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orchestra players with clocks of their own. “All your
organs have rhythms,” says Steven Lockley, an associ-
ate professor of medicine at Harvard Medical School
who studies circadian rhythms and sleep. “There’s
a clock in your heart, a clock in the lungs, a clock
in the kidneys.” Just about everything in the body—
metabolism, hormones, the immune system, repro-
ductive function and the way DNA is translated—is
influenced by a circadian rhythm, he says.
And not everybody’s is the same. People’s internal
clocks are often hours off from one another, Lockley
says. “The range of individual differences is much
bigger than anyone really understands yet.”
The body’s complex clock system has implica-
tions for both healthy people and those with medical
conditions, and scientists are already seeing glimpses
of how they can time certain tests and treatments
to get more accurate or potent results. A cholesterol
reading, for example, might be affected by what time
of day you go to the doctor’s office, because the liver
(which makes cholesterol) has a circadian rhythm.
“The time of day at which you measure something
could make someone look clinically abnormal, even
though they’re not,” Lockley says.
Medicine might also be more effective if taken at a
certain time. Because they’re metabolized in the liver,
“drugs change their effects throughout the day,” Lock-
ley says. Other circadian bodily processes, like cell
function, can also affect how medication acts. Early
research suggests certain drugs— including some for
colorectal cancer, pain and asthma— perform better
or are less toxic when taken at different times of day.
Exercise, which can be as powerful as medicine
for some conditions, is good for you whenever you
do it. “But I do think that the time of day may have
an influence, on top of the effects of exercise, on our
metabolic health,” says Juleen Zierath, professor of
physiology at Karolinska Institute in Sweden. In one
small study published in 2018 in the journal Diabe-
tologia, Zierath and her team started 11 men with
Type 2 diabetes on a high- intensity interval train-
ing program. The men exercised either in the morn-
ing (around 8 a.m.) or the afternoon (4 p.m.) for
two weeks, then switched schedules. The research-
ers expected that regardless of the time of day, men
in both groups would see improvements in blood-
sugar levels. But “when they exercised in the morn-
ing, they actually had slightly higher levels of blood
sugar [than baseline], which we didn’t expect at all,”
Zierath says. It’s not clear to what extent the type of
exercise and other variables matter, but the study
provides an intriguing hint that time of day might
make a difference for exercise.
Scientific knowledge is nascent when it comes to
optimizing testing and treatment by the clock. Our
understanding of individual circadian time is even
more primitive. But Lockley believes it’s the key to
personalized medicine; he and others are exploring
ways to measure a person’s internal circadian time
through simple clinical tests. “Hopefully in the next
five to 10 years, you’d go to the doctor, give a breath
test or a pee sample, and the doctor would know your
biological time,” he says. “Then all your test results
and treatments could be based on your real internal
time, which is going to be very different between you
and me based on our internal clocks.”
For now, even the understanding that our bod-
ies often operate according to different clocks is a
big scientific advance. It’s already changed the way
Chou sleeps, lives and works. “I’ve just become more
comfortable with accepting my sleep,” she says. She
now asks her employees about their sleep schedules
to anticipate when each will be at their peak. She also
informs everyone about her own abbreviated sched-
ule, so they know she doesn’t expect an immediate
response to an email she sends at 4:05 a.m. “That’s
just when my brain is working,” she says. □