All creatures, great and small, are governed by the earth’s 24-hour cycle of light and dark, day and night. Light reaching the back of our eyes is the main stimulus that helps the circadian clock, and thus circadian rhythms, to synchronize with the 24-hour day. Circadian rhythms are every rhythm in our body that repeat at approximately every 24 hours, such as the sleep-wake cycle. In the absence of this daily light- dark pattern, the circadian system will run with a period slightly greater than 24 hours (on average, 24.2 hours) and, therefore, will be desynchronized with the local time on earth. Light of the appropriate quantity, spectrum, duration, and timing can have a profound effect on sleep, alertness, performance, and mood. Irregular light-dark patterns or exposure to light at the wrong circadian time may lead to circadian disruption. Short-term circadian disruption leads to poor sleep and poor performance. Circadian disruption over many years, such as that experienced by night shift workers, has been associated with health risks, including diabetes, obesity, cardiovascular disease, and cancer.
Lighting characteristics affecting the circadian system, as measured by acute melatonin suppression and phase shifting of dim light melatonin onset (DLMO), are different than those affecting visibility. When specifying lighting for the circadian system, it is important to consider light level, spectrum (color), timing and duration of exposure, and photic history (previous light exposures). To estimate if any lighting system can provide the levels of circadian light necessary for circadian entrainment, it is important to know the spectral irradiance distribution of the light incident at the cornea. From this spectral irradiance distribution it is then possible to calculate circadian light (CLA), irradiance at the cornea weighted to reflect the spectral sensitivity of the human circadian system as measured by acute melatonin suppression after 1 hour exposure, and circadian stimulus (CS), the effectiveness of the spectrally weighted irradiance at the cornea from threshold to saturation.
Introducing a regular 24-hour pattern of light and dark by modulating the amount of electric lighting (and daylight) we are exposed to daily can have many positive benefits. Electric lighting in senior housing facilities, for example, is typically dim and constantly operating, 24-7. Several field studies have shown that introducing bright light during the day and dim light at night enables seniors to consolidate their sleep during the night and reduce their napping during the day. Better sleep quality and quantity improve mood and behavior. This same, simple principle of modulating the amount of electric lighting over 24 hours has been shown to be effective for improving sleep among crew members in U.S. Navy submarines and may also be effective for improving health outcomes in premature infants. Seniors, submariners, and premature infants are certainly special cohorts, but there is every reason to believe that a regular 24-hour pattern of light and dark can improve the health and wellbeing of anyone.