The average American spends at least 90% of her time indoors. Recent research suggests that this relatively new living condition in human history has a profound effect on cognition, physical performance and general health.
Lighting and views, ventilation, air quality, temperature, moisture, dust, pests, water quality and noise are now seen as controllable risk factors for disease or vehicles for promoting health.
Let’s look at lighting only today.
Nothing has had a more profound effect on human civilization than the introduction of electric light in the late 19th century. Night, nature’s curfew (Old French couvre-feu, meaning “cover fire”), was eliminated with the flip of a switch. Untethered from the sun, humanity created its own rhythms. The biological consequences of this independence are only beginning to be appreciated.
Until recently, all buildings were daylit. Even the early skyscrapers were designed for the use of natural light. A 1916 NYC zoning regulation prescribed setbacks and street widths to allow light and air to penetrate living spaces. However by the 1950’s, mechanical cooling and electric lighting had largely disconnected people from the outdoor world.
We have remained in the dark about light because we thought of it as illumination only. The equally important non-visual effects of light serve to set our internal clock. Shifts in the quality of light during the day regulate our physiology on every level, from the cellular to the systemic. This circadian rhythm (changes that follow a 24 hour-cycle driven by environmental light and darkness) defines optimal times for different functions.
Lighting and Weight Gain
Recent studies have shown that a meal consumed at 8 a.m. is not processed the same way as an identical meal consumed at 8 p.m. All mammals possess this metabolic circadian rhythm for good reason.
Energy requirements are defined by activity level. The human day (until relatively recently) consisted of an active daylight phase and a resting night phase. This resulted in the evolution of a metabolic day shift, so to speak, bearing little resemblance to the night shift.
Daytime metabolism excelled at eating, energy harvesting and storage while nighttime metabolism was designed for fasting and accessing stored energy. Melatonin, the body’s expression of darkness, orchestrates these changes.
Traditionally, melatonin was considered primarily as a sleep cue. It is now recognized as the most powerful messenger to switch from active to resting metabolic function, a chemical signal of day’s end. Darkness triggers the secretion of this ancient hormone. Light, whether natural or artificial, blocks melatonin secretion.
The widespread adoption of electric lighting ended sunlight-entrained human circadian rhythms. Illuminated night blocks the natural transition to a fasting state. Energy (calorie) acquisition and storage continue. Even dim light at night has been shown to significantly decrease melatonin secretion in humans. This disconnection from nature’s clock is now considered an important cause of our struggle with weight gain.
Seeing The Light
Metabolism is not the only system affected. Sleep-wake cycles, concentration, immune function, hormone regulation, mood, healing and regeneration are all orchestrated by our internal clock. Exposure to natural light has been associated with improved mood, reduced mortality among patients with cancer, and reduced length of hospitalization for patients after heart attack. Even pain perception can be modulated with light. Researchers found that spinal surgery patients recovering on the brighter side of a hospital unit perceived lower stress, less pain and took 22% less pain medication.
Intensity, spectrum and timing all contribute to the biological effects of light.
Visible light is a sliver of the electromagnetic spectrum sandwiched between the shorter wavelengths of gamma rays, x-rays and ultraviolet rays, and the longer infrared and radio waves. Different wavelengths of visible light are distinguishable by color. Running from shortest to longest (~400 - ~700 nanometers), we see violet, indigo, blue, green, yellow, orange, red. But all wavelengths affect our biology. For instance, skin exposure to ultraviolet light is necessary for the production of vitamin D. Infrared radiation provides the heat-generating quality of natural light. Sunlight contains the full spectrum.
The eye uses different systems for visual and temporal perception. The visual system is most sensitive to green light whereas the temporal system has a peak sensitivity to bluish light. This explains the activating effect of blue light and why it should be decreased as one approaches bedtime.
Ain’t Nothin Like The Real Thing
Traditional artificial lighting has a uniform color and brightness that bears no resemblance to natural light. Its intensity is also very different. Outdoor light intensity on a clear day is approximately 10,000 lux. This drops to 1,000 lux when indoors next to a window. Moving just a few feet away from the window reduces light intensity by more than 50%. Traditional recommendations for artificially lighting a normal work station target 500 lux.
The dramatic increase in nearsightedness (myopia) provides a powerful example of the consequences of reduced exposure to natural light. 50% of young adults in the US and Europe are myopic, double the prevalence of 50 years ago. In East Asia the increase has been even more dramatic now affecting more than 90% of children. Scientists studying hunter-gatherer societies in the 1930’s found almost no myopia. Between 1970 and 2000 the prevalence of myopia in the US nearly doubled. The leading theory suggests that inadequate light intensity is the problem. Dopamine secretion in the retina is triggered by intense light. Dopamine in turn signals the eye to stop growing thereby preventing myopia. A University of Michigan study observed that an average child in 2002 spent half as much time outdoors as in 1981.
For those curious about their light exposure, a light meter app is now available for $1.99.
The good news is that innovative technology is beginning to provide a better simulation of sunlight. Programmable systems change the light quality over the course of the day. This holds the potential to improve health, increase productivity and provide greater safety. It is a curious step in the right direction. The increasing sophistication of artificial light is an attempt to recreate the environment that made us in the environment we have made.