In 2000, the World Health Organization (WHO) estimated that depression (unipolar depressive disorders) caused 4.4% of the disability adjusted life years (DALYs) worldwide and an estimated 12% of the total life years lived with disability.1
A multinational study estimated that approximately 7% of Europeans suffered from major depression that substantially impaired their working or social lives; the prevalence of major depression ranged from 3.8% in Germany to 9.9% in the United Kingdom.2
Injuries are also a leading cause of the global burden of disease. In 2000, WHO ranked falls as 15th among leading causes of disease burden—accounting for an estimated 3.4 million DALYs—in adults aged 30–44 years.3
Among the high-income countries included in that study, falls ranked as the 13th and 14th leading causes of morbidity for people aged 15–44 years.3
Together, depression and falls constitute major portions of the global burden of disease. However, WHO recently concluded that insufficient evidence exists regarding light in housing and its relationship to mental and other health effects.4
Falls and depression may have a commonality related to inadequate residential light, but the evidence to date has been insufficient to establish the link.
The relationship between lack of light and depression has been well documented, and the evidence that light is a potent neurobiological agent seems clear.5,6
The role of light as a major synchronizer of circadian rhythms has been established for alertness, plasma melatonin, body temperature, and sleep/wakefulness.7,8
Light therapy has been used to treat seasonal affective disorder (SAD) since the 1980s, when Rosenthal et al. found that artificial light was effective in treating the disorder.6
The light intensity of 2,500–10,000 lumens per meter squared (lux) used during therapy is much brighter than normal indoor light, which is usually 300–500 lux, but not as bright as summer sunlight, which can be as bright as 100,000 lux.9
A consensus has been reached concerning the efficacy of light to treat seasonal depression, based on independent studies from around the world that show an average decrease of 20%–25% in depressive symptoms.10
Depressive symptoms are determined using both observer rating scales, such as the Hamilton Depression Rating Scale, and self-assessment of symptoms.11
Few studies have compared artificial with natural light. However, in a study conducted in Switzerland, researchers compared the use of low-intensity artificial light, defined as half an hour of artificial light at 2,800 lux, with one hour of outdoor light.12
The study concluded that outdoor light was more effective than artificial light, with outdoor light causing a 50% reduction in depressive symptoms. A statistically significant reduction of 25% in depressive symptoms, as measured by the doctor-administered Hamilton Depressive Rating Scale, occurred in the group receiving the low-dose artificial light, although self-reported depressive symptoms did not improve for this group. In another study, low levels of light increased the likelihood of depression when depressed patients reportedly were exposed to 40% less moderate light (100 to 1,000 lux per day), compared with a non-depressed control group.13
Light therapy results in a rapid decrease in depressive symptoms, and few researchers have followed participants over long periods. However, in the few studies that followed patients for longer than one week, positive response rates increased with duration of the light intervention.
The salutary effect of light has been most extensively studied in relationship to seasonal depression, although studies of light's effect on individuals with nonseasonal depression, late luteal phase dysphoric disorder, and bulimia nervosa also have shown promise.14
Three main hypotheses have been proposed: (1
) light's effect on circadian phase shift, (2
) light's effect on the major monoamine transmitters, and (3
) an individual's genetic vulnerability. However, the causal pathway for depression is undoubtedly complex, as shown by (1
) the conflicting results of different studies, (2
) the independent effects of light and standard antidepressant pharmacotherapy, (3
) the mediation of the relationship between light and depression by whether daily behavior followed a predictable pattern, and (4
) evidence of reduced retinal contrast perception in depressed compared with non-depressed individuals.13–15
We undertook this study in part to determine if there is an association between self-reported adequacy of natural light in housing and depression.
In the United States, falls are a significant cause of home injuries across all age groups; an estimated 5.6 million nonfatal falls required medical attention in 1999.16
Risk factors for falls among the people aged 65 years and older have been well-studied and include arthritis, foot problems, medications, and cognitive and motor impairment.17
Environmental hazards do not seem to be strong predictors for risk of falls among the elderly, and results of environmental mitigation have been disappointing.18
The prevalence of falls among adults aged 18 years and older is similar to the prevalence in both older and younger people. In 1998 in the U.S., it was estimated that 38% of nonfatal, unintentional fall injuries occurring at home were among people aged 25 to 64 years.16
To date, no studies have examined housing factors that predict falls in this age group, and more detailed studies of precisely which housing factors are most predictive of falls are needed. Lack of adequate natural light may be one such housing factor, as poor light can prevent individuals from seeing hazards for tripping and falling in their environment.
In this article, we describe the association between self-reported natural residential light and the risk for depression and serious nonfatal falls among study participants aged 18 years and older in the Large Analysis and Review of European Housing and Health Status (LARES) survey.19