Ever wonder why you tend to wake up in the morning at around the same time? Even when you are desperately trying to sleep in on a lazy Sunday morning? It’s almost as if you have an internal alarm clock that is just as accurate as your fancy rainforest sounds-emitting timepiece on the bedside table or the latest alarm app on your smartphone. It turns out you are not imagining it at all. Your body keeps its own time.
Humans, like many other organisms, have internal timers called circadian clocks. As inhabitants of a planet that rotates around the sun every 24 hours, we experience 24-hour day-night cycles that bring predictable changes to our environment. Naturally, it’s to our advantage that our bodies can anticipate those changes and better synchronize all aspects of our physiology.
We can think of our body as analogous to a well-managed factory that keeps a precise schedule. There are multiple coordinating assembly lines, just like the different organ systems in our body. All of them rely on on-time shipments of raw materials and parts to produce their respective products, similar to our reliance on food consumed at regular meal times to maintain energy levels and allow for tissue growth and repair. At the conclusion of a workday, the factory shuts down for the night, and workers go home to rest up for the next day of hard work. This is like the need to sleep and restore our mind and body at night. The master circadian clock, located in the hypothalamus region of our brain, would be equivalent to the manager of such a factory.
Research indicates pretty much all physiological processes are at least partially under the control of our internal clock. The most prominent example is the timing of our sleep-wake cycle. The circadian clock determines when we feel sleepy by generating a rhythm of melatonin secretion by the pineal gland in our brain, which limits this sleep-promoting hormone to the night. When morning approaches, the melatonin level drops, and the body’s alertness and wakefulness are restored.
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The ability of our immune system to combat bacteria and viruses is also tightly controlled by our internal clock. This is because the circadian clock controls the amount of surveillance molecules in our body that are capable of detecting pathogens. Intuitively, it makes perfect sense to limit the production of these and other molecules in our body, for example those important for regulating heart rate, food digestion, cell repair and metabolic waste detoxification, to only certain times of the day to conserve resources. The trick, however, is activating the production of molecules at the times when they are needed by the body. In the case of the immune surveillance molecules, that would be when they are most likely to encounter pathogens. Luckily for us, the circadian clock has evolved for this task.
Our internal clocks also determine when we will have a better appetite, the time of the day when we are more likely to go to the bathroom and even the ideal times for us to exercise. The hormone leptin communicates to our brain to inhibit eating, and its levels generally peak at night, while the “hunger hormone” ghrelin encourages food consumption and normally rises to its highest level during the day. Since the circadian clock controls the timing of the production of these hormones, it has a big influence on when we feel hungry. Bowel movements are similarly controlled by the clock through timely production of a different set of hormones, which promote gastrointestinal movement soon after you wake up. Muscle physiology and energy use also vary over the day, making late afternoon a perfect time for your after-work exercise routine.
Unfortunately, humans have been recently changing the conditions that our circadian clocks depend on to function properly. We’ve modified our lifestyles so much it’s getting difficult for our circadian clocks to adapt and maintain optimal performance. For hundreds of thousands of years, our human ancestors evolved and organized their daily activities under the restriction of day-night cycles without artificial light. It was only within the last 100 to 200 years that it has become common for humans to be indoors throughout the day and then stay active under artificial light well into the night. This translates to our bodies no longer receiving sufficient daytime light and being bombarded by light at night. Since the circadian clock relies heavily on the timely exposure and absence of light, our circadian clocks often don’t get “entrained” or set properly, leading to loss of synchronization between our internal clock and the natural day-night cycles.
If you have ever experienced jet lag after an intercontinental flight or even just flying from coast to coast in the United States, you know the uncomfortable experience when your body clock is out of whack with natural day-night cycles. It’s not surprising that scientists have shown circadian clock disruptions are linked to numerous human diseases, including obesity, metabolic syndrome, diabetes, sleep disorders, depression and even cancer.
We have the opportunity now to live a healthier life with the knowledge we’ve accumulated about circadian clocks and especially about the health effects of modern lifestyles and innovations such as computers, smartphones, intercontinental flights, dark indoor offices and lecture halls and restaurants that stay open until 2 a.m. Better aligning your daily activities with natural day-night cycles will surely help, but that will take discipline.
You can start by exposing yourself to more sunlight during the day, especially early in the morning, to reset your circadian clock to a new day, as well as avoiding artificial light at night. In the long run, to promote healthier living and reduce the economic burden of health care costs, we will likely need significant changes in school and work routines as well as in government policies to take into account scientific findings about circadian biology. To begin with, getting rid of daylight savings time and other circadian clock-disrupting nuisances would prove both popular and healthy.
UC Davis professor of Entomology and Nematology Joanna C. Chiu, Ph.D., studies the genetic control of circadian timekeeping using the fruit fly, Drosophila melanogaster, as an animal model.