Biological Rhythms, Shiftwork, and Occupational Health

“… and God divided the light from the darkness, and God called the light day, and the darkness He called night. And the evening and the morning were the first day” (Genesis 1:4–5, King James Version). Thus, as has been recognized for millennia, Homo sapiens, as well as all other living creatures on earth, are destined to live in a regular cycle of light and darkness, that is, the 24-h solar day. For diurnal species, such as human beings, the sunlight portion of the day is the time of activity and the dark, nighttime portion the time for sleeping. Periodicity is an integral part of life. Although we are under the influence of environmental rhythms, such as the daylight–night cycle, we are also under the physiological influence of our own internal biological clock. Normally the synchronization of our biological rhythms with each other and with environmental rhythms (external time cues) maximizes our waking and sleeping performance and promotes overall well-being. Night work is opposed to the innate drive to sleep at night and work during the daytime. This unnatural mismatch of environmental and internal temporal influences is of concern for shiftworkers due to the often disruptive effect of schedule-related time shifts on the normal synchronization of individual biological rhythms with each other as well as with the external time cues. This chapter reviews basic chronobiological principles as they relate to shiftworker safety and health. Studies dealing with the effects of time shifts on sleep and alertness are discussed as well as performance rhythms. Research exploring the consequences of shiftwork on physical and mental health is reviewed. Countermeasures for minimizing adverse health and safety effects of sleep deprivation and biological rhythm disruption are presented, including work scheduling considerations and medical surveillance. Industrial hygiene considerations related to control of worker exposure to potential toxins during extended and rotating shifts are presented. Finally, international and U.S. regulatory policy regarding shiftwork scheduling and special provisions for shiftworkers are reviewed. Keywords: Circadian rhythms; Biological rhythms; Biological clock; Time shifts; Zeitgebers; Melatonin; Photoreceptors; Shiftwork; Performance; Safety; Sleep; Sleep deprivation; Rotating schedule job; Accidents; Errors; Public disasters; Transportation incidents; Medical disorders; Depression; Medical screening; Surveillance; Gastrointestinal disorders; Cardiovascular morbidity; Mental health; Reproduction; Scedule changes; Regulation; Countermeasures; Caffeine; Diet; Exercise