epinephrine—also known as adrenaline—which
floods the blood and initiates a number of short-
lived physiological changes. Heart rate and sweating
increase, and blood flows more freely to the limbs.
The airways of the lungs expand to bolster breath-
ing capacity, while the movement of food through
the digestive system slows. Throughout the body,
energy and other resources are temporarily reallo-
cated in order to help it meet any physical trials.
While all this is going on, the cells of the im-
mune system release pro-inflammatory chemicals
called cytokines, Kertes says. Because inflamma-
tion works against injury and the spread of infec-
tion, this is part of the immune system’s way of
gearing up for battle.
The fight-or-flight response also affects think-
ing and cognition. Stress can help you stay focused
and block out distraction. But at the same time,
your ability to multitask may be temporarily im-
paired. Research has also shown that during peri-
ods of high stress, some aspects of memory storage
and recall take a hit while decision-making, for bet-
ter or worse, speeds up.
Your ability to think critically also suffers. “The
prefrontal cortex is the part of the brain that helps
you use reasoning to guide your behavior,” Kertes
says. When you’re experiencing strong emotions
or urges, the prefrontal cortex can help keep those
emotions and urges in check. “But under condi-
tions of stress, we see less activity in this region,
and people seem like they’re not acting rationally,”
she explains. Raw emotion tends to take over, and
this can lead to choices that seem helpful in the
moment—throwing a punch, for example—but that
have negative long-term consequences.
While short-term, adrenaline-driven stress re-
sponses are occurring, Kertes says, the hypothala-
mus also activates a second “stress system” to pre-
pare the body for action. This second system is
slower-acting but longer-lasting, she says, and it
involves the hormone cortisol. “When we’re talk-
ing about the long-term effects of stress, we’re usu-
ally talking about the effects of excessive cortisol
circulating in the body,” she explains. “When stress
goes on too long or occurs very frequently, it’s this
system that can negatively affect immune function,
memory, fear and risk for disease.”
Cortisol is often talked about as a “stress hor-
mone.” The body’s cortisol levels naturally rise
and fall even in the absence of stress. “On a dailyto that external threat. Once the threat goes away,
stress should go away.
Anxiety, on the other hand, is the internal alarm
or concern that arises even when an external threat
is not physically present. Anxiety is also the antici-
pation of a threat that never materializes. “The pro-
totypical example is that you work for a nasty boss
who is conflictual,” Slavich says. “You know you
have a meeting with this boss at the end of the week,
and your body has the ability to mount its immu-
nologic stress response every day between now and
Friday whenever you’re thinking about your boss.”
Even if the meeting is canceled—meaning you’re
never actually confronted with the source of your
anxiety—you’ve still weathered the effects of stress.
It may seem odd that your body would react
to the threat of a browbeating boss with the same
systems that defend it from a physical attack. But
Slavich says it makes a lot of sense when viewed
through the lens of human history and evolution.
“The brain and body developed the ability to scan
their environment for social conflict or social isola-
tion and to respond to these social dangers because,
in the past, these often led to serious insult or in-
jury,” he explains. Human beings who possessed
a measure of anxiety and who were on guard for
potential threats likely survived in greater num-
bers than their devil-may-care companions, and so
passed on their anxiety genes to later generations.
Even today, a little anxiety can be beneficial. It
can help you anticipate trouble and avoid it. But
when anxiety and stress run rampant, many of the
body’s core protective and restorative processes are
disrupted. The results of that disruption can be dire.
THE BODY’S STRESS RESPONSE
stress begins in the brain. More specifi-
cally, it begins in the brain’s limbic system, which
is home to the amygdala. While no part of the brain
works independently, the amygdala is sometimes
called the “emotional center.” One of its roles is to
detect and assess potential threats and, if need be,
notify the hypothalamus, which triggers the body’s
fight-or-flight response. “The fight-or-flight re-
sponse mobilizes our body to meet whatever im-
mediate threat we’re dealing with,” says Darlene
Kertes, an associate professor of psychology at the
University of Florida, whose lab studies the effects
of stress on health and human development.
That mobilization involves the hormone