Your Body’s Witching Hours
Circadian rhythms help determine
when we are most vulnerable to disease
By Melinda Beck in the Wall Street Journal
Heart attacks often occur in the
morning. Epileptic seizures peak in the late afternoon. Asthma attacks get
worse and more deadly between 11 p.m. and 3 a.m.
Researchers are finding that
circadian rhythms, which cycle every 24 hours or so, drive virtually every
system in the human body, from circulation and cognition to metabolism, memory
and mood. And they play a big role in determining when we are most vulnerable
to disease.
Chronobiology—the study of these
internal clock mechanisms—has exploded in recent years thanks in part to the
discovery of specific genes, to which scientists have given names like Clock,
Period and Cryptochrome. Those genes help keep our biological systems in sync
with light and darkness, which makes for a rush hour of chemical changes at
dawn and dusk.
Understanding biorhythms is helping
doctors direct treatments, including the best times to take various
medications. It is also suggesting new treatment strategies, such as adjusting
the light in nursing homes to help people sleep better.
Of course, people often disrupt
their circadian cycles with jet travel, erratic work schedules, watching TV and
Web surfing long into the night. That can raise the risk for a variety of
health problems, including heart disease and diabetes.
The body’s master time keeper is a
group of neurons in the hypothalamus, located behind the eyes, called the
suprachiasmatic nucleus, or SCN. In darkness, the SCN prompts the pineal gland
to release melatonin, the hormone that facilitates sleep. Other chemical
changes reduce body temperature, blood pressure and heart rate, all of which
are at their lowest overnight.
Other systems are highly active at
night. Stomach-acid production peaks between 10 p.m. and 2 a.m., digesting the
evening meal and possibly exacerbating heartburn. The liver dumps glucose into
the bloodstream just before dawn. Leptin, a hormone that damps appetite, is
high, so people tend to be least hungry when they wake up. And the immune
system may be overactive, inflaming airways in asthma sufferers and swelling
arthritic joints.
In the morning, light on the retinas
signals the SCN to shut off melatonin. Cortisol, a stress hormone, rises
instead, preparing the body for the day’s demands. Blood pressure and heart
rate start increasing. A substance called PAI-1, which makes blood clot more
readily, peaks around 6:30 a.m. Experts think PAI-1 may have protected early
humans from bleeding to death from an injury as they set off foraging for food.
But in modern humans with plaque-lined arteries, the substance raises the risk
of blockages that cause heart attacks and strokes. Those peak around 9 a.m.,
studies show.
Exposure to bright light at a time
the body isn’t used to can advance or delay the circadian cycle. But that
doesn’t happen right away, as jet-lag sufferers know. An earlier study that
lasted 11 years found that heart attacks among vacationers in Hawaii corresponded
more closely to early morning back home than they did to local time.
Eating, sleeping and other
behavioral patterns, of course, also affect the body’s daily changes. But
various biological systems, including blood pressure, heart rate, blood
clotting and adrenaline, rise and fall in circadian rhythms of their own,
according to research at the Medical Chronobiology Program at Brigham and
Women’s Hospital in Boston and at other centers. If those behavioral patterns
and circadian rhythms get out of sync, a variety of health problems can result,
as researchers have shown in experiments with lab animals and observed in
humans.
For example, when rodents are fed
during their usual sleep time, their liver and pancreas adjust to that feeding
schedule but their SCN master clock doesn’t. Even if the amount of food is the
same, they will gain more weight and fat than rodents fed at the normal time,
says Frank Scheer, the chronobiology program’s director.
Shift workers—who make up about 10%
of the U.S. workforce—are vulnerable to similar problems when they eat, sleep
and work out of sync with their circadian rhythms. Many have higher rates of
heart disease, diabetes, obesity, high blood pressure, stroke and other
disorders.
Millions of other people develop
what experts call “social jet lag” by staying up late and sleeping late a few
days each week. “If you do that Friday, Saturday and Sunday, then by Monday
morning, when you have to go to work, you are waking up at a much earlier
circadian time, which can have consequences for days to come,” says Dr. Scheer.
Even the one-hour shift that occurs with setting clocks forward in the spring
raises the rate of heart attacks for a week, studies show.
Surveys of tens of thousands of
adults by the University of Munich’s Institute of Medical Psychology have found
that the greater the difference between sleep time on weekdays and weekends,
the more likely people are to be obese.
Many people with depression, bipolar
disorder and other mental-health issues have disrupted circadian rhythms and
erratic sleep patterns. Patients with bipolar disorder tend to have cycles that
are longer than 24 hours, says psychiatrist Michael McCarthy, a member of the
University of California, San Diego’s Center for Circadian Biology. There is
growing evidence that these problems may be related to defects in clock genes.
Sleep cycles become earlier, shorter
and more fragmented as people age. “The studies all show that having disrupted
circadian rhythms puts older people at greater risk for cognitive impairment,
depression and mortality,” says Sonia Ancoli-Israel, a professor emeritus of
psychiatry at the UCSD center.
Such issues are even more pronounced
in people with Alzheimer’s disease, whose sleep, core temperature and other
biorhythms don’t follow predictable patterns. The increased restlessness,
irritability and confusion known as “sundowning” that some Alzheimer’s patients
experience in late afternoon isn’t well understood. Another mystery is the
so-called witching hour surge in crankiness that babies often demonstrate at
the end of the day.
“At the extremes of life, in babies
where the brain hasn’t fully developed or in dementia patients, when you are
getting degeneration, the time connections in the brain may not be fully
coordinated,” Dr. McCarthy suggests. “And fatigue makes everything worse.”
Researchers are starting to develop
treatment strategies to take advantage of circadian rhythms or restore them
when they are out of sync—a field called chronotherapy.
“It’s a very promising area but the field
is very young. More clinical trials need to be done,” says Steven Shea, a
professor of public health and preventive medicine at Oregon Health and Science
University.
Taking medications at night that
help prevent heart blockages, such as long-acting beta-blockers and ACE
inhibitors, may lower the risk of early morning heart attacks, experts say.
Taking acid-blocking drugs at night might be more effective, given the
overnight surge of stomach acid.
Light therapy has been shown to be
effective at improving sleep among older people in nursing homes, according to
several randomized, controlled trials. “Many older adults are not getting
sufficient sunlight,” says Dr. Ancoli-Israel, of UCSD. She says nursing-home
patients typically get only a few minutes of bright sunlight a day.
Administering melatonin to aid sleep
also has been studied, but with mixed results. And melatonin can worsen glucose
tolerance, so it isn’t useful for everybody.
In general, experts say going to
bed, getting up and eating meals at the same time every day, getting lots of
light in the morning and avoiding it at night can go a long way to improve
health and mood. “It’s the same advice we’ve always given,” says Dr. McCarthy.
“But now we understand a lot more about why that’s true.”
The entire post with
all the graphics can be found at: http://www.wsj.com/articles/your-bodys-witching-hours-1433198297
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