Negative health effects

Living out of sync with your circadian clock can make you feel tired/alert or hungry at the wrong times of day and result in a more unpleasant morning awakening. However, there’s more than just discomfort at stake: Chronic misalignment between inner and outer clock bears serious health risks that will be discussed in this article.

Whether you are a shift-worker, frequent trans-meridian traveller or a night-owl adhering to fixed morning schedules: Your circadian rhythm is systematically put out of sync with your external schedule. Circadian misalignment, also called circadian disruption when misalignments are drastic (shift work, jet lag) has been shown to be associated with a variety of negative health consequences:

Mental Health

A delayed circadian clock appears to correlate with likelihood and severity of winter depression, scientifically called seasonal affective disorder (SAD). People with delayed sleep phase disorder (DSPS), where sleep timing is delayed by at least 2 hours compared to societal norm, were found to be 3.3. as likely to have SAD as the control group1. A disrupted sleep schedule is also associated with non-seasonal depression and bipolar disorder. Although the causal relationship not entirely clear, a study with mice has shown that circadian disruption led to symptoms of depression2. For seasonal as well as non-seasonal depression in humans, chronotherapy (measures to re-establish a healthy alignment between inner and outer clock) show significant benefits in depression and are a recommended treatment for SAD3.

Cancer

Does circadian disruption cause cancer? Night shift work is classified “probably carcinogenic” by the International Agency of Research on Cancer
(IARC)4. The classification is based on “limited evidence of cancer in humans, sufficient evidence of cancer in experimental animals, and strong mechanistic evidence in experimental animals”. Various studies found a positive association between extensive night shift work and breast or prostate cancer. On the other hand, there are also large cohort studies that did not find significant differences. In mice, repeated 8-h time shifts in the dark-light cycle led to an increase in cancer compared to 12-h light and dark schedules. Moreover, increased tumor cell proliferation was observed in rats with chronically disrupted circadian rhythm. 4

Cardiometabolic Disorders

Even without shift work, inconsistencies in sleep-wake schedule are a common phenomenon: About a third of the Central European population displays 2 hours of social jetlag, which is the difference between sleep timing on workdays and free days. A study showed that the severity of social jetlag correlates with body mass index (BMI) in the overweight population. Sleep timing was found to be a similarly important predictor of obesity as was sleep duration5.
Moreover, there are multiple studies showing a greater insulin resistance and risk for diabetes in people with later sleep-wake timing6.

Conclusion

The relationship between circadian misalignment and health is a topic of ongoing research. While in many areas, there is a lot yet to be understood, the benefits of a proper circadian alignment are striking in other areas, e.g. winter depression (SAD). Overall, the existing evidence suggests that keeping your rhythm consistent and in-sync with your environment may not only make you happier and more performant, but also keep you healthier in the long term.

[1] Lee, H. J., Rex, K. M., Nievergelt, C. M., Kelsoe, J. R., & Kripke, D. F. (2011). Delayed sleep phase syndrome is related to seasonal affective disorder. Journal of Affective Disorders, 133(3), 573–579. https://doi.org/10.1016/j.jad.2011.04.046

[2] Hamo, M. Ben, Larson, T. A., Duge, L. S., Sikkema, C., Wilkinson, C. W., De La Iglesia, H. O., & González, M. M. C. (2016). Circadian forced desynchrony of the master clock leads to phenotypic manifestation of depression in rats. ENeuro, 3(6). https://doi.org/10.1523/ENEURO.0237-16.2016

[3] Wirz-Justice, A., Benedetti, F., Terman, M., & Basel, S. (2010). Chronotherapeutics for Affective Disorders: A Clinician’s Manual for light and Wake therapy. Annals of Clinical Psychiatry, 22(1), 67.

[4] Ward, E. M.; Germolec, D.; Kogevinas, M.; McCormick, D.; Vermeulen, R.; Anisimov, V. N. et al. (2019): Carcinogenicity of night shift work. The Lancet Oncology 20 (8), pp. 1058–1059. DOI: 10.1016/S1470-2045(19)30455-3.

[5] Roenneberg, T., Allebrandt, K. V., Merrow, M., & Vetter, C. (2012). Social jetlag and obesity. Current Biology, 22(10), 939–943. https://doi.org/10.1016/j.cub.2012.03.038

[6] Abbott, S. M., Malkani, R. G., & Zee, P. C. (2020). Circadian disruption and human health: A bidirectional relationship. European Journal of Neuroscience, 51(1), 567–583. https://doi.org/10.1111/ejn.14298