Did you know that your body has its own internal network of clocks? These biological clocks help you feel alert during the day, hungry at mealtimes, and sleepy at night. Keeping your body’s daily cycles, or circadian rhythms, in sync is important for your health.

“Circadian rhythms are big influencers in the body,” explains NIH’s Dr. Michael Sesma, an expert in circadian biology. “They affect almost every part of your physiology in one way or another. Learning how the rhythm is generated is critical for understanding health.”

Our natural daily rhythms are synchronized with the sun. A “master clock” in the brain receives direct input from the eyes and coordinates all the biological clocks in the body. During the day, it sends signals to other brain regions to make hormones that will help keep you awake, boost your heart rate, and give you energy. In the evening, when less light enters your eyes, it triggers production of a hormone called melatonin. Melatonin makes you feel drowsy and helps you stay asleep.

“Many of your body’s functions and normal daily activities—like sleeping, waking, eating, and going to the bathroom—are patterned around this 24-hour cycle,” Sesma explains.

Your biological clock’s “settings” are determined by specific genes. These settings can affect body temperature, blood pressure, activity level, inflammation (your body’s protective response to injury or infection), fertility, mood, and brain functions. Even the timing of health-related events can be related to your biological clocks. For instance, heart attacks are more likely to occur early in the morning, when the level of a hormone called cortisol starts its daily rise.

Circadian rhythms can influence eating habits, digestion, and metabolism (how our body uses and stores energy), too. Researchers have found that eating later in the day, closer to when melatonin is released, can disrupt the body’s natural rhythms. This can lead to increased body fat and weight gain, which are often associated with obesity, heart disease, and diabetes.

Time of day has also been shown to impact the effectiveness and side effects of certain medications, including those used to treat cancer.

Be mindful about how you may be altering your circadian rhythms. “Our around-the-clock society creates challenges for our internal clocks,” Sesma says. “There are lots of modern situations that can disrupt our rhythms, and some may contribute to health problems.”

For instance, shift workers who must be on the job after the sun goes down are at odds with their biological clocks. They may be tired at work and have trouble falling or staying asleep during daylight hours after work. Studies show that shift workers have increased risk for heart disease, digestive disturbances, cancer, depression, and other health problems.

Traveling across time zones can also disrupt your circadian rhythms. The brain has trouble adjusting when the time of day suddenly changes. The result is jet lag.

“Researchers are considering time of day and how to sync up with the body’s clocks in all aspects of health, even the best time to have surgery on specific parts of the body,” Sesma says. These studies may lead to new insights for a range of clock-related disorders, from insomnia and jet lag to diabetes.

    Stick to a regular sleep schedule every day of the week.
    Sleep in a dark, quiet, and comfortable place.
    Avoid heavy meals two to three hours before bedtime.
    Avoid caffeine, nicotine, and alcohol late in the day.
    Exercise daily, but not within two hours of bedtime.
    Limit the use of electronics with bright screens before bedtime.

Written by the NIH Office of Communications and Public Liaison
Editor: Harrison Wein, Ph.D.
Managing Editor: Tianna Hicklin, Ph.D.
Illustrator: Alan Defibaugh