Abstract
Light Anchors Us to the 24-H day
A circadian timing system, which is a set of clocks throughout the body that keeps daily time, controls many aspects of our physiology, metabolism, and behavior. It controls many internal biological rhythms, including body temperature, hormones, alertness and performance, and metabolic processes. These rhythms are generated spontaneously and have a natural rhythm near to—but not exactly—24 h. The circadian system also influences our behavior, such as the time when we can fall asleep and when we wake up naturally. The 24-h light and dark pattern, detected by the eye, is the key environmental time cue that resets, or synchronizes, the circadian system to the external, 24-h day. An example of this resetting process is observed when the light-dark cycle changes suddenly, as can happen when traveling to a new time zone or during shift work, when the circadian system little by little adapts to the new light-dark cycle. Our circadian system can typically shift up to 1 h each day without additional intervention. Light is the brain’s way of telling time.
How is light information transmitted to the clock?
About 20 years ago, a new light detector (or photoreceptor) was discovered in the mammalian eye that contains melanopsin, a light-sensitive chemical, that is especially sensitive to short-wavelength 480-nm light (also often referred to as “blue light”). This photoreceptor tells the central clock in the brain if it is light or dark, with supporting signals from other photoreceptors. This is how the circadian system synchronizes with the Earth’s 24-h day.
In addition to resetting the circadian system, these photoreceptors can also change the size of the pupil, reduce nighttime melatonin levels (the biochemical signal of darkness, with high levels at night and low levels during the day), and improve alertness, performance, and mood. This alerting response is why light close to bedtime can make it difficult to fall sleep. The physiological effects of light also depend on personal characteristics, such as a person’s “light history” (how much time was spent earlier in the day in a dimly lit room or outside), individual differences in sensitivity to light, and the timing of an individual’s central clock. Most totally visually blind individuals who have no light perception (or those who lack eyes) are unable to send this light information to the central clock. As a result, their clocks cannot reset to the 24-h light-dark cycle and instead follow their non–24-h, internal clock time. This mismatch with environmental time leads to circadian rhythm disruption and sleep problems.
What happens when the circadian system is exposed to light when it does not expect it?
The circadian system evolved in the presence of natural light and darkness. With the invention of artificial light sources, we can fool the brain into thinking it is daytime for longer than the natural day at the flick of a switch. When exposed to light after dusk, our circadian rhythms are pushed later, or delayed. When we are exposed to light before dawn, our clocks shift to an earlier time. Furthermore, when the brain is exposed to light at night, it interprets this to mean that it is “daytime” and consequently increases alertness, lowers levels of melatonin, elevates heart rate and body temperature, and tries to shift the circadian clock to a new time.
The widespread use of electric light means that we are exposed to more variation in the timing of daily light exposure compared with the natural light-dark cycle, as well as too much light at night. We also spend relatively more time indoors; thus, we often expose ourselves to less light during the day than we would naturally experience outdoors. Changes in light exposure levels, timing, and regularity can send confusing signals to the circadian system and disrupt internal timing. Evidence shows that disrupting the timing of the circadian system regularly is bad for health. The higher rates of chronic disease among shift workers (as compared with those working a more standard 9-to-5 schedule) are, in part, attributed to this disruption.
Does the light source matter?
The source of light—natural or artificial—does not really matter. Any light can serve as a signal to the brain. Light level, its spectrum, and the timing and duration of exposure are particularly important. White-appearing light is made up of many wavelengths (like a rainbow) and can vary by light source. Of two light sources with the same light intensity, the one with more short-wavelength light will have a greater impact on the circadian system. While the source does not matter, note that natural daylight, for example, has high levels of light and a relatively high share of short wavelengths, especially in the middle of the day. In contrast, a typical indoor fluorescent light emits less light with less short wavelengths. Thus, the indoor fluorescent light will have less effects on circadian and sleep physiology than daylight. Ensuring exposure to higher intensity light rich in short wavelengths in the day provides the strongest signal to the circadian and alerting systems in the brain and can be achieved by exposure to daylight or man-made light.
General Principles for Better Light Hygiene: A Chronobiologist’s Perspective
A simple rule of thumb is to expose yourself to “bright days, dark nights.”
Daytime light exposure is crucial for resetting your clock. Seek out natural daylight or white light rich in short wavelengths during the day (e.g., outdoor lunch breaks, morning bike rides, or walks outside)—even a short “light shower” helps.
The timing of light is also important. Light exposure after dusk, in the hours leading up to bedtime, delays the circadian clock and alerts the brain, making it difficult to fall asleep and reducing good-quality deep sleep. After dusk, avoid bright light and choose a white light that is lacking or low in the short wavelengths for evening use, and make the light as dim as possible. Use night-dimming modes or software to reduce the light emitted by electronic devices for as long as possible before bed. Make your sleeping space as dark as possible, or use a sleep mask that covers your eyes. Light in the early morning shifts the clock earlier, and so getting up and seeing light earlier can help you keep an earlier schedule, if needed.
Regularity of light exposure is important. Keep a regular daily light exposure pattern.
Light not only influences the circadian system but also affects mood and learning, and it has a direct effect on sleep. Bright days and dark nights will help with these too!
Kids are even more sensitive to light prior to bedtime. Avoid exposing children to bright light or electronic devices as they get ready for bed.
Individuals who spend a lot of time indoors (such as in nursing homes or hospitals) will benefit from lighting systems that are bright and/or contain a lot of short-wavelength light during the daytime, as this helps to mimic outdoor lighting conditions.
