tool making, conversing, foraging, and hunting could be effectively carried out over a wide range of luminous conditions. In contrast, reading and computer work require a much greater degree of visual acuity that may be more difficult in some daylit environments. Yet a uniformly lit environment that may be appropriate for office work lacks the psychological, and perhaps biological, value of daylight.



A study of office workers in a Seattle high-rise building asked respondents to compare the relative merits of daylight and electric light for psychological comfort, general health, visual health, work performance, jobs requiring fine observation, and office aesthetics (Heerwagen and Heerwagen, 1986).

The results show that the respondents rated daylight as better than electric light for all variables, especially for psychological comfort, health and aesthetics. They rated daylight and electric light as equally good for visual tasks.

At the time of this study in 1986, there was little evidence connecting daylight to health. Since that time, however, there has been a surge of research on the link between light and health, much of it focusing on the circadian system and seasonal affective disorder. Much of this work has been conducted in clinical settings with phototherapy. Since a review of this topic is provided elsewhere in this issue, it will not be addressed here.

However, it is worth noting a laboratory study that investigated lighting preferences of subjects with Seasonal Affective Disorder (SAD) compared to subjects that did not experience seasonal changes in mood or other behaviours (Heerwagen, 1990). Those who experienced seasonal changes chose significantly higher levels of brightness for all lighting sources compared to those who did not. This suggests that people experiencing SAD may indeed be ‘light hungry’ and could benefit from indoor environments with high daylight levels, such as atria, sunrooms and locations adjacent to windows.

But what do we know about other health impacts of daylight in the builtup environment? Research in hospital settings, looking at the relationship between room daylight levels and patient outcomes, found that bi-polar patients in bright, east-facing rooms stayed in the hospital 3.7 fewer days on average than those in west-facing rooms (Benedetti and others, 2001). Similar results were found by Beauchamin and Hays (1996) for psychiatric in-patients; those in the brightest rooms stayed in the hospital 2.6 fewer days on average. However, neither of these studies provides data on the actual light levels in the patient rooms or light entering the retina, so it is difficult to draw conclusions about exposure levels.

More recent research in a Pittsburgh hospital actually measured room brightness levels. Walch and others (2005) studied 89 patients who had elective cervical and spinal surgery. Half the patients were located on the bright side of the hospital, while the other half were in a hospital wing with an adjacent building that blocked sun entering the rooms. The study team measured medication types and cost as well as psychological functioning the day after surgery and at discharge. The researchers also conducted extensive photometric measurements of light in each room, including light levels at the window, on the wall opposite the patient’s bed, and at the head of the bed (which presumably would have been at or near the patient’s eye level). The results showed that those in the brighter rooms had 46% higher intensity of sunlight. Patients in the brightest rooms also took 22% less analgesic medicine/hr and experienced less stress and marginally less pain. This resulted in a 21% decrease in the costs of medicine for those in the brightest rooms. The mechanisms linking bright light to pain are currently unknown, however.

Other potential benefits of indoor daylight include improved sense of vitality, decreased daytime sleepiness and reduced anxiety. For example, a large-scale survey of office worker exposure to light during the winter in Sweden shows that mood and vitality were enhanced in healthy people with higher levels of exposure to bright daylight (Partonen and Lönngvist, 2000). Another study shows that a halfhour exposure to bright daylight by sitting adjacent to windows reduced afternoon sleepiness in healthy adult subjects (Kaida et al., 2006). In that study, daylight levels ranged from about 1,000 lux to over 4,000 lux, depending upon sky conditions. Kaida et al. found that daylight was almost as effective as a short nap in reducing normal post-lunchtime drowsiness and increasing alertness.


E.O. Wilson popularised the term “biophilia” in 1984 with the publication of his book, Biophilia. In it he describes biophilia as the human tendency to affiliate with life and life- like processes. Wilson never fully explained what he meant by “life-like processes.” However, if we consider the characteristics of life, we can look at daylight as sharing some of these features. Daylight grows over the course of the day as the sun moves across the sky, it changes in colour and intensity, it provides sustenance for life, its absence at night provokes behavioural change, and the lengthening day after the long winter months evokes joy and a sense of well being.

Wilson and others describe biophilia as an evolved adaptation linked to survival. The evidence cited in this article suggests that daylight, in addition to being “life-like,” has deeply-seated health and psychological benefits that may be difficult to support in electrically-lit environments. Clearly, we can design interiors with electric light that changes intensity and colour over time and that mimics other features of daylight. But will it feel the same? Can electrically-lit environments provide the same biological benefits as daylight? We don’t yet know the answers to these questions, but we do know that such efforts would be more energy intensive and more costly.

The life-like and life-supporting qualities of daylight strongly suggest that daylight is a basic human need, not a resource to be used or eliminated at the whim of the building owner or designer. The presence of daylight and sunlight in buildings clearly affects our psychological and physiological experience of place. Its absence creates lifeless, bland, indifferent spaces that disconnect us from our biological heritage.


Judith Heerwagen, Ph.D., is an environmental psychologist in Seattle, Washington. She is an affiliate faculty member in the Department of Architecture at the University of Washington where she has cotaught seminars on bio-inspired design and sustainability. She has written and lectured widely on the human factors of sustainable design, including health, psychological impacts and productivity. She is co-editor of Biophilic Design: The Theory, Science and Practice of Bringing Buildings to Life (Wiley, 2008).

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