Light is the major environmental time cue that resets the circadian pacemaker in the the mammalian hypothalamus. Light information is captured exclusively by the eyes using specialized cells containing a blue-light sensitive photopigment. Each day the light-dark cycle resets the internal clock, which in turn synchronizes the physiology, psychology, and behavior controlled by the clock. Failure to receive this light-dark information, as experienced for example by totally blind individuals, causes the circadian pacemaker to revert to its endogenous non-24-hour period and possibly become desynchronized from the 24-hour light-dark cycle. Exposure to irregular light-dark cycles, as experienced for example by psychiatric patients with irregular sleep-wake cycles, can also disrupt circadian rhythms. Light also suppresses the hormone melatonin and has a direct arousal effect on the brain, improving alertness and performance. This property of light can be useful as a non-pharmacological treatment for fatigue in a number of conditions, and if timed appropriately, these effects can complement the circadian phase resetting effects of light, for example in treating shiftwork and jet-lag disorders, to help maintain alertness at the correct time and subsequently improve sleep. The results of our experiments in which gradual vs. slam-shift changes in schedule along with continuous or intermittent light exposure are tested for their effects on circadian rhythms, sleep, hormones, subjective alertness, and objective performance will be applicable to conditions such as jet lag, and shift-work or night-work. Millions of workers in the safety, security, transportation, healthcare, and industrial sectors are affected by these conditions yearly, with effects on health and safety.
The development of (i) mathematical models of circadian rhythms, sleep, alertness, and performance, and (ii) software based on these models to facilitate schedule design, can improve performance and alertness and thereby effectiveness and public safety for people who work at night, on rotating schedules, on non-24-hr schedules, or on extended duty schedules (e.g., pilots, train and truck drivers, shift workers, healthcare workers, public safety officers). Attempting to sleep at adverse circadian phases is difficult, resulting in poor sleep efficiency. Similarly, attempting to work at adverse circadian phases, and/or after a long time awake, results in poor worker performance and productivity and leads to an increase in errors. For example, the accidents at the Chernobyl and Three Mile Island nuclear reactors and the Exxon Valdez grounding were all partially attributed to employees working at adverse circadian phases and the FAA (Federal Aviation Administration) reports of air traffic controllers sleeping while scheduled to work at night are related to their work schedule. The mathematical models and the available software that implements these models can be used to simulate and quantitatively evaluate different work and light exposure schedules to predict the expected circadian phase, subjective alertness, and performance in an individual.
Our software has been requested by members of NASA, academia, government, and industry, including airline, safety, medical, and military applications. Its use could help produce improved work schedules for both astronauts and ground-crew. The mathematical modeling efforts and software have also been used in educational programs and in the popular press to teach students and teachers about circadian rhythms and sleep and their effects on alertness and performance. NIRS monitoring may be useful in identifying individuals who might be at increased risk of sleep-related errors and occupational injuries. The cost-effective and minimally intrusive NIRS (Near Infrared Spectroscopy) assessment of regional brain activity may be applicable in personnel in safety-sensitive occupations, for better understanding the physiology underlying attentional failures, and for developing countermeasures for these failures.