Circadian rhythms: It's that (biological) time again!

Michael Cardinal-Aucoin
9 May 2013

Above: Image ©

My sister always had trouble waking up in the morning, especially in high school. "Hurry up!" my mother yelled down the hall. "Get your lazy butt out of bed young lady. You're going to be late for school again." My sister rolled over in her bed, groaned, and finally slid onto the floor, before slowly shuffling her way to the bathroom and getting ready for school. I also struggled to get out of bed during high school, but overall I've generally been more of a morning person. My sister definitely prefers the night. Maybe you’re like her and have trouble waking up early, focusing on that trig problem first thing in the morning, or falling asleep when your parents tell you it's lights out. You're probably not to blame and you're definitely not alone.

Did You Know? The adjective circadian is derived from the Latin words circa (about) and dies (day).We all have biological clocks ticking away inside us, ensuring that all our bodily functions are organized and coordinated into a sort of ballet that plays 24 hours a day. Metabolism, immune system, hormone levels, and cognitive abilities: all of these things vary throughout the day, every day. We often refer to these rhythms as circadian. Most of the time we pay little attention to our circadian clock. We tend to consider things like our sleep-wake cycle to be obvious and mundane. We wake up, eat, work, play, go to sleep, and do it all again the next day without realizing that there are clocks inside us making sure everything happens at the right time.

Sometimes, however, our internal clocks get disturbed. For example, if you travel by plane across multiple time zones, you will experience jet lag. The symptoms are often just fatigue and insomnia, but they may also include constipation, diarrhea, headache, nausea, irritability, sweating, and dizziness. These symptoms result from the discord between your internal clock time and the external local time, a situation referred to as internal desynchronization. These sorts of disruptions can even affect outcomes in professional sports. Using baseball stats, it has been clearly demonstrated that the East-Coast teams can expect to score more runs at home if the visiting team is from the West Coast! Athletes are therefore very interested in reducing the effects of jet lag on their competitive edge.

One way to help your clock synchronize to local time more rapidly is by taking a substance called melatonin. This hormone, produced naturally by the body at night, has long been available to travellers in pill form. Recent evidence shows that taking a drug called sildenafil may have a similar effect. However, I think the use of sildenafil (also known by the trade name Viagra) by travelling athletes might produce other distracting side effects.

Our clocks can be also disturbed by other factors, such as shift work and light at night. Sometimes, these disturbances can prove highly detrimental to health. Recent evidence shows that they can even lead to diseases such as diabetes, depression, and cancer. For example, the disturbances to the body clock caused by shift work have been shown to significantly increase the risk of breast cancer. In fact, in 2008, Denmark recognized breast cancer caused by night-shift work as an industrial injury and began offering compensation to those affected.

Did You Know? In a study of baseball teams based in the Eastern and Pacific time zones, it was found that “the home team could expect to score 1.24 more runs than usual when the visitor had just completed eastward travel."Knowledge of the body clock is therefore crucial to understanding health and disease. It can also be applied during medical treatment. A new discipline called chronopharmacology seeks to synchronize the administration of drugs with the patient's internal biological time. In this way, treatments are made more effective and patients suffer fewer side effects. The biological clock can even affect surgical outcomes. A 2010 study found that patients who underwent liver transplantation surgery at night had a longer operative time and a greater risk of early death than those who underwent surgery during the day. If I ever need a liver transplant, sign me up for the early morning!

As it turns out, internal biological clocks are not unique to humans. Scientists have found that essentially all organisms have a biological circadian clock, including bacteria, fungi, plants, flies, and humans. This suggests that such clocks are very important to survival and must have originated in the distant evolutionary past. The detailed genetic and molecular gears of these clocks have been studied in many organisms and are now well understood. In fact, there is a condition called familial advanced sleep phase disorder, in which one of the clockwork genes is mutated. Those who possess a mutated copy wake up and go to sleep much earlier than those who do not possess the mutation.

In fact, the timing of everyone’s body clock, or their chronotype, is determined by genetic factors. People often refer to larks and owls to describe early and late chronotypes. But just as different people grow to different heights, there is also a smooth distribution of circadian chronotypes within the human population. This means that while most of us lie somewhere in the middle range, there are also more extreme chronotypes. Furthermore, your chronotpye changes with age. Babies have very early chronotypes that gradually become later, before reaching a late peak during adolescence. After that, they slowly get earlier as people advance in years. In other words, adolescents and young adults tend to be more owl-like than either children or older adults.

So if you're having trouble waking up early or focusing on math first thing in the morning, and your parents tell you you're staying up too late, just explain that it's not your fault. It's your internal biological clock and there's nothing you can do about it. And if you're interested in learning more about your chronotype, why not complete an online chronotype questionnaire? Your answers will also help chronobiologists better understand the human body's internal biological clock.


Scholarly articles

Agostino PV, et al. 2006. Sildenafil accelerates reentrainment of circadian rhythms after advancing light schedules. Proceedings of the National Academy of Sciences. 104:9834-9839. Foster RG, Roenneberg T. 2008. Human responses to the geophysical day, annual and lunar cycles. Current Biology. 18:R784-R794. Foster, RG, Wulff K. 2005. The rhythms of rest and excess. Nature Reviews Neuroscience. 6:407-414. Lonze BE, et al. 2010. Operative start times and complications after liver transplantation. American Journal of Transplantation. 10:1842-1849. Phillips, ML. 2009. Of owls, larks, and alarm clocks. Nature. 458:142-144. Recht LD, et al. 1995. Baseball teams beaten by jet lag. Nature. 377:583.


Binkley S. 1990. The Clockwork Sparrow. Prentice Hall, Upper Saddle River, NJ. Foster RG, Kreitzman, L. 2004. Rhythms of Life. Yale University Press, Princeton, NJ. Moore-Ede MC, et al. 1982. Harvard University Press, Cambridge, MA. Palmer, JD. 2002. The Living Clock. Oxford University Press, Oxford, UK. Roenneberg T. 2012. Internal Time. Harvard University Press, Cambridge, MA.

Michael Cardinal-Aucoin

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