The time change can have effects on your body’s internal clock that last beyond the first night that you “spring forward.” You can think of your circadian rhythm as the internal schedule your body follows, which helps keep daily bodily functions (not just sleeping and waking, but things like metabolism, too) regularly happening at the right times. Your internal clock is accustomed to daylight and darkness consistently happening at certain times of the day. It can take time to adjust to changes, which is why jet lag happens.
“The body doesn’t do very well when it’s asked to live in a different time zone,” says Elizabeth B. Klerman, MD, PhD, a professor of neurology at Harvard Medical School and a sleep research investigator at Massachusetts General Hospital in Boston. Changing the clocks for DST has effects similar to jet lag, which we know can be damaging to health in the long term.
Many body systems and processes (like sleep, digestion, and heart function) follow circadian rhythms, or patterns that stay consistent from day to day (roughly every 24 hours). These circadian rhythms trigger physical, mental, and behavioral changes in the body that keep us healthy — hormones get released in the morning to boost energy and rev up digestion, for example, according to the National Institute of General Medical Sciences (NIGMS).
Cues from the sun generally keep these circadian rhythms on track (via a network of cells in the brain referred to as the “body clock”), Awad says. But our behaviors — sleeping in much later than usual on a Saturday, spending a lot of time in the evening in front of bright screens (which the body registers as stimulating sunlight), or eating a big meal in the middle of the night — can send mixed messages to the body and misalign the body’s various circadian rhythms.
Traveling across time zones and switching the clocks for DST have similar effects, misaligning your body’s clock to the one on the wall and the rotation of the sun, Awad explains. DST results in more hours of darkness in the morning, and more hours of daylight in the evening.
“Nighttime light and morning darkness shift your body to a later time, but the social clock has moved to an earlier time, so it’s harder to fall asleep, and people get less sleep,” Dr. Klerman says. “And we know that many bad things happen from not getting enough sleep,” she adds.
DST’s Body Clock Effects on Heart Health
Our bodies will, of course, adjust to the new schedule after a few days or a week, but there’s evidence that the change may still affect our health, particularly heart health.
Some previous studies have pointed to an increased risk of heart attacks immediately after DST, but a study of data from nearly 169,000 people found no link between the time change and heart attacks. It might be more accurate to think of the effects of DST on cardiovascular health as a longer-term, slower type of damage.
For example, in a study at Stanford University, researchers used mathematical models to predict that not changing clocks for DST would help to prevent 300,000 strokes and 2.6 million cases of obesity every year in the United States. They explain that the long-term effects of the seasonal DST change may place chronic stress on the cardiovascular system and the way the body uses energy.
The mechanisms behind this phenomenon are still up for debate, and research is ongoing. But it may be that DST disrupts the regular release of the stress hormone cortisol, which may make existing heart problems worse and increase the risk of heart problems in older adults, according to Jamie M. Zeitzer, PhD, a research professor at the Stanford Center for Sleep Sciences and Medicine in Palo Alto, California.
Normally, your cortisol levels rise around the same time every morning to give you the energy you need to ease out of sleep and into a wakeful state. “If you’re getting up too early, that cortisol spike hasn’t happened yet,” Dr. Zeitzer says. Without cortisol to help, your heart has to work a lot harder to get you moving again, increasing your risk of heart problems.
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