The contemporary world, with its deep-core buildings and around-the-clock work schedules, may be impacting our circadian system—the biological functions that run on about a 24-hour sequence, such as the sleep-wake cycle. Sleep typically occurs about two hours after the onset of melatonin, a hormone produced in the evening and under conditions of darkness, which acts as a timing messenger signaling to the body when it is nighttime.
Like a clock that needs to be set daily for accurate time-telling, the brain's circadian clock requires a rhythmic pattern of light-dark exposure to orchestrate all of the body's complex biological functions, one of which is melatonin production. The human biological clock runs with a period slightly longer than 24 hours; therefore, each morning when you receive sufficient light, your biological clock is “reset” to match the 24-hour solar day. A natural cycle of bright days and dark nights is the ideal regulating rhythm for the circadian system.
However, patterns of light and dark in today's built environment are often inconsistent with the natural rhythm of bright days and dark nights. Surprisingly, electric light in buildings pales in comparison to outdoor light levels during the day—even under cloud cover or during the winter. This absence of suitable light exposures during the day may induce “circadian darkness” because the indoor electric lighting can be insufficient to strongly stimulate the circadian clock. Likewise, too much light in the evening, either through electric lighting or extended daylight hours in the spring and the summer, may be too bright to signal darkness to the brain's clock. Lack of exposure to morning light, or exposure to too much light in the evening, can delay melatonin production onset, and therefore delay sleep, resulting in circadian disruption.
Long-term effects of circadian disruption may be impacting our health, well-being, and performance. In fact, recent studies using animals (e.g., mice and rats) showed that circadian disruption by irregular light-dark patterns are associated with increased mortality, higher risks for developing diabetes, obesity, cardiovascular disease, and even cancer. Epidemiological studies in humans show that those who work swing shifts for an extended number of years—20 to 30 —are more likely to experience circadian disruption, are at higher risks of diseases such as breast and colorectal cancer.
CIRCADIAN LIGHT This irregular light-dark pattern exposure may have an acute effect on teenagers. Sleep restriction is common in adolescents and has received growing attention in the past few years. Pubertal changes in sleep regulation mechanisms are believed to underlie the tendency toward later bed and rise times experienced by adolescents. Rigid school schedules require teens to be in class very early in the morning, yet schools are not likely to provide adequate electric light or daylight to stimulate a student's circadian system, which regulates body temperature, alertness, appetite, hormones, and sleep patterns. The circadian system responds to light much differently than the visual system and is much more sensitive to short-wavelength (“blue”) light. Therefore, having enough light in the classroom for teenagers to read and study does not guarantee that there is sufficient light to stimulate their biological systems. Because daylight can provide a robust circadian stimulus of the correct spectrum (rich in short-wavelength radiation), quantity, timing, and duration, it is reasonable to suppose that daylight can affect circadian timing and thus sleep onset at night and rise time in the morning.
Although daylight is an ideal source to synchronize the circadian system, its impact on scholastic performance is still unresolved. To begin to establish a scientific foundation in the context of linking daylight to scholastic performance, three important issues must be addressed. First, it is necessary to identify an underlying mechanism that could plausibly support such a link. Second, it is necessary to measure the stimulus needed to trigger that mechanism. And finally, a formal hypothesis of the presumed biophysical relationship between the measured stimulus and a measured response from the mechanism must be tested. For validation, it is also important to demonstrate that the application of the stimulus consistently produces a predictable response.
The scientific journals Neuroendocrinology Letters, Chronobiology International, and Lighting Research & Technology recently published results1, 2, 3 from field studies done by Rensselaer Polytechnic Institute's Lighting Research Center (LRC) at Smith Middle School in Chapel Hill, N.C. These studies examined how morning light exposure may impact the sleep patterns and self-reports of well-being of teenagers. These studies were also a first attempt to systematically explore the possibility that the human circadian system might be the underlying mechanism in the proposed link between daylight and scholastic performance. Smith Middle School is housed in a building that uses south-facing roof monitors. This is done to deliver daylight to the interior spaces, thereby exposing students to some of the highest light levels of daylight found in an indoor classroom environment.4 In one of the Smith Middle School studies conducted in 2009, the LRC showed that when the same group of 11 teenage subjects wore special orange glasses in the morning to remove short-wavelength (“blue”) light from reaching their eyes, their melatonin onset was delayed by about 30 minutes compared to the previous week, when they did not wear the glasses.
In a larger study at Smith Middle School, 22 students participated before and during school hours for a week. Half the students wore the orange glasses that minimized short-wavelength light exposure needed for circadian-system stimulation, while a control group did not wear the orange glasses. Melatonin onset was delayed (approximately 30 minutes) for those students who wore the orange glasses compared to the control group. Sleep durations were slightly, but not significantly, curtailed in the orange-glasses group. Performance scores on a brief, standardized psychomotor vigilance test (the test lasted about 5 to 7 minutes and subjects had three types of performance tests: reaction times, forced-choice reaction time, and short-term memory task) and self-reports of well-being were not significantly different between the two groups.
In both instances, each student wore a Daysimeter, a small, head-mounted device developed by the LRC to measure an individual's exposure to daily “circadian light,” as well as rest and activity patterns. The definition of circadian light is based upon the potential for light to suppress melatonin synthesis at night, as opposed to measuring light in terms of how it stimulates the visual system.