Living out of sync with your circadian clock can lead to feeling tired/restless or hungry at the wrong times of the day and make waking up in the morning feel unpleasant. However, it’s more than just discomfort: chronic misalignment between your internal and external clock (=”circadian misalignment”) carries serious health risks, which will be discussed in this article.

Whether you’re a shift worker, frequent flyer across multiple time zones, or a night owl trying to stick to a fixed morning schedule, your circadian rhythm is systematically thrown out of balance with your external schedule. Circadian misalignment, also known as circadian disruption when the misalignment is extreme (shift work, jet lag), has been shown to be associated with a variety of negative health outcomes:

How your circadian clock influences your Mental Health

A delayed circadian clock appears to correlate with the likelihood and severity of winter depression, scientifically known as Seasonal Affective Disorder (SAD). People with Delayed Sleep Phase Syndrome (DSPS), where sleep is delayed by at least 2 hours compared to the societal norm, were 3.3 times more likely to develop SAD than the control group [1]. A disrupted sleep rhythm is also associated with non-seasonal depression and bipolar disorder. While the causal relationship is not fully clear, a study on mice showed that circadian rhythm disruption leads to symptoms of depression [2]. For both seasonal and non-seasonal depression in humans, chronotherapy (measures to restore healthy alignment between internal and external clocks) shows significant benefits for depression and is a recommended treatment for SAD [3]. Read more about SAD in our dedicated blog article.

Cancer

Does circadian disruption cause cancer? Night shift work has been classified as “probably carcinogenic” by the International Agency of Research on Cancer (IARC) [4]. This classification is based on “limited evidence of cancer in humans, sufficient evidence of cancer in experimental animals, and strong mechanistic evidence in animal studies.” Several studies found a positive correlation between extensive night shift work and breast or prostate cancer. On the other hand, there are also large cohort studies that found no significant differences. In mice, repeated 8-hour time shifts in the dark-light cycle, compared to 12-hour light-dark schedules, led to an increase in cancer. Additionally, in rats with chronic circadian rhythm disruption, increased tumor cell proliferation was observed [4].

Cardiometabolic Disorders

Even without shift work, inconsistencies in the sleep-wake cycle are a common phenomenon: about a third of the Central European population experiences a social jetlag of 2 hours, the difference between sleep timing on workdays and free days. One study showed that the severity of social jetlag correlates with Body Mass Index (BMI) in the overweight population. It was found that sleep timing is as important a predictor of obesity as sleep duration [5].
Furthermore, several studies show greater insulin resistance and a higher risk of diabetes in people with later sleep-wake timings [6].

Conclusion

The relationship between circadian misalignment and health is the subject of ongoing research. While there is still much to understand in some areas, the benefits of correct circadian alignment are well-established in others, such as winter depression (SAD). Overall, the existing evidence suggests that a consistent rhythm, synchronized with the environment, not only makes you happier and more productive but also keeps you healthier in the long run.