Squashing versus Nudging Circadian Rhythms with Artificial Bright Light: Solutions for Shift Work?

SQUASHING VERSUS NUDGING CIRCADIAN RHYTHMS WITH ARTIFICIAL BRIGHT LIGHT: SOLUTIONS FOR SHIFT WORK? CHARMANE I. EASTMAN* Circadian Rhythms and Shift Work Humans, like other animals, are ruled by stubborn, autonomous body clocks. These internal oscillators produce circadian rhythms in almost all variables that have been measured, such as body temperature, hormones , neurotransmitters, DNA synthesis, sleepiness, performance, and so on [I]. Although the daily cycle of sleep and waking may be the most conspicuous circadian rhythm, it is not responsible for producing the circadian rhythms in the other variables. Experiments in which subjects are deprived of sleep for several days demonstrate the persistence of circadian rhythms [2]. The typical student experiences the power of these circadian oscillations when staying up all night studying for an exam. When body temperature reaches its minimum (about 4—6 a.m.), it may seem almost impossible to fight off sleep. Later in the morning, as body temperature rises, she or he will become more alert and start to feel better, despite the further accumulation of sleep deprivation. Our modern technologically oriented society encourages us to ignore the clock within. Jet travelers arrive in a new time zone with body clocks still set to home time. Insomnia, daytime sleepiness, and gastrointestinal problems persist until the stubborn internal oscillators slowly and gradually shift (by 1-11A hours/day) to match the new time zone [3]. Similar symptoms are common in shift workers whose schedules include nights. The internal circadian rhythms rarely shift completely to match the work and sleep schedule of the night shift [4]. Thus, the workers are asked to sleep, work, and eat at the "wrong" phases of their circadian Work supported by NIH grant ROl NS23421. The author expresses gratitude to Mary Simon, Barbie McCluskey, and Glenn Good for their technical assistance. *Biological Rhythms Research Laboratory, Rush-Presbyterian-St. Luke's Medical Center , 1653 West Congress Parkway, Chicago, Illinois 60612.© 1991 by The University of Chicago. All rights reserved. 0031-5982/91/3402-07l3$01.00 Perspectives in Biology and Medicine, 34, 2 ¦ Winter 1991 \ 181 cycle. One reason for the incomplete adaptation of circadian rhythms is that the sleep-wake (SW) schedule is not consistently maintained at the new phase; workers revert to sleeping during conventional hours on days off. Another reason is that many of the Zeitgebers or time cues, such as the natural light-dark (LD) cycle and social cues, maintain their original phase position and may oppose the shifting of circadian rhythms. Thus, whilejet lag eventually abates, "shift lag" [5] is a more or less permanent affliction. Airline pilots and flight attendants suffer the double whammy ofjet lag plus shift lag. The "graveyard" shift poses the greatest hazard to the individual and society. Sleepiness on the night shift is common and can cause accidents [6]. Well-known instances of night-shift catastrophes include those at Three Mile Island and Chernobyl. Health problems are most likely to occur with night work [7, 8]. The prevalence of shift work in industrialized countries has increased dramatically over the decades. This is not surprising when we consider the many industries and service occupations that require night work: nuclear power plant operators, factory workers, computer users, airline personnel, police, military personnel, nurses and other health care professionals, and the like. In 1980, about 26 percent of male and 18 percent of female American workers rotated between day and night shifts [9], If we include workers on permanent night shifts, then the proportion of people who work nights is even larger. Although the problems of shift work have been extensively documented [10—12], relatively few interventions based on circadian rhythm principles have been attempted [13—15]. The Discovery of the Power ofBright Light A recent breakthrough has put a new tool—artificial bright light— into the hands of modern science. It is as if we can trick Mother Nature into believing that the artificial bright light is sunlight and shift circadian rhythms faster than ever before. Dramatic preliminary results have led the media and public to expect immediate solutions to modern man's circadian rhythm problems. Scientists are usually more cautious. It all began when Dr. Alfred Lewy returned home from a trip to Australia, nine...