Czeisler’s original area of study wasn’t sleep, but biochemistry. The Chicago native, who attended Harvard for his undergrad work and Stanford for grad school, quickly found himself, in the early 1970s, pulled into the burgeoning field of chronobiology and circadian rhythms. These include, among other things, the fluctuations of blood pressure and body temperature, the sleep-wake cycle, and the secretion of hormones, including cortisol—which was the subject of Czeisler’s first research project. “I became fascinated by the effect of sleep on the release of these hormones,” he says. “That’s how I came into the sleep field.”
In the late ’70s, he began a study of sleep patterns in which he allowed several human subjects to sleep whenever they pleased, freeing them of any “time cues.” “There were many studies in the 1930s and ’40s exploring the circadian rhythms of plants and small organisms, and in the 1950s and ’60s scientists were beginning to understand the effects of light on circadian rhythms in other organisms,” he says. But the prevailing wisdom at the time, largely drawn from mid-century German research, was that humans proved to be the exception to the rule—that somehow we, unlike every other organism, are untethered to the basic rhythm of a 24-hour day.
“One of the weird things that happens when people are living free of time cues, however, is that they exhibit periods of circadian rhythms about an hour longer than the 24-hour day,” he says. “They get to bed an hour later every day and wake up an hour later every day.” With the experiment, Czeisler determined that a human’s duration of sleep is dependent on what time of day it is in the body rather than on how long the person has been awake. “Czeisler demonstrated that the period of the human [circadian system] regulates our biological timing just like other animals,” says UPenn’s Dinges. “It was just a tour de force in science.”
When Czeisler and his fellow researchers published his conclusion in the journal Science in 1980, the article ended with a flash of scientific speculation that the findings may have an impact on “shift workers.” “Suddenly I’m in my apartment and I’m getting calls from The London Times about shift schedules,” he says.
The story found an audience among factory owners, and he got a call from Ogden, Utah’s Great Salt Lake Minerals and Chemical Co., a major player in the mining of potash, an ingredient used in fertilizers. He learned that the company’s shift schedules recessed “counterclockwise,” a common industrial practice at the time. Employees worked the night schedule (midnight to 8 a.m.) for a week, then the evening schedule (4 p.m. to midnight) for a week, then the day schedule (8 a.m. to 4 p.m.) for a week. According to Czeisler, this practice was harmful to worker productivity for several reasons.
If you work the 4 p.m.-to-midnight shift, your circadian rhythm dictates that you go to bed sometime around 3 a.m. and wake up sometime around 11 a.m. If you then rotate counterclockwise—to a shift beginning at 8 a.m.—that will require you to force yourself onto an earlier bedtime to wake up at roughly 6 or 6:30 a.m. to get to work. (For obvious reasons, this is not ideal.) But if you rotate in a clockwise direction, from a shift beginning at 4 p.m. to one beginning at midnight, “you are halfway there,” Czeisler says. “The circadian system in humans runs a little more than 24 hours”—demonstrated by his research subjects who unknowingly went to bed an hour later each night—so it’s much easier for your body to roll forward in that direction. For any corporate road warrior who has flown from New York to Los Angeles and adjusted easily to West Coast time—or suffered the consequences of the reverse—this observation will sound familiar.
After the company in Utah implemented his suggestions, production increased and employees reported happier working conditions. Czeisler then founded the Center for Design of Industrial Schedules, a “fatigue risk-management consulting” nonprofit organization that led him to 10 years of “working among smokestacks in Texas” and eventually with professional sports teams. He went on to consult for entities ranging from police departments to Delta Air Lines to the Department of Homeland Security and NASA. He also advised the investment bank Morgan Stanley. “Bankers,” he says, “have the worst sleep schedules of all.”