A sleep study published by a group of Weill Cornell Medicine researchers found that a disruption to the circadian clock may cause obesity. The research suggests external factors, such as stress, may promote fat cell growth when the body's internal clocks are disrupted. 

As senior author of the study and associate professor of biochemistry, Dr. Mary Teruel says, "A lot of forces are working against a healthy metabolism when we are out of circadian rhythm." Therefore, the more we, as medical experts, understand how a disruption to the circadian clock may cause obesity, the more we can do something about it. 

Understanding the Circadian Clock 

The circadian clock, or circadian rhythm, is the 24-hour internal process that dictates the sleep-wake cycle and plays a role in physical, mental, and behavioral health. The circadian rhythm primarily responds to light and dark, which describes why it is easy to fall asleep at night and stay awake during the day. During light exposure, the body sends signals to the brain that initiate alertness to keep us awake and active. On the contrary, during times of dark (night), the body's internal clock promotes melatonin production, encouraging sleep. 

The circadian rhythm can vary based on the biological and master clocks. Biological clocks are defined as the "natural timing devices" regulating circadian rhythms. Almost every tissue and organ in the body contains biological clocks coordinated by the body's master clock. The master clock is a group of ~200,000 nerve cells in the brain. The master clock is directly impacted by the environment, especially light, which explains why the circadian rhythm relies heavily on the day-night cycle. 

When the circadian rhythm is adequately aligned, individuals can achieve restorative health during sleep hours. On the contrary, when the circadian rhythm is disrupted, they may experience trouble falling and staying asleep, potentially jeopardizing their health. 

Causes of Disruption to Your Circadian Clock

There are several reasons the circadian clock may be disrupted and can occur over a short or long period. One common and explanatory cause is sleep disruption through jet lag or shift work. Other explanations may include advanced or delayed sleep phase disorders.

As the name might suggest, delayed sleep phase disorder is characterized by people who like to stay up late at night and sleep late in the morning. Although relatively rare among the general population, delayed sleep phase disorder affects up to 16% of teens and is primarily a result of behavioral habits. 

Advanced sleep phase disorder is the opposite of delayed sleep phase disorder and is a condition that causes people to get tired early in the evening (when it is still light) and wake up early in the morning (when it is still dark). This condition affects about 1% of older adults and is likely an inherited genetic trait.  

Finally, other causes of disruption to the circadian clock that may cause obesity is related to hormonal health. Particularly, during times of high stress, sleep-wake cycles can be negatively impacted. The elevated cortisol levels boost fat cell growth and can lead to weight gain. 

Disruption to The Circadian Clock May Cause Obesity

It is understood that chronic stress and other conditions, like Cushing’s disease, can cause significant fluctuations in glucocorticoids, a class of stress-linked hormones. Glucocorticoids play an important role in fighting inflammation and controlling metabolism and are said to influence the sleep-wake cycle. A disruption in glucocorticoid levels through chronic stress may disrupt the circadian clock, causing weight gain and obesity long-term. 

In the study performed at Weill Cornell Medicine, they injected mice with pellets that released glucocorticoids for 21 days. They found that when the body’s normal daily release cycle of glucocorticoids was disturbed, the mice were likely to have higher fat and insulin levels even though those mice were on a healthy diet. Surprisingly, a temporary protective mechanism resulted from the metabolic changes and boosted fat cell growth and insulin production while reducing excess blood sugar and fat levels in the bloodstream and liver. 

It was concluded that a disruption to the circadian clock may cause obesity as it relates to fluctuations in glucocorticoid levels. This research may lead to further developmental drugs that help reset circadian clocks in patients as a treatment option for obesity. However, more research is needed to understand how to synchronize the body’s cellular and master circadian clocks and promote weight loss.