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(The second of a three-part story on how sleep impacts
health)
DENNIS
CORRIGAN sometimes questions his decision to switch to a
night shift 12 years ago. By working nights, the UPS
truck driver from West Covina, age 52, avoids the
physical demands of the day shift, when lifting boxes is
part of the job, plus the worst of L.A. traffic. The
10:45 pm-to-11 am shift also allowed him to attend all
of his son’s high-school football games.
But
Corrigan now sleeps only about six hours a day. He has
put on weight and gets less exercise than before the
switch and was diagnosed with diabetes five years ago.
“The
rough part is, when I come home, I’m hungry,” he says.
“I eat a heavy meal before going off to bed. You’re not
supposed to do that. It’s a worry.”
His
circadian rhythms may be to blame. Those rhythms
determine when certain body processes take place. For
example, melatonin, the hormone that aids sleep, is
released at night; the hormone cortisol is low at night
and pours out in the morning, jump-starting the body’s
daytime functions. But in night workers, melatonin
continues to peak at night—even though they’re awake—and
cortisol levels continue to peak in the early morning
hours, even when night-shift workers are eager to get
some sleep.
Those
disrupted circadian rhythms are why night-shift workers
sleep less and with poorer quality, Van Cauter says:
they try to sleep when their bodies want to be awake.
Chronic
sleep deprivation may carry some of the same risks as
disrupted circadian rhythms, she says. Today, Americans
average about one hour less of sleep per night than they
did 30 years ago.
Liberty
Bunag, the registered nurse, feels the effect of
night-shift work on her days off. If she tries to sleep
at night, she often wakes around 3 am and is alert until
dawn, when she falls back to sleep, often for 10 hours.
On work days, she sleeps about six hours during the day
but still awakens tired.
“My
problem is not while working but on my days off,” she
says. “I feel unproductive because all I do is sleep all
day, and I’m up the whole night when nothing much can
get done.”
She
finds herself less willing to socialize these days and
worries that her irritability may border on depression.
She also wonders about the long-term health consequences
of her schedule. “I want to be able to sleep normally at
night when the body does all of its detoxifying,
cleansing, repairing and recharging. But I haven’t
figured out what’s going to work for me.”
Her concerns are well-founded.
Night-shift workers have a 40 percent to 50-percent
increased risk of heart disease compared with day
workers, various studies have found.
People
who get five hours of sleep, common among night-shift
workers, are 50 percent more likely to be obese than
normal sleepers, Columbia University researchers have
found. Several dozen other studies have tied sleep loss
to weight gain as well.
Women
night-shift workers have higher rates of miscarriage,
preterm birth and low birth-weight babies.
Night-shift workers show increased rates of breast (by
50 percent) and colon (by 35 percent) cancer in
numerous, independent studies. And animal studies have
shown that exposure to dim light during the night-time
can substantially increase tumor development.
“It’s
been known for years that there is an increased risk of
a variety of medical conditions in the population of
shift workers,” says Dr. Diane Boivin, associate
professor of psychiatry and director of the Center for
Study and Treatment of Circadian Rhythms at Douglas
Mental Health University Institute in Montreal. “What is
difficult to parcel out is the exact contribution of
this circadian misalignment and sleep deprivation. We
think it’s major, but it’s very difficult to be sure.”
Genes
that keep time
SCIENCE
is inching closer to understanding how a lack of
sleep—or sleep at the wrong time—can wreak biological
havoc. In the last few years, researchers have made
surprising discoveries about the body’s sophisticated
time-keeping.
Scientists once assumed the body’s sole “clock” was
nestled in a place in the brain called the
suprachiasmatic nucleus, or SCN. Light—particularly
sunlight—is the primary synchronizer for circadian
rhythms. When we open our eyes each morning, light
reaches photoreceptors in the retina and creates signals
that travel to the SCN to jump-start the body’s
hormones, neurotransmitters, temperature and metabolism
for the new day.
But
that’s not the body’s only timepiece. Circadian
time-keeping genes can be found in organs all over the
body. These peripheral clocks control the activity of
many cellular processes and biological functions, and
their presence may explain why sleep dysfunction seems
to have such a broad effect on overall health. Light
sets the circadian clocks in the SCN, but scientists
still aren’t sure what compels the body’s peripheral
clocks to work in unison. After all, the liver and the
digestive tract don’t have direct access to sunlight.
“The SCN
is like a musical conductor and the peripheral clocks
are the instruments that need to play their music with
peak activity at certain times of the day to get good
harmony across the body,” Boivin says. “They must be in
sync.”
The SCN
somehow “talks” to these peripheral clocks by using
hormones or other messengers, says Eve Van Cauter, a
professor of medicine at the University of Chicago.
For
example, among its many functions, the nighttime release
of hormone melatonin is thought to inhibit tumor growth.
Melatonin production usually peaks in the middle of the
night. But it stops being secreted when light reaches
the eye in the morning—or when a light is switched on
during the night.
This
disruption could influence the genes involved in tumor
development, says Paolo Sassone-Corsi, chairman of the
department of pharmacology at UC Irvine, and provide a
possible explanation for why cancer rates appear higher
in shift workers.
(Next week: How to prevent sleep-deprivation damage.) |
