Science Watch

Nearly 15 million Americans work a permanent night shift or regularly rotate in and out of night shifts, according to the Bureau of Labor Statistics. That means a significant sector of the nation’s work force is exposed to the hazards of working nights, which include restlessness, sleepiness on the job, fatigue, decreased attention and disruption of the body’s metabolic process.

Those effects extend beyond the workers themselves, as many of us share the road with night-driving truckers, count on the precision of emergency-room workers and rely on the protection of police and national security personnel at all hours.

Now, psychologists are gaining a better understanding of how exactly night and shift work affect cognitive performance and which interventions and policies could keep shift workers and the public safer.

"The basic take-home is that fatigue decreases safety," says Bryan Vila, PhD, a sleep expert and criminal justice researcher at Washington State University–Spokane. Learning healthy sleeping practices is "just as important as occupational training," he says.

Natural rhythms

Poor scheduling, combined with unhealthy attitudes about the need for sleep, can cause major problems for night workers. That’s because working at night runs counter to the body’s natural circadian rhythm, says Charmane Eastman, PhD, a physiological psychologist at Rush University in Chicago. The circadian clock is essentially a timer that lets various glands know when to release hormones and also controls mood, alertness, body temperature and other aspects of the body’s daily cycle.

Our bodies and brains evolved to relax and cool down after dark and to spring back into action come morning. People who work the night shift must combat their bodies’ natural rest period while trying to remain alert and high functioning. It doesn’t matter whether they get enough sleep during the daytime, she says. All the sleep in the world won’t make up for circadian misalignment.

That’s especially dangerous for people whose jobs require them to be on high alert and make split-second, life-or-death decisions during the night, such as medical personnel or police officers. It’s common for police departments, for example, to require rookies and lower-ranking officers to bear the brunt of night shifts. They’ll often work a few days during normal daytime hours, then either work an extra-long shift that carries on until the morning, or take a day off, rest, then work a full night shift. But that seesaw scheduling approach is a doubly bad idea, says John Violanti, PhD, an organizational psychologist who was a New York state trooper for 23 years. Not only are these highly stressful, performance-draining shifts being foisted upon the least experienced officers, but the young officers aren’t given time to adjust their sleep schedules for night work. Also, many officers seek night shifts to get overtime pay, he says. According to Vila’s research, roughly 40 percent of the nation’s 861,000 police officers work more than 12 hours a day — and a similar proportion suffer from a sleep disorder such as insomnia or excessive sleepiness.

Working against a person’s natural sleep cycle causes such sleep disorders, as well as fatigue. Fatigue, in turn, worsens moods, decreases cognitive abilities and reflexes, and makes people more vulnerable to disease, says Vila. That resulting crankiness and warped perspective can interfere with one’s ability to make sound decisions and manage people effectively, and can increase the frequency of negative encounters.

That’s not a recipe for good decisions, says Vila, especially when officers must make decisions about whether to use deadly force — often in ambiguous, fast-paced, high-risk situations. Neither is it safe for more routine activities, such as driving. "When you’re drowsy, local parts of the brain shut down for milliseconds or seconds at a time, then come back online," he says.

Night work and fatigue may also contribute to the risk of heart disease and cancer, according to research by Violanti, Vila and colleagues (Policing, Vol. 30, No. 2). Working with 98 Buffalo, N.Y., police officers, the researchers looked for metabolic syndrome — a combination of symptoms that contribute to poor heart health and diabetes, including large waist circumference, elevated triglyceride levels, high cholesterol, high blood pressure and high levels of glucose when not eating. They found that the officers who most frequently worked the 8 p.m.-to-4 a.m. shift had the highest prevalence of metabolic syndrome symptoms. Those who commonly worked that shift and also averaged fewer than six hours of sleep were four times more likely than other officers to have metabolic syndrome.

Vila and Violanti hope their findings will spur changes in shift scheduling, both for the public’s safety and the officers’.

Of course, police officers aren’t the only night-shift workers suffering from circadian misalignment. A number of studies have found that fatigue due to prolonged work hours or being called into work in the middle of the night can result in lapses in judgment and impaired motor skills among medical workers. For instance, a 2008 article in the American Journal of Surgery (Vol. 195, No. 2) reports that, when performing "surgery" on a virtual patient, well-rested surgeons were significantly smoother in their hand motions and made fewer errors than did their sleep-deprived counterparts.

Fatigue is also risky for the physicians themselves, other research has found. A 2005 report published in the New England Journal of Medicine (Vol. 352, No. 2) found that medical interns whose shifts lasted longer than 24 hours were more than twice as likely to have a car crash and five times as likely to have a driving near-miss after leaving work as interns who worked shorter shifts.

In fact, any worker who must drive while tired — whether it’s on the job or driving home after a night shift — is at risk. In 2004, the National Institute for Occupational Safety and Health reported that car crashes are the top cause of occupational fatalities, accounting for 22 percent of work-related deaths between 1992 and 2001. In 7 percent of those cases, drowsiness or falling asleep while driving was cited as a primary factor in the crash. But other factors such as driver inattention, speeding and running off the road can also be influenced by fatigue, the institute found.

Another report from 2004 found that workers across a variety of occupations who worked 12-hour night shifts were more likely than their day-shift-working colleagues to experience physical fatigue, smoke and abuse alcohol.

Possible solutions

Of course, many workers can’t give up the night shift entirely. So the question is, how can night shift workers adapt to their schedules?

There are two ways, says Rush University’s Eastman. One is through symptomatic relief by using such stimulants as coffee and caffeine pills to stay awake during the night, then taking sedatives to sleep in the morning. The other way is to shift the body’s circadian clock so that it better tolerates working at night and sleeping during the day.

Eastman and her team are exploring the latter approach. "The circadian clock is very stubborn and hard to push around," she says.

Previous research has established that you can delay the circadian clock by about one or two hours per day. To determine that, researchers measure the body’s circadian rhythm by monitoring "dim-light melatonin onset," or the time at which the pineal gland begins to secrete melatonin, which is triggered by the circadian clock. Normally, it kicks in a couple hours before people are ready to sleep. "It’s an output that’s a way of seeing what the circadian clock is doing," Eastman says. "It’s a very good marker of the phase of the time of the clock."

By exposing experimental subjects to intermittent bright light during their night shifts and having them wear sunglasses on their way home and sleeping in very dark bedrooms, Eastman and her team have found that within about a week, they can shift someone’s circadian rhythm to align perfectly with working a night shift and sleeping during the day.

But that’s unrealistic for most people, she says. "The problem is that adapting completely physiologically would leave you a nocturnal person, unable to sleep until very late on days off and being out of phase with regular day-working people."

So Eastman and her team have developed a compromise system in which people who work permanent night shifts — say, 11 p.m. to 7 a.m. — adapt their circadian rhythms just enough to function well at night, but still be lively during their days off. It works like this: On his or her day off, the worker goes to sleep as late as possible (in Eastman’s experiment, participants went to bed at 3 a.m. and woke up at noon). On a workday, he or she would be exposed to intermittent bright light, wear sunglasses on the way home from the night shift, then go to sleep as early as possible. "So the difference between sleep on their night shift days and their days off would likely be less than what most shift workers have now," Eastman says.

Last year, she tested the cognitive skills of workers who tried this compromise approach and compared those who fully adapted to a night-shift schedule to those who only partially adapted. Both groups performed about the same, and both scored almost as well as they did when they first took the tests when on a normal daytime schedule (Sleep, Vol. 32, No. 11). "You don’t have to be fully adapted to the night shift to get the benefits of shifting your circadian clock," Eastman says.

However, Eastman points out she doesn’t yet have a solution for workers who have a combination of night and day shifts because it’s impossible to keep shifting their circadian rhythms to keep up with an ever-changing work schedule. She says it falls on employers to assign shift work in blocks, giving workers enough time to adapt.

"You can’t phase shift the circadian clock with a rapidly rotating shift schedule because the clock can’t move that fast," she says. "The only thing you can do is symptomatic relief, meaning you’ll have very sleepy people working at night, which is dangerous."


For more information on improving the health of workers, go to APA’s Psychologically Healthy Workplace Program site.