A life without synchronization with your circadian clock can lead you to feel tired, restless, or hungry at the wrong times of the day, making morning wake-ups unpleasant. But it’s about more than just discomfort: A chronic misalignment between your internal and external clock (="circadian misalignment") carries serious health risks.

Whether you are a shift worker, a frequent flyer across multiple time zones, or a night owl adhering 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, occurs when the misalignment is drastic (shift work, jet lag) and has been shown to be associated with a variety of negative health consequences.

How your circadian clock affects your mental health

A delayed circadian clock seems to correlate with the likelihood and severity of winter depression – scientifically known as Seasonal Affective Disorder (SAD). In individuals with Delayed Sleep Phase Disorder (DSPD), where the sleep schedule is shifted by at least 2 hours compared to the societal norm, the likelihood of developing SAD was 3.3 times higher than in the control group [1]. A disrupted sleep rhythm is also associated with non-seasonal depression and bipolar disorders. Although the causal relationship is still not entirely clear, a study with mice has shown that a disruption of the circadian rhythm leads to symptoms of depression [2]. In both seasonal and non-seasonal depression in humans, chronotherapy (measures to restore a healthy alignment between internal and external clocks) shows significant benefits and is considered a recommended treatment for SAD [3]. Learn more about SAD in our follow-up blog post.

Cancer

Does disruption of the circadian system cause cancer? Night shift work is classified as "probably carcinogenic" by the International Agency for Research on Cancer (IARC) [4]. This classification is based on "limited evidence of cancer in humans, sufficient evidence of cancer in animal studies, and strong mechanistic evidence in animals." Various studies have found a correlation between intensive 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 shifts in the dark-light cycle compared to 12-hour light-dark schedules led to an increase in cancer. Additionally, an increased tumor cell proliferation was observed in rats with chronically disrupted circadian rhythms [4].

Cardiometabolic disorders

Even without shift work, inconsistencies in the sleep-wake rhythm are a common phenomenon: About one-third of the Central European population experiences a social jet lag of 2 hours, which is the difference between the sleep time on workdays and on days off. One study showed that the severity of social jet lag correlates with the Body Mass Index (BMI) in the overweight population. It has been found that sleep timing is a similarly important predictor of overweight as sleep duration [5]. Furthermore, several studies have shown greater insulin resistance and a higher risk of diabetes in people with later sleep-wake timing [6].

Conclusion

The relationship between circadian shift and health is the subject of current 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 findings suggest that a consistent rhythm that aligns with your environment not only makes you happier and more productive, but also keeps you healthier in the long run.