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Have we forgotten about our circadian clock?

Posted by Dr. Sushrut Jangi  December 6, 2013 08:30 AM

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"Night shifts really affect me," Jamie Horton tells me.  

A nurse employed on one of the busiest floors of a Boston hospital, Jamie typically works two weeks of day shifts before switching to a row of nights. The sudden change in schedule palpably affects her health. 

"I typically refer to it as feeling like I am hung over for days. I get sicker much easier," she says. In the dark winters of Boston, when ambient light is low and the days are mercilessly short, her discomfort is often magnified: "It's already dark when I wake up and only sunny for a very short time before I go to bed. As far as my mood goes, I am miserable." 

But Jamie's getting enough sleep: this isn't straight up sleep deprivation. What bothers her is the abrupt change in her sleep-wake cycle: one day she goes off to work during sunrise, another day she goes to bed at dawn. Her schedule, however unpalatable, is not uncommon in this country: nearly 10 percent of the American workforce participate in disorienting, rotating work schedules that lead many shift workers to describe symptoms of malaise, depression, or inattention, which can, at their worst, impair workers' performance. 

Just earlier this week, a Metro-North Railroad train derailed in the Bronx, killing four passengers and injuring more than 70. It was reportedly operated by an engineer who "nodded off" moments before the crash. While sleep deprivation may be to blame, so may be the consequence of his early work shift. That morning, his shift began around 5 a.m., before sunrise. Two hours later, he was still "in a daze. I don't know what happened," he reportedly told investigators. 

Jamie's symptoms during her changes in the shift and the train accident in the Bronx do not come as a surprise to Dr. Frank Scheer, who has made studying the physiology of shift workers part of his research focus. "Do you know about the suprachiasmatic nucleus?" he asked me. 

Dr. Scheer directs the Medical Chronobiology Program at Brigham and Women's Hospital. His question is a valid one: I don't think most doctors know an answer when we are evaluating a patient's health. The suprachiasmatic nucleus, Dr. Scheer describes, is our master clock. 

A tiny part of the brain (about the size of a grain of rice) located just behind the eyes, the master clock is a fleshy, pink network of neurons linked by a series of timed chemical reactions instead of cogs, sprockets, and second-hands. Aesthetically, it's an unconventional time-piece but a functioning clock all the same--this bundle of neurons wakes us up, tells us when we're hungry, adjusts our hormones, regulates our body temperature, and reminds us when it's time for bed. Hundreds of other clocks, located throughout the body's cells, respond to the master clock the way a symphony follows the lead of a conductor. All fine when the master clock works--but what keeps the suprachiasmatic nucleus running on time? 

Daylight. The moment we open our eyes, little photons from a sunray sail through the chambers of the eye, striking the film-like retina and sending a message to the master-clock:  "Time to reset!" the sun says. Eventually, after a series of such mornings, the master clock adjusts to the solar cue. The body's peripheral clocks then adjust to the change in the master clock. 

But now imagine Jamie's schedule--her internal clock has no time to orient to the earth's light and dark cycles; consequently, her master clock is thrown out of wack, and with it, all of the body systems it helps to regulate, which disrupts mood, digestion, and even menstruation. 

"If you are doing rotating shift work, the internal body clock just can't keep up," Scheer says. Living and working on an irregular schedule, Scheer says, is akin to jet lag. It's like suffering the effects of changing time zones without leaving the country. What's worrisome is when the body's clocks stay unsynchronized chronically, and lots of systems begin to go haywire. Over time, "people can develop glucose intolerance, high blood pressure, or suffer cardiovascular events. They may even have trouble losing weight,"  Scheer warns.  

Exactly how glucose intolerance or high blood pressure could result from a disrupted circadian clock is still being worked out. The field of chronobiology is mysterious, if not previously hokey, in the medical community. But lately, surprising findings are emerging: several groups have found that some medications, including chemotherapies, may be more effective when administered at specific times of the day. A diagnostic test can give varied results when blood is drawn in the morning rather than in the afternoon. 

"We're used to thinking of ourselves as computers," Janis Anderson says, a Harvard psychologist who studies the effect of light on human biological rhythms. "But we can't be just turned on and off. We operate on a cycle," she reminds me. 

Perhaps in the era of Starbucks and smartphones it's easy to forget that human beings don't come with an on-off switch. For the last tens of thousands of years, we have carried out our lives under the watchful eye of the sun and the dark expanse of the night sky. 

"The best thing to do," Scheer says, "is to maintain regularity. Try to wake up, take your meals, and sleep regularly, even on weekends." A corollary of his advice: listen to your body - tune into its cycles rather than working through them.  It's good advice. Things will always get in the way of course: shifts, deadlines, families, even recreation. But - as much as you hate to hear it - our body and mind are tethered to the goings-on of the natural world. 

Here's the alternative. Some laboratories have managed to destroy the master clocks of rats. These animals wake up whenever. They sleep haphazardly. They don't really like to rest. Many get sick, even growing tumors faster than normal animals. The rats work their exercise wheels indiscriminately, no matter if it's night or day. Look at them running ceaselessly. Remind you of anyone?
This blog is not written or edited by Boston.com or the Boston Globe.
The author is solely responsible for the content.

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About the author

Sushrut Jangi is an internist at Beth Israel Deaconess Medical Center and an editorial fellow at The New England Journal of Medicine. More »

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