Many diseases have regular daily variations in risk or symptoms tied to the body’s internal clock.

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Your circadian rhythms can affect your health throughout the day. Getty Images

Like others in the modern world, you probably spend a lot of your day living by the clock.

Your body does the same thing with many of its internal functions, except that the clock it uses isn’t on a smartphone.

The body actually has many biological “clocks” that create the body’s circadian rhythms — the physical, mental, and behavioral changes that follow a daily cycle. And now researchers are uncovering how treatment for conditions can be improved with by working with these “clocks.”

There’s a master clock in the hypothalamus in the brain. This is set by the light and dark cycle in your environment. There are also many peripheral clocks composed of molecules in cells throughout the body.

Scientists think that, in general, the circadian system optimizes the functioning of the body. But for people with certain diseases, the circadian system can make symptoms worse at specific times of the day.

Several diseases show regular daily variations in their risk or severity of symptoms.

Cardiovascular disease. The risk of having a heart attack or stroke is highest in the morning. There’s also a second, but lower, peak in the evening for stroke.

These patterns coincide with changes in factors that can affect cardiovascular function, such as stress hormone levels, heart rate, or activity of the autonomic nervous system.

Colds or infections. One study found that fever peaked in the evening for bacterial infection and in the morning for viral infection.

In another study, nasal secretions during a cold were highest in the early morning, decreased during the day, and increased a little in the evening.

Asthma. For most people, asthma symptoms are worse at night than during the day. This coincides with a worsening of lung function.

Seasonal allergies. Seasonal allergy symptoms — sneezing, stuffy nose, and red, itchy eyes — are more common in the morning compared to the rest of the day.

Although many diseases follow daily patterns, factors other than circadian rhythms can also play a part.

For example, with nighttime asthma, lying down or sleeping may contribute to a person’s symptoms.

Steven Shea, PhD, a circadian rhythm researcher at Oregon Health and Science University in Portland, said these factors, along with circadian rhythms, “add together to make asthma symptoms even worse for some people during the night.”

With heart attacks, the mental stress of getting ready for work could contribute to the higher risk of heart attacks in the morning. This may vary throughout the week.

“Monday morning is the worst time for heart attacks because it’s also the first day of the workweek back at work,” said Courtney M. Peterson, PhD, an assistant professor of nutrition at the University of Alabama at Birmingham’s Nutrition Obesity Research Center.

“This is an example of a combined effect due to the circadian rhythm, or biological clock, and behavior or what’s going on with your life,” said Peterson.

Shea’s lab runs controlled studies to further understand how circadian rhythms contribute to daily disease patterns.

In one study, the researchers found that the increase in the hormone epinephrine after exercise was twice as high at 8:30 a.m. as it was at 4:30 a.m.

Epinephrine plays an important role in the cardiovascular system’s response to stress. It causes a number of physiological changes, such as increased heart rate and blood pressure, and more rapid breathing.

This study was done in healthy individuals, but Shea is now doing the same research in people at risk of heart problems, such as older adults and people with obesity or high blood pressure.

“We’re now looking at people with sleep apnea and asking them to exercise at different times of day and night in the lab,” said Shea, “looking at their physiological responses to different challenges at different times.”

Understanding how circadian rhythms affect disease severity can also help doctors treat diseases more effectively — what’s known as chronotherapy.

Some of this has to do with timing medications to match the circadian rhythms.

One study found that taking high blood pressure medications at night may have a bigger effect on lowering blood pressure.

Other research has looked at whether people’s symptoms of asthma and allergies could be improved if they took medications at certain times of day.

Shea said rather than taking a drug so you have the highest dose in the blood throughout the day, you take the medication so it peaks at the time of day when it’s most effective.

“By doing this, you could reduce the side effects and the cost of the drugs,” said Shea, “but you may also improve the efficacy.”

Another approach is to give people vaccinations when their immune system is most likely to produce a beneficial immune response.

One study found that older adults produced more antibodies in response to the influenza vaccine if they were vaccinated in the morning compared to the afternoon.

The timing of medications and vaccinations is only one type of chronotherapy.

Peterson studies how shifting when you eat affects health.

“There’s more and more evidence that the time of day that you eat has an effect on health,” said Peterson.

In one study, she put men with prediabetes on either a 12-hour or 6-hour feeding schedule. Men on the shorter schedule — known as time-restricted feeding — finished dinner by 3 p.m. each day.

Eating earlier and over a shorter period improved the men’s insulin sensitivity, lowered their blood pressure and, surprisingly, lowered their hunger in the evening.

This study combined both eating according to circadian rhythms and intermittent fasting, so it’s difficult to know the circadian effects alone.

But Peterson said that other research has found that eating more of your daily calories for breakfast and lunch — even without changing the time of meals — improves blood sugar control and other risk factors for obesity and type 2 diabetes.

This dietary research is still in its early stages, with no large clinical trials yet. But as larger studies are done, the effects of mealtime on health will become more clear.

“I expect in the next 10 years that we’ll probably have clear national dietary guidelines on meal timing,” said Peterson.

Researchers are learning more about how your circadian rhythms affect your overall health. The body has many biological “clocks” that create the body’s circadian rhythms — the physical, mental, and behavioral changes that follow a daily cycle.

Data has shown that heart attacks are more likely to occur in the morning, while asthma symptoms and fever are more likely to be worse in the afternoon and evening.

Researchers are hoping to discover ways people can stay healthy by harnessing their circadian rhythms.